BE894970A - TABULAR SILVER HALIDE GRAIN EMULSIONS WITH ORIENTED SENSITIZATION SITES - Google Patents

Description

       

  Tabular silver halide grain emulsion bearing

  
oriented awareness sites.

  
The present invention relates to a silver halide emulsion comprising tabular silver halide grains and a dispersing medium.

  
A. Tabular silver halide grains.

  
Silver halide photography uses radiation-sensitive emulsions, which include a dispersing medium, usually gelatin, into which micro-crystals (referred to as grains) of halides of d are incorporated. sensitive to radiation.

  
Different grain patterns, regular and irregular, have been observed in photographic silver halide emulsions. Regular grains are often cubic or octahedral in shape. The grain edges can be rounded due to ripening effects and, in the presence of strong ripening agents such as ammonia, the grains can even be spherical or have the form of thick, almost spherical tablets, as described for example in United States patent 3,894,871 and by Zelikman and Levi in "Making and Coating Photography Emulsions", Focal Press, 1964, pp. 221-223.

   We have frequently observed in variable proportions rod-shaped or tabular-shaped grains associated with grains of other shape, especially when the pAg (i.e. the negative logarithm of the silver ion concentration) of the emulsions were modified during precipitation, as is the case, for example, in single-jet precipitation processes.

  
The tabular silver bromide grains have been the subject of numerous studies, but the grains thus studied were often large grains without photographic utility. What is understood in the present description to be a tabular grain is a grain delimited by two parallel crystal faces 111 or substantially parallel which each have a surface which is considerably larger than any other face of the crystal constituting the grain. The shape index, i.e. the ratio of the diameter to the thickness of a tabular grain, is therefore clearly greater than 1: 1. Silver bromide emulsions! high shape index tabular grains have been described by De Cugnac and Chateau in "Evolution of the Morphology of Silver Bromide Crystals During Physical Ripening", Science et Industries Photographiques

  
Flight. 33, N [deg.] 2 (1962), pp. 121-125.

  
From 1937 until around the 1950s, the Eastman Kodak Company sold a film for radiography called "Duplitized" and whose reference was "No Screen X ray

  
Code 5133 ". This product included on each side of a film support a sulfur-sensitized silver bromide emulsion. Since the emulsions were intended for direct exposure to X-rays, they were not spectrally sensitized. were tabular, had an average shape index of 5 to 7: 1 and these tabular grains represented more than 50% of the projected surface, while the non-tabular grains represented more than 25% of the projected surface. several times, we find that in the emulsion where the shape index is the most

  
  <EMI ID = 1.1>

  
an average thickness of 0.36 µm and an average shape index of 7: 1. Other reproductions of these emulsions have produced thicker, smaller diameter grains which have a lower average shape index.

  
Tabular grain emulsions of silver bromoiodide i

  
have been described in the prior art but none in fact has a high average shape index. The question of tabular grains of silver bromoiodide is discussed by Duffin, Photographie Emulsion Chemistry, Focal Press, 1966, pp. 66-72 and by Trivelli and Smith, in "The Effect of Silver lodide Upon The Structure of

  
  <EMI ID = 2.1>

  
Journal, Vol. LXXX, July 1940, PP. 285-288. According to Trivelli and Smith, a marked decrease in grain size and shape index is observed as iodide is introduced.

  
Gutoff, in "Nucleation and Growth Rates During the Precipitation of Silver Halide Photographie Emulsions", Photo-

  
  <EMI ID = 3.1>

  
August 1970, pp. 248-257, describes the preparation of bromide and silver bromoiodide emulsions by a single jet process using a continuous precipitation apparatus.

  
Methods for preparing major-part emulsions of silver halides in the form of tabular grains have recently been described in publications. U.S. Patent 4,063,951 describes the formation of limited tabular silver halide crystals

  
  <EMI ID = 4.1>

  
(calculated in relation to the edge length) is between 1.5 and 7: 1. Tabular grains have a square shape

  
  <EMI ID = 5.1>

  
U.S. Patent 4,067,739 describes the preparation of silver halide emulsions consisting of

  
mostly twinned crystals of the octahedral type. these crystals are formed by first preparing seed crystals which are then grown by Ostwald ripening in the presence of a solvent of silver halides and the growth of the grains is completed without renucleation or maturation of Ostwald by controlling the pBr (negative logarithm of the bromide ion concentration). U.S. patents 4,150,994, 4,184,877 and 4,184,878, as well as British patent 1,570,581 and German patent application publications 2,905,655 and 2,921,077 relate to the formation of grains of octal tabular twin silver halides from seed crystals with iodide content of at least
90% by mole.

   In several of these references, the increase in covering power is mentioned and it is indicated that the emulsions obtained are useful for black and white and color films. According to United States patent 4,063,951, the upper limit of the shape index of these tabular grains is 7: 1; but, according to the very low shape indices obtained in the examples (2: 1), this upper limit of 7: 1 appears to be overestimated. It is clear, as the reproduction of the examples and the examination of the published photomicrographs show, that the grain shape indices obtained according to the others

  
  <EMI ID = 6.1>

  
U.S. Patent 4,150,994, although not limited to the use of crystal seed seeds of silver iodide. In addition, this publication only relates to the formation of tabular grains of silver chlorobromide containing less than 50 mol% of chloride. No specific example of such an emulsion is provided; however, from the indications given, it seems that the emulsion obtained according to this publication comprises only a relatively small proportion of tabular grains, the shape index of which is, moreover, not higher than that grains described in US Patent 4,150,994.

  
The data relating to the percentages of halides are expressed relative to the silver present in the emulsion, the grain or the part of grain considered; for example a grain of bromoiodide containing 90 mol% of iodide contains
10% by mole of bromide.

  
B. Composite silver halide grains.

  
The idea of combining halides in the same grain structure to combine the advantages produced separately by these halides is old; applications have even been made without the idea itself having been expressed.

  
German patent 505,012 relates to the formation of silver halide emulsions which once developed provide a green tone. This result is obtained with an emulsion from which the silver halides have been precipitated by successive introduction of potassium iodide and sodium chloride. Examination of the emulsions thus prepared reveals the formation of very small silver iodide grains, the average diameter of which is approximately less than

  
0.1 µm. Separate grains of silver chloride are also formed, and examination of electron micrographs further suggests the formation of an epitaxial deposit of silver chloride on the grains of silver iodide.

  
  <EMI ID = 7.1>

  
we observe that the desired green tone turns brown.

  
There are indications similar to those appearing

  
in German Patent 505,012, in Photographische Industrie, "Green-and Brown-Developing Emulsions", Vol. 34, pp. 764, 766, and 872, (1938).

  
British Patent 1,027,146 describes a technique for forming composite silver halide grains. We form silver halide nuclei (or heart), then

  
on these nuclei, one or more contiguous layers of silver halide are deposited. Composite grains contain

  
silver chloride, silver bromide, silver iodide or mixtures of these halides. For example,

  
it is possible, on a silver bromide core, to deposit a layer of silver chloride or of a mixture of silver bromide and silver iodide, or alternatively, on a silver chloride core, a layer of silver bromide can be applied.

  
The deposition of silver chloride on silver bromide, according to English patent 1 027 146, constitutes a means of obtaining

  
the spectral response of silver bromide together with the developability of silver chloride.

  
In U.S. Patent 3,505,068,

  
describes the use of the techniques of the English patent

  
1,027,146 to prepare a slow emulsion layer intended to be associated with a faster emulsion layer, to attenuate the contrast of a dye image. The silver halide grains of the slow emulsion layer have a core consisting of silver iodide or a silver halo-iodide and a shell without iodide, consisting for example of silver bromide, silver chloride or silver chlorobromide.

  
Research has been directed towards obtaining composite silver halide grains comprising a first and a second silver halide, but distributed according to a structure other than that where the first halide forms

  
the core of the grain and the second a shell coating this core. US Patent 4,094,684 describes the deposition by epitaxy of silver chloride on silver iodide in the form of truncated bipyramides (hexagonal structure of Wurtztite type). According to this reference, it is possible to combine in these composite grains, both the characteristics of silver iodide relating to the absorption of light and the characteristics of silver chloride relating to developability. US Patent 4,142,900 gives the same indications with this difference that after the epitaxial deposition, the silver chloride is converted to silver bromide by conventional halide conversion techniques. British patent application 2,053,499 A also provides the same information, except that silver bromide is directly deposited

  
by epitaxy on silver iodide. European patent application 0019917 (published December 10, 1980) describes the epitaxial deposition of silver halide which contains less

  
10% by mole of iodide on silver halide grains containing 15 to 40% by mole of iodide.

  
U.S. Patent 3,804,629 mentions that the protection of silver halide emulsion layers against the effects of dust, especially metal, can be improved if, after physical maturation and washing but before maturation chemical

  
of the emulsion, either a silver chloride emulsion is added to it, or silver chloride is precipitated on this emulsion. According to this reference, the silver chloride deposited in this way on the preexisting grains of silver bromide is in the form of small nipples. In "Surface Structures and Epitaxial Growths on AgBr Microcrystals", Journal of Applied Physics, Vol. 35, No. 7, July
1964, pp. 2165-2169, Berry & Skillman describe the growth of silver chloride on silver bromide. In octa-dric form, silver bromide forms growths

  
) over the entire surface of the crystals which, in this case, are

  
more reactive than cubic crystals. Cubic crystals mainly react at the corners and along the edges. On tabular twin crystals, the growths are distributed randomly over all of the main faces, with a preference for regions close to the edges. In addition, protuberances may have a linear distribution after the emulsion layers have been folded.

  
C. Speed, granularity and awareness.

  
During the photographic exposure, latent image centers are formed by absorption of only a small number of quanta of radiation which make a whole grain selectively developable. It is this property that precisely gives silver halide photography its exceptional speed, compared to many other imaging means.

  
Different types of chemical sensitization have been developed, for example by means of noble metals such as gold, by chalcogens such as sulfur and / or selenium and by reduction sensitization. These sensitizations, used alone or in combination, make it possible to improve the sensitivity of the emulsions to silver halides. When the chemical sensitization is pushed beyond an optimal level, relatively small gains in speed are obtained, which are further accompanied by a sharp drop in image discrimination (maximum density minus minimum density). This decrease results from an increase in the haze (minimum density). Optimal chemical sensitization therefore consists in finding the best compromise between speed, image discrimination and minimum density, for a determined photographic application.

  
In general, when the silver halide emulsions are sensitized chemically, the extension of sensitivity which is obtained beyond the spectral region of natural sensitivity is completely negligible. To sensitize the silver halide emulsions over the entire visible spectrum and even beyond, spectral sensitizers are used which are methine dyes. The sensitivity of the emulsion is extended from the region of natural sensitivity as a function of the concentration of spectral sensitizing dye and this until reaching an optimum beyond which in general this sensitivity then decreases rapidly (conf. Mees, Theory of the Photography Process, Macmillan, 1942, pp. 1067-1069).

  
In the range of dimensions of the silver halide grains of common photographic products, the maximum speed that can be achieved for optimal sensitization increases linearly with increasing grain size. We can consider that the number of quanta that a grain must absorb to become developable is independent of its size. However, the density that a given number of grains forms during development is directly related to the

  
  <EMI ID = 8.1> size of these grains. If the objective is to produce a maximum density of 2 for example, less is needed

  
  <EMI ID = 9.1>

  
you. In total, less radiation was necessary to make a smaller number of grains developable.

  
Unfortunately, the density produced is concentrated on a smaller number of sites and the point variations in density are greater. One who examines the image in itre a perception which is called the grain

  
of the image. The objective measurement of these same variations constitutes granularity. Although quantitative granularity measurements could have been made in different ways, the most common measurement is that known as RMS granularity (root mean square) which can be defined as the standard deviation of density through a micro opening (of As soon as the maximum acceptable granularity has been determined for a given emulsion, the maximum speed that can be obtained with this emulsion is also determined.

  
The real improvements, as regards the sensitivity of the emulsions, are those which make it possible to improve the speed without sacrificing the granularity, or to decrease the granularity without sacrificing the speed, or which make it possible to improve both the speed and the granularity. The usual vocabulary of technology has summarized this definition of improved sensitivity in expression: improvement of the speed / granularity relationship of an emulsion.

  
FIG. 1 schematically represents a graph of the variation of the speed as a function of the granule-

  
  <EMI ID = 10.1> 4 and 5 of the same composition, which each differ in grain size, while also being sensitized, coated and treated identically. The emulsions have respectively a maximum speed and a different granularity, but there exists between these emulsions a predictable linear relationship, as the line shows

  
A representing the variation of the speed as a function of

  
granularity. All emulsions whose representative points can be aligned on line A have the same speed / granularity relationship. Emulsions whose sensitivity is really improved, have a representative point located above the line A. For example,

  
emulsions 6 and 7 whose representative point is located on the speed / granularity curve B have a better speed / granularity relationship than that of any of emulsions 1 to 5. Emulsion 6 has a speed greater than that of l emulsion 1, but its granularity

  
is not stronger. The speed of emulsion 6 is identical to that of emulsion 2, but with a granularity

  
very inferior. The emulsion 7 has a speed greater than that of the emulsion 2, but its granularity is less than that of the emulsion 3 which, itself, has a speed less than that of the emulsion 7. The emulsion

  
8, the representative point of which is located below line A has the worst speed / granularity relationship in FIG. 1. The emulsion 8 indeed has

  
the highest photographic speed of all the emulsions represented, but this speed can only be achieved at the cost of a disproportionate increase

  
granularity.

  
Given the importance of the speed / granularity relationship in photography, significant efforts have been made to be able to determine this relationship in all cases quantitatively. Compare with

  
series

  
  <EMI ID = 11.1> which differ by a single characteristic, for example

  
the grain size of silver halides is normally an easy problem. We have often compared the speed / granularity relationships of photographic products whose characteristic curves are similar. However, quantitative comparisons of speed / granularity relationships of photographic products have not generally been made, since such comparisons have a subjective character which becomes more marked as the other photographic characteristics. become different.

   In addition, comparisons of speed / granularity relationships between photographic products forming analog images, for example, products for black and white photography, and products forming color images, involve many considerations other than Silver halide grain sensitivity issues. Indeed, the substances responsible for the formation of density and therefore responsible

  
of granularity, are of very different origin and nature. Regarding the granularity measurements in silver and color imaging systems, see: "Understanding Graininess and Granularity", Kodak Publication N [deg.] F-20, Revised 11-79 , which can be obtained from Eastman Kodak Company, Rochester New-York 14650; Zwick, "Quantitative Studies of Factors Affecting Granularity", Photography Science and Engineering, Vol. 9, N [deg.] 3, May-June 1965; Ericson and Marchant, "RMS Granularity of Monodisperse Photography Emulsions", Photography Science and Engineering, Vol. 16, N [deg.] 4; July-August 1972, pp. 253-257; and Trabka "A random-Sphere Model Dye Clouds", Photography Science and Engineering

  
Flight. 21, N [deg.] 4, July-August 1977, pp. 183-192.

  
U.S. Patent 3,320,069 describes a silver bromoiodide emulsion with an iodide content of between 1 and 10 mol% and which has <EMI ID = 12.1> a very exceptional speed / granularity relationship for the formation of argentic images (that is to say, black and white images). The emulsion is sensitized to sulfur, selenium or tellurium and it is applied in a layer on a support at the rate of 33 to 110 mg of silver per square decimeter, then it is exposed in an intensity scale sensitometer and treated at 20 [deg.] C in the Kodak DK-50 developer

  
  <EMI ID = 13.1>

  
quinone) for 5 minutes. Under these conditions, the emulsion provides a logarithm of rapidity from 280 to 400. On the other hand, the index calculated by subtracting the value of the granularity from the logarithm of rapidity, is between 180 and 220. Preferably, one gold is used in combination with the sensitizer belonging to the sulfur group and the silver halides are precipitated in the presence of thiocyanate. However, if desired, this thiocyanate can be added to the silver halide at any time before washing. The use of thiocyanate during precipitation and sensitization of silver halides is described in US Patents 2,221,805, 2,222,264 and 2,642,361.

   The emulsions of the United States patent of 3,320,069 also provide exceptional speed / granularity relationships for photography.

  
in color, although the granularity values for the dye images are not available.

  
U.S. Patents 3,656,962,

  
3,852,066 and 3,852,607 describe the incorporation of mineral crystalline substances in silver halide emulsions. The intimate physical association between the silver halide grains and these crystals can, according to

  
the patent, altering the light sensitivity of silver halides. The patent of the United States of America

  
3,140,179 mentions that the speed and contrast of an optically sensitized emulsion can be improved

  
if a layer of a silver chloride emulsion is placed under this emulsion, the speed of which is sufficiently low that it does not form any image during the exposure and the development of the sensitized emulsion

  
  <EMI ID = 14.1>

  
a slow silver chloride emulsion with an optically sensitized emulsion of silver chloride or bromoiodide.

  
British patent application 2,038,792 A indicates that cubic grains limited by crystalline faces

  
  <EMI ID = 15.1>

  
truncations, in each case showing faces 111 111 # . Silver chloride is then deposited

  
  <EMI ID = 16.1>

  
can be selectively chemically sensitized at sites where silver chloride has been deposited. This awareness in specific places increases photosensitivity. Composite crystals exhibit the sensitivity of silver chloride but, during development, fixing and washing, they behave as if they were only made of silver bromide. English patent application 2,038,792 A gives no indication of how to carry out such awareness-raising on selective sites.

  
  <EMI ID = 17.1>

  
Suzuki & Ueda, in "The Active Sites for Chemical Sensitization of Monodisperse AgBr Emulsions", 1973, SPSE Tokyo Symposium provides similar indications, except that on the corners of cubic grains of silver bromide,

  
Ostwald maturation of very fine grains of silver chloride is carried out.

  
The subject of the present invention is a silver halide emulsion having improved photographic properties and, in particular, better sensitivity.

  
The emulsion according to the invention comprises a dispersion medium and silver halide grains and it is characterized by the following points:

  
(1) at least half of the total projected surface of the silver halide grains consists of tabular silver halide grains whose thickness is

  
  <EMI ID = 18.1>

  
and the average shape index greater than 8: 1;

  
(2) these tabular grains are limited by opposite and parallel main crystal faces (or practically

  
  <EMI ID = 19.1>

  
(3) these grains carry awareness sites whose orientation with respect to the grain is selective.

  
The shape index is the ratio of the diameter to the grain thickness and the grain diameter is the diameter of a circle having the same surface as the projected surface

  
grain.

  
The present invention provides significant advantages over the prior art. According to one embodiment, very high sensitivities are obtained with tabular grain emulsions according to the invention which have not been sensitized according to conventional techniques of chemical sensitization, for example sensitization to gold (noble metal) and / or sulfur (chalcogen) or by reduction. The present invention also offers several other advantages which can be linked directly to the presence of the epitaxial deposit of silver salt. These advantages will be described in detail below. The emulsions according to the invention finally exhibit a better photographic response than conventional, non-tabular emulsions, the grains of which also carry an epitaxial deposit of silver salt.

  
The sharpness of the photographic images can be improved by means of the photographic products of the present invention, in particular if grains of large average diameter are used. When they are sensitized spectrally outside the part of the spectrum where they have their natural sensitivity, the emulsions used according to the present invention exhibit a clear separation between the sensitivity in the blue region of the spectrum where they have their natural sensitivity and the sensitivity in the spectral awareness region. The silver bromide and silver bromoiodide sensitized emulsions for the blue minus used according to the present invention are much less sensitive to blue light.

  
  <EMI ID = 20.1>

  
filter tection to obtain blue minus recordings when exposed with neutral light, for example

  
  <EMI ID = 21.1>

  
of the present invention, it is still possible to obtain very significant increases in speed in blue compared to the natural sensitivity in blue, if spectral sensitizers are used for blue.

  
The emulsions according to the present invention are also useful for all photographic applications, for example for producing radiographic products carrying a layer on each side of a support transparent to radiation and in which the exposure of each layer is controlled through the support . If one compares radiographic products comprising emulsions according to the invention with similar radiographic products but produced with conventional emulsions, it is found that this parasitic exposure through the support can be reduced thanks to the use of emulsions according to invention and with lower silver titles;

  
the silver titer is the amount of silver halide per unit area of the layer.

  
Another form of improvement consists, thanks to

  
7 emulsions according to the invention, to produce products having the same level of exposure through the support but with a reduced silver titer and / or by improving the speed / granularity relationship. Photographic products according to the invention, in the form of elements for image transfer photography, make it possible to obtain better image clarity, faster access to the visible transferred image and better contrast of the image. image transferred for a shorter development time; they also make it possible to reduce the silver titer and to make better use of the dye formers and finally they allow advantageous product structures with elimination of the yellow filters and a less dependence of the image formation with respect to the temperature.

  
FIG. 1 is a graph representing the variation of the speed as a function of the granularity, FIG. 4 is a diagram relating to the phenomena of light scattering and, FIGS. 2, 3 and 5-26 are photomicrographs of emulsions.

  
D. Tabular grains and their preparation.

  
The tabular grains according to the present invention are limited by parallel and opposite crystal faces

  
  <EMI ID = 22.1>

  
hexagonal or triangular rations.

  
According to the present invention, the term "high shape index" describes grains of silver halides of which

  
  <EMI ID = 23.1>

  
of average form greater than 8: 1 and they represent at least
50% of the total projected surface of the silver halide grains.

  
The preferred silver halide tabular grains according to the present invention are those whose thickness is <EMI ID = 24.1>

  
medium in shape is greater than 12: 1 and preferably 20: 1.

  
The silver halide grains which have the characteristics of diameter and thickness indicated above represent at least 70% and preferably at least 90% of the total projected surface of the silver halide grains.

  
The thinner the tabular grains representing a given percentage of the projected surface, the higher is the average shape index of the emulsion. In general, the average thickness of the tabular grains is at least 0.03 µm and,

  
preferably at least 0.05 µm, although in principle even thinner tabular grains can be used, for example grains of 0.01 µm thick. For use in image transfer photographic films, it may be desirable to use thicker tabular grains (up to 0.5 µm). In what follows, it is indicated that one can use average thicknesses of grains

  
  <EMI ID = 25.1>

  
blue. However, to obtain high shape indices without excessively increasing the diameter of the grains, it is considered that normally the average thickness of the tabular grains according to the invention should not exceed 0.3 μm. The values given in the present description for the thickness of the tabular grains correspond to the thickness of the carrier grain, excluding the additional thicknesses resulting from the epitaxial deposition of silver halide.

  
The grain characteristics of the silver halide emulsions indicated above can be readily demonstrated by methods well known in the art. As indicated, the expression "shape index" designates the ratio of the diameter of the grain to the thickness of these grains. The term "diameter" itself is defined as the diameter of a circle having an area equal to the projected area of the grain as it appears on a photomicrograph or on an electron micrograph of the sample. emulsion. From the shadows cast by an electron microscopy image of an emulsion, it is possible to determine the thickness and diameter of each grain and to identify those of the tabular grains whose

  
  <EMI ID = 26.1>

  
given, the shape index of each of these tabular grains can be calculated and we can average these shape indices which constitutes the average shape index. According to this definition, the average shape index is the average of the shape indices of each grain. In practice, it is generally simpler to obtain an average thickness and an average diameter of the tabular grains having a thickness of

  
  <EMI ID = 27.1>

  
diameter of at least 0.6 µm and calculate the average shape index which is then the ratio of these two means. Whatever the evaluation method chosen, and taking into account

  
of the size measurement tolerances, the values obtained for the average shape index do not differ significantly.

  
One can add up the projected surfaces of the tabular grains of silver halides which satisfy the conditions of thickness and diameter then separately, one can add up the surfaces of the other grains of silver halides of the photomicrography; from these two respective sums, one can obtain the percentage of the total projected surface occupied by the tabular grains of silver halides whose thickness and diameter satisfy the conditions expressed above.

  
For the above assessments, a reference tabular grain was chosen; this grain has a thickness of less

  
  <EMI ID = 28.1>

  
is intended to distinguish tabular grains of small thickness from thicker tabular grains whose characteristics

  
"7 photographic terristics are smaller. A grain diameter of 0.6 µm has been chosen as a reference since for smaller diameters it is not always possible to distinguish grains which are tabular from those which are not. are not.

  
The term "projected area" is used in the same sense as the terms "projective area" or "projection area", commonly used in the art (see for example James & Higgin s, Fundamentals of Photography Theory, Morgan & Morgan New- York, page 15.

  
High form index tabular grain emulsions can be prepared as follows. A dispersion medium is introduced into a conventional reactor for the precipitation of silver halides, equipped with an appropriate stirring device. In general, the dispersion medium thus introduced initially into the reactor represents at least about 10% and preferably from 20 to 80% of the total mass of the dispersion medium which will be present in the emulsion at the end of the precipitation.

   The dispersion medium can be removed from the reactor by ultrafiltration during the precipitation of the silver bromoiodide grains, according to the indications of Belgian patent 886,645 and French patent 2,471,620; however, the volume of dispersion medium initially present in the reactor can be equal to or even slightly greater than the volume of the silver halide emulsion which will be in the reactor at the end of the precipitation of the grains. Preferably, the dispersion medium introduced at the start consists of water or of a dispersion of peptizer in water possibly containing other substances, for example one or more maturing agents or one or more metal dopants, described in more detail below. When a peptizing agent is present

  
at the start, its concentration represents at least 10% and preferably at least 20% of the total peptizing agent present at the end of the precipitation of silver bromoiodide. An additional quantity of the dispersion medium is added to the reactor with the silver salt and the halides and optionally by means of a separate jet. Usually, the proportion of dispersing medium is adjusted, in particular in order to increase the proportion of peptizing agent, once the addition of the salts is complete.

  
A small part, generally less than 10% by mass of the bromide used to form grains of silver bromoiodide, is present at the start in the reactor to adjust the concentration of bromide ions at the beginning of the precipitation of silver bromoiodide. In addition, at the start of precipitation, the dispersion medium contained in

  
the reactor should in principle not contain iodide ions; in fact, the presence of iodide ions before the introduction of silver salt and bromide promotes the formation of thick grains which are not tabular. When in

  
the present description, it is indicated that the reactor does not in principle contain iodide ions, this means, with regard to the ingredients present in the reactor, that if the iodide ions are present in the reactor, their concentration relative to the bromide ions is insufficient to cause precipitation of a distinct phase of silver iodide. This is why, in practice, it is preferred to maintain the iodide concentration in the reactor below 0.5 mol% relative to the total amount of halides present in the reactor before the introduction of the silver salt. If the pBr of the dispersion medium is initially too high, the tabular grains of silver bromoiodide produced will be comparatively thick and have

  
therefore a low form index. So we watch

  
  <EMI ID = 29.1>

  
  <EMI ID = 30.1>

  
On the other hand, if the pBr is too low, this promotes the formation of silver bromoiodide grains which are not

  
¯1 tabular; this is why the pBr must be kept equal

  
or greater than 0.6 and preferably greater than 1.1. It will be recalled that pBr is the negative logarithm of the concentration of bromide ions and that the pH, pCl, pi, pAg are respectively defined in the same way for the concentrations of hydrogen, chloride, iodide and silver ions.

  
During precipitation the silver salts, bromide and iodide are added to the reactor according to well known techniques. In general, a solution of a soluble silver salt, for example a solution of silver nitrate, is introduced into the reactor, together with solutions of bromide and iodide. Bromide and iodide are also introduced in the form of an aqueous solution of one or more ammonium or alkali metal salts, for example sodium or potassium, or alkaline earth metal salts, for example magnesium or calcium. The silver salt, at least initially, is introduced into the reactor by a separate jet from that of the iodide. The iodide and bromide can be added to the reactor separately or as a mixture.

  
When a silver salt is introduced into the reactor, the nucleation phase of the grain formation process is started. We form a population of germs capable of constituting precipitation sites for arom bromide.

  
  <EMI ID = 31.1>

  
silver salt, bromide and iodide salt continues to be added. The precipitation of bromide and silver iodide on existing germs constitutes the growth phase of grain formation. The shape index of the tabular grains formed according to the present invention is less influenced by the concentration of iodide and bromide during the growth phase than during the nucleation phase. This is why during the growth phase, it is possible to increase the latitude for fixing the pBr so as to have, during the introduction of the salts, a pBr of between approximately 0.6 and 2.2 and preferably between 0, 8 and 1.6. This latter interval is particularly preferred when the rate of germ formation continues appreciably during the introduction of the salts, as is the case in the preparation of emulsions with high polydispersity.

   If the pBr rises

  
above 2.2 during the growth of the tabular grains, this leads to the formation of thicker grains, but there is in many cases a tolerance which makes it possible to still obtain shape indices greater than 8: 1.

  
A variant for the introduction of silver salts,

  
to introduce bromide and iodide

  
at the start or in the growth phase, in the form of fine grains of silver halides suspended in a dispersion medium. The diameter of the grains is such that their maturation of Ostwald on larger seeds is possible as soon as they are introduced into the reactor, if such seeds are present. The maximum useful grain diameter depends on the specific conditions prevailing in the reactor, for example the temperature and the presence of a solubilizing or maturing agent. It is thus possible to introduce grains of silver bromide, of silver iodide and / or of silver bromide; since bromide and / or iodide preferentially precipitates over chloride, it is also possible to use grains of silver chlorobromide and silver chlorobromoiodide; these silver halide grains are preferably very small, that is to say

  
  <EMI ID = 32.1>

  
Subject to the conditions relating to pBr set out above, the concentration and the flow rate of the jets of bromide silver salts and of iodide can be produced in the appropriate conventional forms. The silver salts and the halides are preferably introduced at a concentration of between 0.1 and 5 moles per liter, although wider intervals can be envisaged ranging from

  
  <EMI ID = 33.1> preferred cipitation are those which allow the shortest precipitation times to be obtained by increasing the quantities of salts..silver and halide introduced during precipitation. This increase can be obtained <EMI ID = 34.1> salt jets. We prefer to increase the flow rate of the jets, but

  
by keeping it below a threshold from which the formation of new germs would be favored, i.e.

  
say from which re-nucleation would occur according to the indications given to the patents of the United States of America. 3,672,900, 3,650,757, 4,242,445, as well as on publication

  
from German patent application 2,107,118, to European patent application 80102242 and by Wey in Photographic Science and Engineering, Vol. 21 N [deg.] 1, January / February 1977, p. 14 et seq. By avoiding the formation of additional germs in the growth stage, it is possible to obtain relatively monodisperse populations of tabular silver halide grains. Emulsions with coefficients of variation of less than about 30% can be prepared. We

  
  <EMI ID = 35.1>

  
grain diameter multiplied by a hundred and divided by the average grain diameter. If deliberately promoting renucleation during the growth phase, it is of course possible to form polydispersed emulsions whose coefficient of variation is significantly higher.

  
The concentration of iodide in the silver bromoiodide emulsions according to the present invention can be controlled by acting on the introduction of iodide. Any conventional iodide concentration can be used. Even very low concentrations, for example as low as

  
  <EMI ID = 36.1>

  
geuses. Preferably, the emulsions according to the invention contain approximately at least 0.1 mol% of iodide. Silver iodide can be incorporated into tabular grain emulsions of silver bromoiodide until the solubility limit of silver iodide in silver bromide is reached at the grain formation temperature. Thus, for precipitation temperatures of 90 [deg.] C, the concentration of silver iodide can reach 40 mol% in the tabular grains of silver bromoiodide. In practice, the precipitation temperature can be lowered almost to room temperature, i.e. up to 30 [deg.] C, but in general, it is preferred to carry out precipitation at

  
  <EMI ID = 37.1>

  
in photographic applications, the maximum iodide concentration is limited to about 20% and preferably does not exceed about 15%.

  
The respective amounts of iodide and bromide introduced into the reactor during precipitation can

  
be maintained in a fixed ratio, so as to obtain tabular grains of silver bromoiodide whose iodide concentration has a uniform profile, but this profile can be varied to obtain various photographic effects. Particular advantages can be obtained by increasing the proportion of iodide in an annular or peripheral region of the silver bromoiodide grains with a high shape index; this means that in these regions of the tabular grain, the iodide concentration is higher than it is in the center of the grain.

   The iodide concentration in the center of the grain can be between 0 and 5 mol% and, at least in a peripheral annular region, the iodide concentration can go up to the solubility limit of silver iodide in silver bromide; preferably, the iodide concentration can reach up to 20 mol%, and optimally up to about 15 mol% in this annular region.

  
The tabular silver bromoiodide grains according to the invention may have concentration profiles

  
uniform or variable iodide and this variation

  
can be controlled if desired to promote a higher iodide concentration either inside or near the surface of, or on, the tabular grains of silver bromoiodide.

  
The preparation of tabular grain emulsions of high shape index has been described with reference to a process providing emulsions which are neutral or non-ammonia; but the present invention is not limited to this particular process. According to another method, the silver halide seeds are initially present in the reactor. The concentration of silver iodide in the reactor is lowered below 0.05 mol per liter and the maximum diameter of the grains of silver iodide initially present in the reactor below 0.05 m m.

  
Tabular-grain silver bromide emulsions of high form index without iodide can be prepared by the method described in detail above modified

  
do not use iodide.

  
High bromide tabular grain silver bromide emulsions can also be prepared according to a method based on the indications given by De Cugnac and Chateau in the publication cited above.

  
The examples below provide other modes

  
for preparing silver bromide emulsions with tabular grains of high form index without iodide.

  
The variety of emulsions which can be used according to the invention can be illustrated by the fact that tabular grains of silver chloride can be prepared, the interior of which contains practically neither silver iodide nor silver bromide. To this end, a double-jet precipitation process is implemented, according to which one introduces

  
at the same time chloride and a silver salt in a reactor containing a dispersion medium in the presence of ammonia. During the introduction of the chloride, the pAg in the dispersion medium is maintained between 6.5 and 10 and the pH between 8 and

  
10. At high temperatures, the presence of ammonia tends to cause the formation of thick grains. This is why the precipitation temperature does not exceed 60 [deg.] C to obtain silver chloride emulsions with tabular grains of high shape index.

  
It is also possible to prepare tabular grains containing at least 50 mol% of chloride and, of which two opposite crystalline faces are in crystalline planes.

  
  <EMI ID = 38.1>

  
main. Such tabular grain emulsions can be prepared by reacting aqueous solutions of silver salt and halides containing chloride in the presence of an aminoazaindene in an amount capable of modifying the crystalline form and of an agent. peptizer containing a thioether bridge.

  
  <EMI ID = 39.1>

  
in which the grains contain chloride and bromide at least in one annular region of the grain and preferably throughout the grain. The regions of the tabular grains containing silver, chloride and bromide are formed by maintaining the molar ratio of chloride to bromide between 1.6: 1 and 260: 1 and maintaining the total concentration of halide ions in the reactor between 0.10 and 0.90 N during the introduction of the silver salt, chloride and bromide and optionally iodide, into the reactor. The molar ratio of silver bromide to silver chloride in the tabular grains is between 1:99 and 2.3.

  
The tabular grain emulsions according to the present invention can have very high shape indices. These shape indices can be increased by increasing the grain diameter. One can thus obtain advantages as regards sharpness, but the maximum diameter of the grains is generally limited by considerations of granularity for a given photographic application. Shape indices can be increased by decreasing the average grain thickness. When the silver title is kept constant, the reduction in the thickness of the tabular grains generally leads to an improvement in granularity in direct proportion to the increase in the index.
-of form.

   As a result, the maximum average shape index of the tabular grain emulsions according to the invention depends on the maximum average diameter acceptable for the photo- <EMI ID = 40.1>

  
can get. The maximum shape indices that we have observed are variable depending on the precipitation techniques.

I

  
tion used. The highest form indices that

  
a observed are 500: 1 for the tabular grains still having grain diameters useful from a photographic point of view; these grains were obtained by Ostwald ripening of silver bromoiodide grains; we have: obtained indices of 100: 1, 200: 1 or more with techniques

  
double jet precipitation. The presence of iodide generally leads to a reduction in the maximum average shape indices that can be obtained, but it is also possible to prepare emulsions with tabular grains of silver bromoiodide whose shape indices reach 100: 1, and even
200: 1 or more. It is possible to prepare tabular grains of silver chloride possibly containing bromide and / or iodide, the shape indices of which reach

  
50: 1 or even 100: 1.

  
Modifiers may be present during precipitation of the tabular grains, either initially in the reactor, or added along with one or more of the salts, according to conventional methods. These modifiers can be compounds of copper, thallium, lead, bismuth, cadmium, zinc, medium chalcogen (i.e. sulfur, selenium and

  
tellurium), gold and noble metals of group VIII, according to the indications given to the patents of the United States of America 1 195 432, 1 951 933, 2 448 060, 2 628 167,

  
2,950,972, 3,488,709, 3,737,313, 3,772,031, 4,269,927 and in the journal Research Disclosure, volume 134, June 1975 publication 13452. The journal Research Disclosure and the title that preceded it, Product Licensing Index , are published by Industrial Opportunities Limited; Homewell, Havant; Hampshire, P09; 1EF; United Kingdom. Tabular grain emulsions can be sensitized by reduction inside the grains during precipitation, as described by Moisar et al; Journal of Photographie Science, Volume 25, 1977 pages 19 to 27.

  
30

  
The silver salts and the halides can be added to the reactor by means of surface or sub-surface supply tubes, by gravity feed or by means of devices which allow the regulation of the speed of addition as well as pH, pBr and / or pAg of the reactor contents, according to the indications given to the patents of the United States of America 3 821 002 and 3 031 304 and by Claes in Photographische Korrespondenz, volume 102, n [ deg.] 10, 1967, page 162. To obtain a rapid distribution of the reactants in the reactor, one can use specially adapted mixing devices such as those described in the patents of the United States of America 2 996 287, 3 342 605 , 3,415,650,

  
3,785,777, 4,147,551 and 4,171,224 ,. to British patent application 2,022,431A, to German patent applications

  
2,555,364 and 2,556,885 and in Research Disclosure, volume
166, February 1978, publication 16662.

  
To precipitate tabular grain emulsions,

  
a dispersing medium is initially present in the reactor. Advantageously, the dispersing medium is formed of an aqueous suspension of peptizer. The concentration of peptizer can be between 0.2 and about 10% by mass relative to the total mass of the constituents of the emulsion in the reactor. It is common to keep the peptizer concentration in the reactor below around 6%

  
of the total mass, before and during the formation of the silver halide, and later to adjust the vehicle concentration of the emulsion to higher values (the term vehicle including the binder and the peptizer), by additional vehicle additions to achieve optimal sleeping characteristics. The emulsion initially formed can contain approximately from 5 to 50 g of peptizer per mole of silver halide, preferably approximately from 10 to

  
30 g per mole of silver halide. An additional vehicle can be added later to bring the concentration up to 1000 g / mole of silver halide. Advantageously

  
^ 7 in the finished emulsion, there are more than 50 g of vehicle per mole of silver halide. Once coated and dried

  
  <EMI ID = 41.1>

  
30 to 70% by mass of the emulsion layer.

  
Vehicles can be chosen from the substances usually used in silver halide emulsions for this purpose. The preferred peptizers are hydrophilic colloids which can be used alone or in combination with hydrophobic substances. Suitable hydrophilic vehicles include substances such as proteins, protein derivatives, derivatives

  
cellulose, for example cellulose esters, gelatin, for example gelatin, treated with an alkaline agent (skin or bone gelatin) or gelatin treated with an acidic agent (pig skin gelatin), derivatives of gelatin, for example acetylated gelatin and phthalylated gelatin. These and other vehicles are described in Research Disclosure vol. December 176, 1978, publication 17643, section IX.

  
Vehicles in particular hydrophilic colloids, as well as useful hydrophobic substances combined with them can be used not only in the emulsion layers of the elements of the photographic product of the invention, but also in other layers, such as overlays, interlayers and layers placed below the emulsion layers.

  
The preparation of the silver halide emulsions according to the invention may comprise a step of maturation of the grains and the preferred way consists in carrying out the maturation of the grains in the reactor during at least the formation of the silver halide grains. Solvents of known silver halides are used to promote maturation, such as for example an excess of bromide ion in the reactor. It is known, for example, that the bromide solution introduced into the reactor can itself promote maturation. It is also possible to use other maturing agents, which can be entirely incorporated into the dispersing medium in the reactor before the addition of salt.

  
  <EMI ID = 42.1>

  
the reactor 'together with one or more of the silver salts, halides or peptizer. According to another mode

  
In one embodiment, the curing agent can be introduced independently during the addition of the halide and the silver salt. Although ammonia is a known ripening agent, it is not a preferred ripening medium.

  
for the emulsions according to the invention whose speed / granularity relationship is the highest.

  
Advantageous ripening agents are those which contain sulfur. Thiocyanates can be used in the form of alkali metal salts, usually sodium and

  
potassium, and ammonium thiocyanates. Conventional amounts of thiocyanate can be used, but advantageous concentrations are included in. general between about

  
0.1 and 20 g of thiocyanate per mole of silver halide. The use of thiocyanate as a ripening agent is described in US Patents 2,222,264,

  
2,448,534 and 3,320,069. Thioethers such as those described in US Patents 3,271,157, 3,574,628 and 3,737,313 can also be used conventionally.

  
The tabular grain emulsions with a high shape index are preferably washed to remove the soluble salts, by known techniques such as decantation, filtration and / or by jelling and filtration, like this

  
is described in Research Disclosure, Vol. 176, December 1978; publication 17643, section II. It is particularly advantageous according to the present invention to complete the maturation of the tabular grains by washing after the end of precipitation, to avoid increasing their thickness, reducing their shape index and / or excessive increase in their diameter. Emulsions with or without a sensitizer can be dried and stored before use.

  
The methods of preparing tabular grains described above make it possible to obtain emulsions in which the tabular grains meeting the criteria of thickness and diameter necessary to obtain a high shape index, represent at least 50% of the total surface projected of the total silver halide grain population; but additional benefits can be obtained by increasing the proportion of tabular grains. It is advantageous that at least 70% (and optimally,

  
at least 90%) of the total projected surface is occupied by tabular silver halide grains meeting the criteria of thickness and diameter. Although the presence of small amounts of non-tabular grains is fully compatible with most photographic applications, the proportion of tabular grains can be increased to obtain the full benefits of tabular grains. Larger tabular silver halide grains can be mechanically separated from smaller grains that are not tabular in a mixed population of grains, using conventional separation means, for example, a centrifuge or a hydrocyclone. Separation by hydrocyclone is illustrated in US Patent 3,326,641.

  
E. Epitaxy and sensitization on predetermined sites.

  
A characteristic of the present invention resides in the fact that a tabular grain as defined above by the parameters of thickness, diameter and shape index, also has awareness sites whose orientation relative to grain is predetermined. According to a preferred form, the tabular grains are carriers

  
a deposit of at least one silver salt produced by epitaxial growth. In other words, this silver salt is in a crystalline form whose orientation is determined by the grain of tabular silver halide serving as support for this epitaxial growth. In addition, the silver salt deposited by epitaxy is located in sites distributed in a determined manner on the surface of the grain. The epitaxial deposit of silver salt can be located in the central part of each main face of the tabular grains or else in an annular region of each of these faces, and / or in a peripheral region on the edges delimiting these main faces. According to a preferred variant, the epitaxial deposit of silver salt is located at the tops of the tabular grains, or near these tops.

   Epitaxial deposition can reveal several of these forms simultaneously. For example, the epitaxial deposit can be located both in the central region of the main faces and on the vertices or along the edges of the tabular grains.

  
In all cases, regardless of the location of the epitaxial deposit of silver salt, the result is that in

  
at least a predetermined part of the main faces

  
  <EMI ID = 43.1>

  
silver.

  
It has been discovered that, surprisingly, the localization of the epitaxial deposit of silver salt in determined sites on tabular grains makes it possible to obtain a

  
gain in sensitivity by comparison with what can be obtained with an epitaxial deposit of this same silver salt distributed in some way on the main faces

  
of these tabular grains, as had been observed

  
by Berry and Skillman, "Surface Structures and Epitaxial Growths on AgBr Microcrystals", Journal of Applied Physics, Vol. 35, No. 7, July 1964, pp. 2165-2169.

  
The degree of localization of the silver salt at determined sites and the part of the main faces of the tabular grains which is consequently left without epitaxial deposition of silver salt are very variable within the scope of the present invention. In general, the greatest increases in sensitivity are obtained when the area occupied by the epitaxial deposit decreases. The epitaxial deposition of silver salt can thus be limited to less than half the surface of the main faces of the tabular grain, preferably to less than 25% of this surface; in certain embodiments, for example in the case where the epitaxial deposition is carried out at the apices of the grain, this deposition advantageously represents less than 10% or even less than 5% of the surface of the main faces of the tabular grain.

   In some cases, it has been observed that epitaxial deposition begins to form on the edges of the tabular grain. Consequently, when the epitaxy is limited, it can be located in awareness sites chosen on the edges and effectively excluded from the main faces.

  
This epitaxial deposit of silver salt can be used to create awareness sites on tabular silver halide grains, called "carrier grains"

  
By controlling the epitaxial deposition sites, it is possible to sensitize the carrier grain on selective sites. Awareness is carried out on one or more sites whose arrangement is ordered, that is to say that the arrangement of these sites with respect to the main faces of the crystal and preferably the arrangement of these sites with respect to each other, is not random and obeys

  
to a predictable relationship.

  
By controlling the epitaxial deposition relative to the main faces, one can control both the number and the spacing of the sensitization sites between them.

  
In some cases, sensitization can be detected by selective sites: this is so when silver halide grains are exposed to radiation to which they are sensitive and when superficial latent image centers are formed on the sites. awareness.

  
  <EMI ID = 44.1>

  
/ If the grains bearing latent image centers are fully developed, the latent image centers can neither be located nor counted. But, if we stop the development before it is propagated beyond the immediate proximity of the latent image center, and then we examine the partially developed grain under an appropriate magnification, we see that the partial development clearly appear sites. They generally correspond to the latent image centers which themselves correspond to the awareness sites.

  
This is illustrated in Figure 2, which is a photomicrograph of a sensitized tabular grain according to the present invention and partially developed. The black dots correspond to the developed silver. Although the silver has grown irregularly beyond the grains, it is noted that the points of contact between the silver and the tabular grains are ordered. This means that the location of these contact points at the apexes of the grains is predetermined. Thanks to this location, the contact points are spaced, and their number, for a given grain, is limited.

  
In contrast to this ordered location of the sensitization sites of Figure 2, Figure 3 shows a tabular grain emulsion of high shape index which has not been sensitized in accordance with the present invention. The black dots, corresponding to the developed silver, are distributed in some way on the grains. In many cases, the contact points of the silver developed with a grain edge are very close to each other. We do not observe in FIG. 3 an ordered distribution of the sensitization sites with respect to the main crystal faces of the grain.

  
It is not always possible, as is done with Figure 2, to highlight by stopping the development, the ordering of awareness sites. For example, if the latent image is formed inside the

  
7 grain rather than on the surface or near the surface, it is difficult to visualize the sites by partial development, because the development is accompanied by a dissolution of the grains. It may also happen that the ordering of the awareness sites with respect to the geometry of the grain is not found in the arrangement of the latent image sites which do not appear distributed in any order. When, for example, the sensitization sites arranged in a determined order, function as traps with positive holes, they capture these holes formed by the action of light and sensitize the grains by preventing the recombination of the electrons formed at the same time.

   However, these electrons formed by the action of light remain free to migrate and can form latent images in any favorable place in or on the grain. Thus, the sensitization at distinct and determined sites of the grain according to the present invention can be independent of an ordered or unordered formation of latent image sites on the grain.

  
Sensitization in selective sites according to the invention can often be demonstrated on electron microscopy pictures, without resorting to the technique

  
partial development. For example in FIG. 2, the epitaxial deposit of silver halide used to sensitize the tabular grains on selective sites is clearly visible at the top of these grains. In the emulsion of FIG. 2, it is the deposits of silver salts formed in an orderly fashion at the apices of the grains by epitaxy which provide the selective sensitization sites according to the invention. If the epitaxial deposition is limited, it may not appear on the electron microscopy images of the grains and it is therefore not possible to directly confirm the presence of the selective sensitization sites by this means; one can then give indications on the process of preparation of the emulsions.

  
According to a preferred embodiment, an emulsion

  
silver bromoiodide with tabular grains according to the invention, prepared by the process described above, is sensitized on grain sites arranged in a determined order. The tabular grains of silver bromoiodide have

  
  <EMI ID = 45.1>

  
a spectral sensitizing dye forming aggregates on the faces of the tabular grains by a conventional spectral sensitization technique. The quantity of dye adsorbed must make it possible to produce a monomolecular layer on at least 15% and preferably on at least 70%

  
of the total grain area. Although the amount of dye adsorbed is expressed by reference to a monomolecular layer, it is not necessary for the dye to be distributed uniformly over the surface of the grains. The amount of dye introduced may be greater than that which is adsorbable on the surface of the grain, but in general this possibility is not preferred, because the excess

  
dye does not lead to further improvement.

  
The dye in the form of aggregates is used at this stage

  
sensitization not because of its sensitizing properties, but because of its power to influence the localization of the epitaxial deposit of silver salt on the tabular grains of silver bromoiodide with high form index. Consequently, any other adsorbed substance capable of exerting the same effect on the epitaxial deposit and of being subsequently displaced by the sensitizing dye, can be used. Since the dye in the form of aggregates fulfills both the regulating function of the epitaxial deposit and the spectral sensitization function, and that once applied to the grain there is therefore no need to remove it, it is the preferred substance for regulating epitaxial deposition.

  
When the dye is adsorbed in the form of aggregates on the surface of the silver bromoiodide grains, the silver chloride can be deposited by a conventional technique of precipitation or Ostwald maturation.

  
The epitaxial deposition of silver chloride does not form a shell coating the grain of silver bromoiodide; this silver chloride is also not deposited in any way. In fact, silver chloride is deposited selectively in an orderly arrangement around the tops of the tabular grain. In general, the slower the epitaxial deposition, the fewer the sites where this deposition occurs. We

  
can therefore limit the epitaxial deposition to only part of the vertices of the grain. It is also possible to form an annular silver chloride deposit on the edges of the main crystal faces, the ring possibly being incomplete if the quantity of silver chloride deposited is limited. The epitaxial deposit

  
Silver chloride can itself cause a considerable increase in sensitivity for the composite grains obtained, without additional chemical sensitization.

  
In the foregoing, reference has been made to tabular grains of silver bromoiodide carrying an epitaxial deposition of silver chloride at specific sites. There are other possibilities of combinations of tabular grains and sensitizing silver salt. The halide composition of the carrier tabular grains may be any of those recognized as useful in photography and allowing the formation of emulsion with tabular grains of high shape index. Thus, the tabular grain emulsion which must be sensitized can consist of silver bromide, silver chlorobromide, or silver chloride, possibly with a little silver iodide. The respective proportions of the halides have been indicated above.

  
The sensitizing silver salt deposited on specific sites of the carrier tabular grain can generally

  
be chosen from all the silver salts capable of forming

  
an epitaxial deposit on a silver halide grain and

  
whose photographic utility has been recognized. The anion of this
the silver salt and the tabular silver halide grains differ sufficiently that their respective crystal structures themselves show detectable differences.

  
Surprisingly, non-tabular growths on vertices or edges have been observed when the

  
deposition on the carrier grain is carried out in the presence of a site regulator adsorbed on the grain, even if the halide compositions of the carrier grain and of the deposit at the top or on the edge are identical. Modifiers can be incorporated into the carrier grain or into the repository or into the

  
two, whether their respective halide compositions are the same or different. It is possible to choose the

  
silver salts among those whose utility has been recognized to form the shells of grains of so-called "core i and shell" emulsions. In addition to all photographic silver halides, silver salts can be chosen which can be precipitated on the silver halide grains, for example salts such as silver thiocyanate, silver cyanide , silver carbonate, silver phosphate etc. Depending on the silver salt chosen and the application envisaged, the deposition is carried out in the presence of one of the modifiers described above with regard to the tabular grains.

  
Part of the silver halide forming the carrier grains goes into solution during the epitaxial deposition and is incorporated into the salt thus deposited. For example, the silver chloride deposited on a grain carrying silver bromide generally contains a few bromide ions. The designation of a silver salt constituting an epitaxial deposit on a carrier grain therefore does not exclude the possibility for this deposit to contain a little of the halide of the carrier grain, unless a different indication is expressly given. .

  
It is generally preferred that the silver salt of the epitaxial deposit has greater solubility than the silver halide of the carrier tabular grain. We can thus decrease

  
the tendency of the carrier grain to dissolve during the deposition of the silver salt. This avoids limiting awareness of the conditions which minimize the dissolution of the tabular grain, as would be necessary if, for example, an attempt were made to deposit a less soluble silver salt on a tabular grain formed of a more soluble silver halide . Since silver bromoiodide is less soluble than silver bromide, silver chloride or silver thiocyanate, and can therefore serve as a carrier for the deposition of these salts, it is preferred to use grains carrying bromoiodide d 'silver.

   On the contrary, silver chloride, which is more soluble than bromoiodide or silver bromide, can be deposited by epitaxy on tabular grains made up of any of these halides and it is therefore a salt of preferred money for awareness raising using this technique. Silver thiocyanate, which is less soluble than silver chloride, but much more soluble than silver bromide or bromoiodide can be used in place of silver chloride in many cases. However, to obtain maximum stability, silver chloride is generally preferred to silver thiocyanate. The epitaxial deposit of

  
less soluble silver salts on non-tabular carrier grains consisting of less soluble silver halides, has been described and can be produced according to the present invention. It is possible, for example, to make an epitaxial deposit of silver bromoiodide on silver bromide

  
or thiocyanate or silver bromide on silver chloride. Multiple epitaxies are also possible, that is to say epitaxies in which a silver salt is deposited on a silver salt which has itself been deposited.

  
by epitaxy on a carrier tabular grain. It is thus possible to carry out an epitaxial growth of silver thiocyanate on silver chloride itself applied in the form of an epitaxial deposit on a grain carrying silver bromide or bromoiodide.

Concentration of silver salt of the epitaxial deposit

  
-7 on predetermined sites can be included within fairly wide limits. Sensitivity increases can be recorded with a silver salt concentration as low as 0.05 mol% relative to the total amount of silver present in the sensitized composite grains. On the other hand, the highest sensitivities are obtained when the concentration of silver salt is less than
50% by mole. In general, it is preferred that the concentration

  
epitaxial silver salt is between 0.3 and 25%

  
in moles and advantageously, for optimal sensitization, between 0.5 and 10% in moles.

  
Depending on the silver salt used and the composition

  
  <EMI ID = 46.1>

  
Site readers, adsorbed on the grains, such as dyes in the form of aggregates, can not be used while still regulating the epitaxy on these sites. When the carrier tabular grain contains at least 8 mol% iodide on the surface (and preferably

  
at least 12 mol% of iodide), the silver chloride forms an epitaxial deposit selectively on the regions adjacent to the apexes of the carrier tabular grain, in the absence of adsorbed regulatory compound. Surprisingly, analogous results can be obtained when tabular grains of silver bromide or bromoiodide are contacted with an aqueous iodide solution to incorporate amounts of tabular grains of silver bromide iodide as low as 0.1 mol% before depositing the epitaxial silver chloride. The silver thiocyanate can be used to achieve a selective epitaxial deposition on the edges of the tabular grains of silver halide of any composition described above and in the absence of adsorbed regulatory compounds.

   However, even if this is not necessary, it is often preferred to use an adsorbed regulatory compound so as to restrict the location of the deposited silver salt, on the tops or on the edges.

  
Tabular grains of silver bromoiodide with a high form index, which have a central zone with a lower raw iodide concentration in a peripheral region, can be used. If the peripheral region has a surface iodide concentration of at least 8% by mole, and preferably at least 12% by mole, while the central region contains less than 5% by mole of iodide, it is possible to limit the localization of the sensitization of the tabular grains of silver bromoiodide on a central region of these grains without using an adsorbed regulator. In other words, the surface iodide, in the annular regions of the grains, plays itself the role of regulatory compound for the subsequent epitaxial deposition of a silver salt in the central part of the grains.

   The sensitization surface can be limited simply by reducing the surface of this central zone relative to the peripheral zones surrounding this central zone.

  
The advantage of this technique consists in the central location of awareness sites. Thus, the electron or hole diffusion path generated by the exposure to reach the awareness sites is shorter. As a result, holes and electrons can be trapped more efficiently without the risk of recombination. When the latent image is formed on the awareness sites, the reduction in the number of these sites leads to a parallel reduction in competition for the electrons generated by the exposure. This technique is particularly advantageous in the case of sensitization on selective sites by epitaxial deposition of silver chloride.

  
Alternatively, a silver bromoiodide emulsion is used as described above, without a regulator

  
of adsorbed sites. The central part of the tabular grains of silver bromoiodide is poor in iodide but this central region is itself annular in shape; these tabular grains therefore have a silver bromoiodide center, a

  
7 first annular region called central, poorer in iodide and a second annular region called peripheral. Likewise, as indicated, the central annular region contains less than 5 mole% of iodide, while the center and the peripheral annular region each contain

  
at least 8% and preferably at least 12% by mole of iodide. The epitaxial deposition of silver chloride is carried out exclusively on the parts of the main faces defined by

  
  <EMI ID = 47.1>

  
the central annular region, one can act on the importance of the epitaxial deposit on the main faces of the tabular grains. Of course, if the amount of silver chloride thus deposited is limited, the epitaxial deposit may not occupy the entire area remaining free from the central annular region.

  
The deposition of silver chloride can be limited to a limited number of distinct sites in the central annular region. If there were no annular region with a lower iodide content, the epitaxial deposition of silver chloride would occur directly on the tops of the tabular grains of silver bromoiodide. It is surprising that the silver chloride preferably deposited on the central region. If the silver chloride deposition is accelerated sufficiently, it is then possible to deposit silver chloride.

  
  <EMI ID = 48.1>

  
laires.

  
Depending on the halide composition of the silver salt of the epitaxial deposit and of the carrier grain, the silver salt can sensitize by acting as a trap either with positive holes or with electrons; in the latter case, the epitaxial silver salt also determines the place of the latent image sites formed by exposure.

  
The presence of modifying compounds during the epitaxial deposition of the silver salt is particularly useful for increasing awareness; the compounds

  
7 modifiers are, for example, derivatives of copper, thallium, lead, bismuth, cadmium, zinc, derivatives of chlorogens, that is to say, sulfur, selenium and tellurium, derivatives of gold and group VIII noble metals. The presence in the epitaxial silver salt of metal ions as electron traps particularly favors the formation of internal latent images. For example, a particularly preferred embodiment of the invention consists in depositing silver chloride in the center of a tabular grain of silver bromoiodide rich in iodide as described above, in the presence of a modifier compound promoting the capture of electrons, such as a lead or iridium derivative.

   During the exposure, internal latent image sites are formed in the tabular grain, at the places where it has been sensitized by epitaxial deposition of doped silver chloride.

  
Another way to promote the formation of internal latent image in association with the epitaxial deposition of a silver salt is to carry out the conversion to halide once the silver salt has been deposited. When the silver salt deposited is silver chloride, it can be converted by bringing it into contact with a halide giving a less soluble silver halide such as a bromide, or a mixture

  
bromide and iodide. The chloride ions in the epitaxial deposit are replaced by bromide and possibly iodide ions. The resulting crystal defects are attributed to the formation of the internal latent image. This conversion of an epitaxial deposit of silver salt is described in US Pat. No. 4,142,900 cited above.

  
It was indicated above that the silver salt of the epitaxial deposit can either be located on a few sites distinct from the bearing tabular grains (for example, in the center or at the apices), or form a ring, for example

  
at the periphery of the main faces. When the epitaxial silver salt functions as an electron trap and, therefore, also determines the place of the latent image sites on the grain, it is preferable that the epitaxial deposition is limited to a few sites distinct from the grains, for example at center of the main faces or next to the tops of the bearing grains. In this case, the latent image sites have a reduced probability of forming close to each other and therefore of competing to combine with the electrons generated by the exposure, compared with what occurs when the latent image sites are forming

  
along the edges of the tabular grains, as is

  
  <EMI ID = 49.1>

  
The epitaxial deposition of silver salt on the tabular bearing grain can act as an electron trap or a positive hole trap; therefore, these two functions of the

  
epitaxial silver salt constitute a complementary sensitizing combination.

  
It is for example possible to selectively sensitize the center or a place close to the center of carrier tabular grains with an epitaxial silver salt capable

  
to capture electrons. One can then carry out an epitaxial deposit of silver salt capable of capturing the positive holes, selectively on the tops of the grains. In this case, a central latent image is formed on the epitaxial site of the electron trap, while on the vertices, the epitaxial deposit captures the positive holes and prevents them

  
to recombine with the electrons generated by the exposure, which further increases the sensitivity. We can in this way form an epitaxial deposit of silver chloride on a tabular grain of silver bromoiodide whose

  
central region contains less than 5 mol% of iodide, and

  
The rest of the main faces at least 8% and preferably at least 12% by mole of iodide, as described above. The epitaxial deposition of silver chloride is formed in the presence of a modifier compound promoting the capture of electrons, for example a dopant with lead or with iridium. We can then make an epitaxial deposit of silver salt serving as a trap with positive holes at the vertices of the grain, or along the edges of the main faces so as to form a ring,

  
using an adsorbed regulator. One can for example deposit on the tabular grain carrying silver thiocyanate or silver chloride containing a copper dopant. Other combinations are possible. For example,

  
the central epitaxial deposit can serve as a trap with positive holes while the epitaxial deposit formed on the tops of the carrier grain can serve as an electron trap, if the application of the modifying agents mentioned above is reversed.

  
The epitaxial deposition of silver salt may have other uses than that of serving for sensitization on selective sites described above. The epitaxial silver salt can, for example, improve the stability on drying of emulsions.

  
with tabular grains. It can also facilitate partial grain development and dye image enhancement mechanisms, as discussed in more detail above. The epitaxial deposition of silver salt can also prevent desensitization by dyes. It can promote the formation of dye aggregates, leaving free

  
significant areas of the surface of the silver bromoiodide grains inocuppressed by silver chloride: in fact, many dyes in the form of aggregates are much better adsorbed on silver bromoiodide than on silver chloride. Another advantage is the improvement in developability.

  
Localized epitaxial deposition can also improve contrast.

  
Chemical sensitization can be carried out by conventional means before or after the epitaxial deposition of silver salt on the carrier grain in order to sensitize the latter on selective sites. When silver chloride and / or silver thiocyanate is deposited on silver bromoiodide, a significant increase in sensitivity is obtained by the mere fact of depositing on selective sites. Consequently, to obtain the photographic speed, it is not necessary to resort in addition to chemical sensitization.

  
On the other hand, chemical sensitization makes it possible to further increase the sensitivity and without using high temperatures or extended treatment times, which constitutes an advantage. The amount of sensitizers can be reduced if desired, when (1) the epitaxial deposition itself improves sensitivity or (2) the sensitization is carried out at epitaxial deposition sites.

  
The silver bromoiodide tabular emulsions can be sensitized optimally by epitaxial deposition of silver chloride, without additional chemical sensitization. If silver bromide is deposited on silver bromoiodide, a much more significant increase in sensitivity is obtained with chemical sensitization subsequent to selective deposition, using, moreover, the conventional sensitization times and temperatures.

  
When using an adsorbed regulator which is itself an effective spectral sensitizer, for example, a dye in the form of aggregates, it is no longer necessary to carry out a spectral sensitization afterwards.

  
chemical awareness. However, it is often possible to make a spectral sensitization, during or after chemical sensitization. When a spectral sensitizing dye is not used as an adsorbed regulator, for example, when an aminoazaindene (such as adenine) is used in place of the dye, spectral sensitization is optionally carried out after chemical sensitization. If the regulator is not itself a spectral sensitizing dye, the spectral sensitizing dye must be able to displace the regulator or at least to be placed in sufficient proximity to the surface of the grain to sensitize it spectrally.

   In many cases, when an adsorbed sensitizing dye is used as the site regulator, it is desirable to achieve separate spectral sensitization after chemical sensitization. An additional spectral sensitizing dye can either displace or add to that which served as the site regulator. For example, a spectral sensitizer dye can provide additional sensitization or a spectral oversensitization effect. It is immaterial that the spectral sensitizers introduced after chemical sensitization are capable of playing the role of orienters for chemical sensitization.

  
After the selective epitaxial deposition, any known technique can be used to sensitize chemically. In general, the chemical sensitization to be carried out depends more on the composition of the epitaxial silver salt than on the composition of the tabular carrier grain, since chemical sensitization is supposed to occur in the first place on the sites of the epitaxial deposit or in their immediate vicinity .

  
The tabular-grain silver halide emulsions with high form index, according to the present invention, can be chemically sensitized before or after epitaxial deposition. They can be chemically sensitized with active gelatin, as indicated by T.H. James, The Theory of the Photography Process 4th

  
Ed., Macmillan, 1977, pp. 67-76; chemical sensitization can also be carried out with sulfur, selenium, tellurium, gold, platinum, palladium sensitizers,

  
with iridium, osmium, rhodium, rhenium, or phosphorus or with combinations of these different sensitizers and a pAg of, for example, between 5 and 10 and

  
a pH between 5 and 8, and at a temperature between 30 [deg.] C and 80 [deg.] C, as indicated in Research Disclosure, Vol. 120 - April 1974 - $ 12008, Research Disclosure, Vol. 134 - June 1975 - $ 13452,

  
to the patents of the United States of America 1,623,499, 1,673,522,

  
2,399,083, 2,642,361, 3,297,447, 3,297,446; to patent

  
English 1,315,755, United States of America patents 3,772,031, 3,761,267, 3,857,711, 3,565,633. 3,901,714 and 3,904,415 and English patent 1,396,696; chemical sensitization is optionally carried out in the presence of thiocyanates, as described in United States patent 2,642,361, in the presence of sulfur-containing compounds such as those described in the United States patents America 2,521,926, 3,021,215 and 4,054,457. Emulsions can be chemically sensitized in the presence of chemical sensitization modifiers, i.e. compounds known for their ability to suppress haze and

  
to increase the speed under these conditions; such compounds are for example azaindènes, azapyridazines, azapyrimides, benzothiazolium salts and sensitizers having one or more heterocyclic rings. Examples of modifiers are given in U.S. Patents 2,131,038, 3,411,914, 3,554,757,

  
  <EMI ID = 50.1>

  
by Duffin in Photographic Emulsion Chemistry, Focal Press
(1966), New York, pp. 138-143. In addition to the various chemical sensitizations defined above, or as an alternative to these sensitizations, the emulsions can also be sensitized by reduction, for example with hydrogen, as described in the United States patents. America 3,891,446 and 3,984,249, or by subjecting them to conditions bringing together a low pAg, for example less than 5 and / or a high pH, for example greater than

  
8 or by means of various reducing agents such as stannous chloride, thiourea dioxide, polyamines

  
and borane amines, as described in the patents

  
from the United States of America 2,983,609, 2,518,698, 2,739,060,

  
2,743,182, 2,743,183, 3,026,203, 3,361,564, as well as in Research Disclosure, Vol. August 136, 1975, $ 13654. Surface chemical sensitization or chemical sensitization can be carried out in an area immediately below the surface, as described in U.S. Patents 3,917,485 and 3,966,476.

  
In addition to chemical sensitization, the high form index tabular grain emulsions of the present invention are also spectrally sensitized. With the emulsions with tabular grains of high shape index according to the invention and with other emulsions also: mentioned in the present description, provision is made to use spectral sensitizing dyes which exhibit maximum absorption in blue and minus blue, that is to say in the red and in the green of the visible spectrum. In addition, for certain specific uses, spectral sensitizing dyes can be used which improve the spectral response in the region of the spectrum situated beyond the visible part. One can for example use spectral sensitizers absorbing in the infrared region.

  
Silver halide emulsions can be spectrally sensitized with dyes belonging

  
to various classes, including cyanines, merocyanines, cyanines and complex merocyanines (tri-, tetra-,

  
or polynuclear), oxonols, hemioxonols, styryl dyes, rnérostyryliques and streptocyanines.

  
Cyanine-type spectral sensitizing dyes comprise two heterocyclic rings of basic character linked by a methine bond; these heterocyclic nuclei are derived for example from quinolinium nuclei,

  
  <EMI ID = 51.1>

  
oxazolium, oxazolinium, thiazolinium, thiazolium, selenazolium, selenazolinium, imidazolium, imidazolinium, benzoxazolium, benzothiazolium, benzoselenazolium, benzimidazolium, naphtoxazolium, naphtothiazolium, naphtoselenazidiumaziazide.

  
The spectral sensitizing dyes of the merocyanine type comprise, linked by a methine bond, a nucleus of basic character of the type found in the formula of cyanines and an acid nucleus derived for example from barbituric acid, acid 2 -thiobarbiturate, rhodanin, hydantoin, 2-thiohydantoin, 4-thio-

  
  <EMI ID = 52.1>

  
indan-1,3-dione, cyclohexane-1,3-dione, 1,3-dioxane4,6-dione, pyrazolin-3,5-dione, pentane-2,4-dione, alkylsulfonyl acetonitrile, malononitrile, isoquinolin-4-one and chroman-2,4-dione.

  
One or more spectral sensitizing dyes can be used. Dyes are known with maximum sensitivities for wavelengths distributed over the entire extent of the visible spectrum and providing very different spectral sensitivity curves. The choice and relative proportions of dyes depends on the region of the spectrum to which the grains are to be sensitized and

  
of the shape of the spectral sensitivity curve that we want to obtain. Dyes with partially overlapping spectral sensitivity curves often provide, when used in combination, a curve such that the sensitivity at each wavelength in the overlap region is approximately the sum of the sensitivities of each dyes. Thus, it is possible to use combinations of dyes having different maxima, in order to obtain a spectral sensitivity curve having a maximum situated between the sensitization maxima of each of the dyes.

  
Certain combinations of spectral sensitizing dyes produce an oversensitizing effect, that is to say provide in a region of the spectrum, a spectral sensitization greater than that resulting from the use of one of the dyes alone at any concentration,

  
or resulting from the addition of 'effects / dyes. Oversensitization can be achieved with selected combinations of spectral sensitizing dyes and other additives such as stabilizers, anti-stars, development accelerators or inhibitors, coating aids, optical brighteners and antistatics. Mechanisms for explaining oversensitization and compounds for achieving it, are described by Gilman. In "Review of the

  
  <EMI ID = 53.1>

  
and Engineering, Vol. 18, 1974, pp. 418-430.

  
Spectral sensitizing dyes can also exert other actions on emulsions. These dyes can also play the role of anti-veil, stabilizers, development accelerators or inhibitors, halogen acceptors or electron acceptors, as described in the United States patents. '' America 2 131 038 and 3 930 860.

  
According to a preferred embodiment, the dyes for spectral sensitization also play the role of orienters for the sites during the deposition of the silver salt and the chemical sensitization. Useful dyes of this type are those forming aggregates. Such dyes exhibit an offset absorption by bathochromic or hypsochromic effect when they are adsorbed on the surface of the silver halide grains.

  
Dyes having these characteristics are well known and mentioned for example by TH James in The Theory of the Photographic Process, 4th Ed., Macmillan 1977, Chapter 8 (in particular, F. Induced Color Shifts in Cyanine and Merocyanine Dyes) and Chapter 9 (in particular, H. Relations Between Dye Structure and Surface Aggregation) and by FM Hamer, in Cyanine Dyes and Related Compounds, John Wiley and Sons, 1964, Chapter XVII (in particular,

  
F. Polymerization and Sensitization of the Second Type). Dyes for spectral sensitization, of the merocyanine, hemicyanin, styrylic and oxonol type, forming H aggregates (with hypsochromic displacement of absorption), are known; however, in dyes of this class, J-type aggregates (with bathchromic displacement of absorption) are not common. The preferred spectral sensitizing dyes are the cyanines forming aggregates

  
type H or J.

  
Advantageously, spectral sensitizing dyes belonging to the class of carbocyanines forming J aggregates are used. These dyes are characterized by

  
at least two basic heterocyclic nuclei, linked by

  
a chain of three methine groups; preferably

  
benzene rings are attached to the heterocyclic nuclei in order to enhance the formation of J aggregates. To enhance the formation of J-aggregates, the most advantageous heterocyclic rings are the nuclei of. quaternary salts of quinolinium, benzoxazolium, benzothiazolium, benzosélénazolium, benzimidazolium, naphtooxazolium, naphtothiazolium, and naphtosélénazolium.

  
Table I gives a list of preferred dyes for use as site orienters according to the invention.

  
  <EMI ID = 54.1>

  
  <EMI ID = 55.1>

  
bis (3-sulfopropyl) oxacarbocyanine, <EMI ID = 56.1>

  
9.hydroxide-5,5'-dichloro-3,3'-bis (3-sulfo-

  
propyl) thiacyanine,

  
  <EMI ID = 57.1>

  
In general, we are content to use the natural sensitivity of bromide or silver bromoiodide in blue for recording blue light; however, significant benefits can be obtained by using spectral sensitizers, the main absorption of which

  
is located in the region of the spectrum where emulsions have their natural sensitivity. For example, advantages can be obtained from the use of spectral sensitizing dyes in blue. Even when using silver bromide or bromoiodide emulsions consisting of tabular grains with a high shape index, speed can be greatly increased in blue with spectral sensitizing dyes in blue. When it is planned to expose the emulsions according to the invention in the region of the spectrum corresponding to their natural sensitivity, gains in sensitivity can be obtained by increasing the thickness of the tabular grains.

   For example, according to a preferred form, the emulsions are sensitized in blue and consist of bromide grains and silver bromoiodide, the thickness of which is less than 0.5 μm, the

  
  <EMI ID = 58.1>

  
less than 8: 1 and preferably 12: 1, and these tabular grains represent at least 50%, preferably at least 70% and advantageously at least 90% of the total projected surface of the silver halide grains.

  
Dyes useful for spectral sensitization of emulsions are those listed in Research Disclosure Vol. 176, December 1978, Item 17643, Section III.

  
For the spectral sensitization of non-tabular silver halide grains or of weak shape index, we

  
&#65533;

  
can use conventional quantities. dyes. To take full advantage of the advantages offered by the present invention, it is preferable to adsorb an optimal quantity of sensitizing dyes on the surface of the tabular grains of high shape index. By optimum quantity is meant the quantity sufficient to reach at least 60% of the maximum photographic speed which it is possible to achieve with these grains under the conditions provided for the exposure. The amount of dye to be used depends on the nature of the dye or the combination of dyes chosen, or on the size and shape index of the grains.

   It is known in the photographic technique that optimal spectral sensitization can be obtained with organic dyes when these dyes are used in an amount which makes it possible to produce a monolayer on approximately 25% or more of the total available surface of the silver halide grain with surface sensitivity, as described for example in United States patent 3,979,213 and by West in

  
  <EMI ID = 59.1>

  
Journal of Phys. Chem., Vol. 56, p. 1065, 1952; by Spence et al. in "Desensitization of Sensitizing Dyes", Journal

  
of Physical and Colloid Chemistry, vol. 56 n [deg.] 6, June 1948, pp. 1090-1103. The optimal amounts and concentrations of dyes can be determined by the methods indicated by Mees, in Theory of the Photography Process

  
pp. 1067-1069, already cited.

  
It has been discovered that, surprisingly, the tabular grain emulsions of high shape index which have been sensitized on selective sites in accordance with the invention have higher sensitivities than those which can be obtained by sensitizing comparable emulsions. with tabular grains by known techniques of spectral sensitization. The silver bromoiodide emulsions with tabular grains of high shape index according to the invention present higher speed / granularity relationships than those noted in general before. The best results have been obtained using dyes for spectral sensitization in the blue minus.

  
Although it is not necessary to obtain all their advantages, the emulsions defined according to the invention are preferably sensitized chemically and spectrally in an optimal manner, in accordance with current manufacturing practices. This means that their speed represents at least 60% of the maximum of the logarithm of the speed that can be expected from grains in the spectral region of sensitization and under expected conditions of use and treatment. The logarithm of speed is defined as 100 (1-log E), where E is measured in lux. second

  
at a density of 0.1 above the veil. Once the tabular grains bearing an emulsion layer have been characterized, it is possible to estimate, from other analyzes made on the product and from the evaluation of its performance, whether a layer of emulsion of this product was sensitized chemically and spectrally optimally, in comparison with other comparable commercial products. To get the sharpness benefits presented by

  
the products according to the invention, it does not matter that the silver halide emulsions have been sensitized chemically or spectrally in an effective or ineffective manner.

  
C. Formation of silver images.

  
Once tabular grain emulsions of high shape index have been produced by the precipitation processes, washed and sensitized as described above, the preparation can be completed by incorporating conventional photographic additives, and they can be used in photographic applications requiring the production of a silver image, for example, conventional black and white photography.

  
The photographic products according to the invention intended to form silver images can be tanned sufficiently to avoid having to use additional tanning agents during processing. This increases the covering power.

  
Usually useful tanning agents or pre-tanning agents are described in Researdh Disclosure, Volume 176, December 1978, item 17643, Section X.

  
We can prevent the instability that increases the

  
  <EMI ID = 60.1>

  
negative, that is to say the haze, or which increases the minimum density or decreases the maximum density in the positive-direct emulsion layers, by incorporating stabilizing agents, anti-stars, agents inhibiting the effects of mechanical stresses, latent image stabilizers and similar additives in the emulsion and adjacent layers before coating, as is

  
described in Research Disclosure, volume 176, December 1978, item 17643, Section VI. Most anti-stars effective in emulsions can also be used in developers, and can be classified into a few general categories, as proposed by C.E.K. Mees, in

  
The Theory of the Photography Process, 2nd edition, MacMillan, 1954, pages 677 to 680.

  
When we use aldehyde tanning agents, we

  
can protect layers of emulsion by conventional anti-stars.

  
The emulsion according to the invention can be used to produce photographic products forming negative as well as positive images; for example, products with emulsions forming latent surface or internal images and giving a negative image by development. Photographic products can also be those forming positive-direct images with one-step development. When the composite grains consist of the grain

  
7 carrier and the epitaxial deposit of silver.sel form an internal latent image, we can veil these grains on the surface to facilitate the formation of the positive-direct image.

  
According to a preferred variant, the epitaxial silver salt is itself chosen to form an internal latent image site
(i.e. an internal electron trap) and the surface haze can, if desired, be limited to the epitaxial deposition of silver salt. Another variant consists in that the carrier grain can trap the internal electrons, the epitaxial silver salt then playing the role of trap with positive holes. The surface-veiled emulsions can be used in combination with an organic electron-accepting compound, according to the indications given in the patents of United States of America 2,541,472; 3,501,306; 3,501,305; 3,501,307; 3,672,900; 3,600,180; 3,647,643, to British Patent 723,019 and in Research Disclosure, Vol 134, June
1975, paragraph 13542.

  
The electron accepting organic compounds can be used in combination with a spectral sensitizing dye; a sensitizing dye can itself play the role of electron acceptor, as indicated in US Pat. No. 3,501,310. If the emulsions are internally sensitive, the surface haze and the acceptors can be combined of electrons, according to United States patent 3,501,311, but to produce positive-direct images, neither the surface haze nor the electron acceptors are required.

  
In addition to the characteristics mentioned above, the photographic products comprising emulsions according to the invention may have other conventional features, listed in Research Disclosure, Vol 176, December 1978, paragraph 17643.

  
When the photographic products according to the invention are to be used for radiographic applications, the emulsion and the other layers of the radiographic product can take all the forms described in Research Disclosure publication 18431, cited above. The emulsion layers described here as well as the other conventional silver halide emulsion layers, interlayers, overlays and substratum if the product according to the invention contains them can be applied and dried as is

  
  <EMI ID = 61.1>

  
publication 17643, paragraph XV.

  
It is common practice to mix tabular grain emulsions with a high shape index

  
of the present invention either with another emulsion such as those described above, or with a conventional emulsion, to obtain specific characteristics. For example, it is known to mix emulsions to adjust the characteristic curve of a photographic product to obtain a determined result. The mixture can be used to increase or decrease the maximum densities obtained by exposure and treatment, to decrease or increase the minimum density, and to adjust the shape of the characteristic curve between the foot and the shoulder. For this, it is possible to mix the emulsions used according to the invention with conventional silver halide emulsions such as those described in Research Disclosure, volume 176, December 1979, publication 17643, paragraph I, particularly in paragraph F of the paragraph I.

  
In the simplest form, a photographic product according to the invention is formed of a single emulsion layer containing an emulsion of silver halides with flat grains of high shape index and a photographic support. Of course, the products can comprise more than one layer of silver halide emulsion as well as an overlay, a substratum and interlayers. Instead of mixing the emulsions as indicated above, one can generally obtain the same result by applying the emulsions to be mixed in the form of separate layers. The application of separate layers to obtain the latitude of exposure is well known in the art and has been described by Zelikman and Levi, "Making and Coating Photography Emulsions", Focal

  
  <EMI ID = 62.1>

  
from America 3,662,228 and British Patent 923,045. It is furthermore well known that photographic speed can be increased by applying fast and slow emulsions in separate layers, rather than mixing them. Usually, the fast emulsion layer is applied closer to the source of exposure radiation than the slow emulsion layer. We can extend this means

  
at least three superimposed layers of emulsion. Such layer arrangements are provided in the making

  
products according to the invention.

  
The layers of the photographic products can be applied on various supports. Typical photographic supports include polymer films, wood fiber, for example paper, metal plates and sheets, glass and ceramic supports, provided with a

  
or several substrates to improve the adhesive, antistatic, dimensional, abrasive, antihalo, hardness and friction characteristics, and / or the other surface properties of the support. These supports are well known in the art. See for example Research Disclosure volume 176, December 1978, publication 17643, Section XVII.

  
The emulsion layer or layers are generally applied in the form of continuous layers on supports having opposite planar main surfaces, but this is not always the case. The emulsion layers can be applied as segments of laterally spaced layers on the surface of a flat support. When the emulsion layer or layers are segmented, a microcell support is preferably used. Such supports are described in PCT patent application W080 / 01614 published on August 7, 1980 and in

  
7 corresponding Belgian patent 881 513, as well as the United States patent 4 307 165. The width of the microcells

  
be

  
can / between 1 and 200 µm and the depth reached
1000 µm. It is generally advantageous for the microcells

  
  <EMI ID = 63.1>

  
100 µm in width and depth for ordinary black and white photography applications, especially when the photographic image needs to be enlarged.

  
The photographic products according to the invention can be exhibited photographically in any usual manner. See in particular Research Disclosure, publication
17643, cited above, paragraph XVIII.

  
After exposure, the photosensitive silver halide contained in photographic products can be treated in a conventional manner to form a visible image by bringing this silver halide into contact with an aqueous alkaline medium in the presence of a developer contained in the middle or in the product.

  
Once the silver image has formed in the photographic product, it is common practice to fix the undeveloped silver halide. The tabular grain emulsions with a high shape index of the invention are particularly advantageous in that they allow fixing to be carried out more quickly, which makes it possible to obtain shorter treatment times.

  
G. Formation of dye images

  
The photographic products and techniques described above for producing silver images can be readily adapted to the production of color images, through the use of colorants. Probably the simplest way to obtain a projectable color image is to incorporate a conventional dye into the support of a photographic product, and to form a silver image as described above. In the areas where the silver image is formed, the product can no longer transmit light, and in the remaining areas, the light is transmitted with a color corresponding to the color of the support. One can easily form a colored image this way. The same results can also be obtained by using in conjunction with a product containing a transparent support, a layer or a separate product containing a color filter.

  
Dye images can be formed in silver halide photographic products by destruction or selective dye formation. Developers containing dye image formers can be used such as the couplers described in Research Disclosure, volume 176, December 1978, publication 17643, section XIX, paragraph D. The developer contains a chromogenic developer (for example an aromatic primary amine ) which is capable in its oxidized form of reacting with the coupler to form the dye.

  
One can also, in a conventional manner, incorporate

  
dye-forming couplers in photographic products. They can be incorporated in various quantities to obtain different photographic results. For example, the concentration of coupler relative to the silver content can be limited to values lower than those normally used in emulsion layers of intermediate or faster sensitivity.

  
In general, colorless non-diffusible dye-forming couplers are chosen which are capable of forming primary subtractive colors, that is to say yellow, magenta and blue-green. Dye-forming couplers having different reaction rates in separate layers or in a single layer can be used to achieve the desired results in specific photographic applications.

  
Dye-forming couplers can liberate at the time of coupling photographic groups, such as inhibitors or development accelerators, bleaching accelerators, developers, silver halide solvents, tone modifiers , tanning agents, veiling agents, anti-stars, competing couplers, chemical or spectral sensitizers and desensitizers. Couplers releasing a development inhibitor (DIR) are well known in the art. Dye-forming couplers and non-dye-forming compounds are also known which, by coupling, release various photographic effect groups. You can also use compounds

  
DIR which do not form dye by reaction with oxidized chromogenic developers. We can also use

  
DIR compounds which are cut by oxidation. Relatively light-insensitive silver halide emulsions such as Lipmann emulsions,

  
as interlayers, and overlays, to prevent or control the migration of development inhibitor groups.

  
In photographic products, color chromogenic couplers such as those used can be incorporated.

  
to form embedded masks for negative color images, and / or competing couplers. Photographic products may also contain conventional stabilizers for the dye image. All that has just been described is described in Research Disclosure, volume 176,

  
  <EMI ID = 64.1> December 1978, publication 17643, Section VII.

  
Dye images can be formed or enhanced by methods which include combining with a color-forming reducing agent, an oxidizing agent in the form of an inert complex of a transition metal and / or a peroxide. In the case of tabular grains containing iodide, intensification reactions can be used, for example, those using iodide ions for poisoning a catalyst.

  
Color images can be produced by selective destruction of dyes or dye precursors, for example by bleaching silver dyes.

  
To form color images in silver halide photographic products, the developed silver is usually removed by bleaching. The bleaching can be improved by incorporating a bleaching accelerator or a bleaching accelerator precursor into the treatment solution or into a layer of the product. In some cases, the amount of silver formed by development is small compared to the amount of dye produced, particularly in processes including enhancement of the dye image, as described above, and bleaching can be suppressed money with no significant visual effect. In other applications, the silver image is kept and a dye image is produced to increase or supplement the density provided by the silver image.

   When it is desired to enhance the silver image with a dye, it is usually preferred to form a neutral dye or a combination of dyes which together produce a neutral image.

  
H. Partial grain development

  
It is known that certain photo-detectors have, for the detection of energy quanta, a higher efficiency than that of silver halide products. The study of the fundamental properties of silver halides shows

  
that this is due in large part to the binary nature of the functioning of the silver halide grains rather than to a low sensitivity to quanta. Comments on this can be found in "Multilevel Grains and the Ideal Photography Detector" by Shaw, Photography Ses Enqineer. Flight. 16, N [deg.] 3, May / June 1972, pages 192-200.

  
The functioning of silver halide grains is called binary (or "all or nothing"), because,

  
once a latent image center is formed on a grain, this grain becomes fully developable. In general, the development is independent of the quantity of light received by the grain, above a threshold which corresponds to

  
the quantity necessary for the formation of the latent image.

  
The silver halide grain produces exactly the same result, whether it has absorbed many photons and whether several latent image centers have been formed, or whether it has absorbed only the minimum number of photons and one only latent image center has been formed.

  
Upon exposure to light, latent image centers are formed in and on the high shape index grains according to the invention. Some grains may have only a latent image center, others have many and others have none.

  
However, the number of latent image centers formed is a function of the amount of light received at the time of exposure. The tabular grains have a large diameter and their speed / granularity relationship can be high, particularly in the case of silver bromoiodide sensitized chemically and spectrally optimally; under these conditions, the speed of these tabular grains can be relatively high. Since the number of latent image centers present in or on each grain is directly a function of the quantity of light received during the exposure, the conditions are met for a high efficiency of detection of quanta, if the information n is not lost during development.

  
w

  
According to a preferred mode, each latent image center is developed so as to increase its size without fully developing the silver halide grains. This can be achieved by interrupting the development of the silver halides, well before the usual limit for optimal development in an ordinary application. Another technique is to use a DIR coupler and a chromogenic developer. The inhibitor released during coupling

  
can be used to prevent the complete development of silver halides. The preferred way of implementing this technique is to use an auto-inhibitor developer.

  
Such a developer triggers the development of silver halides, but stops development by itself before the grains have been fully developed. Preferred developers are auto-inhibiting developers containing para-phenylene diamines, such as those described by Neuberger et al in "Anomalous Concentration Effect: An inverse Relationship Between the Rate of Development and Developer Concentration of Some p-Phenylenediamines", Photographic Science and Engineering, Vol. 19, N [deg.] 6,

  
Nov.-Dec. 1975, pp. 327-332. By interrupting development or by developing in the presence of DIR couplers, we can completely prevent the development of grains whose development induction period is longer than

  
that of adjacent developable grains. However, the auto-inhibiting developer has the advantage that the development of a silver halide grain is prevented only after the development of that grain has started.

  
Strengthening the development of latent image centers leads to the formation of metallic silver centers. The size and number of these metal centers are proportional to the degree of exposure of each grain. Since the preferred auto-inhibitory developers contain chromogenic developers, the oxidized developer which forms can react with a dye-forming coupler to give a dye image. However, only a limited part of the silver halide is developed and the quantity of dye thus formed is therefore also limited.

   To remedy this limitation without affecting the proportionality between the dye density and the degree of exposure, it is possible to use a redox reaction catalyzed by silver operating with a peroxide or a complex of a transition metal, and a chromogenic reducer, for example a chromogenic developer. Where the silver halide grains form surface latent image centers, the centers themselves can provide sufficient silver to catalyze a dye image enhancing reaction. Therefore, strengthening the latent image by development is not essential, although it is a preferred variant.

  
According to the preferred embodiment, all visible silver remaining in the photographic product after having formed the dye image therein is removed by bleaching, in the usual manner.

  
The photographic image obtained is a dye image whose point dye density is proportional to the amount of light received during the exposure. As a result, the quantum detection efficiency of the photographic product is high. High photographic speeds can easily be obtained, although the oxidation reactions described above can contribute to increasing the granularity.

  
The granularity can be reduced by using a microcellular support such as that described in PCT application WO 80/01614, already cited. The feeling of granularity is given not only by the size of the individual dye particles constituting the image, but also by the random nature of their arrangements. By applying the emulsion in the form of a network of microcells formed on the support, and by spreading the dye formed in each micro-cell so as to distribute it uniformly, the feeling of granularity is reduced.

  
Although the question of partial grain development has been raised with reference to image formation

  
of dyes, what has been indicated also applies to the formation of silver images. By developing to form a visible silver image, the granularity can be decreased by stopping development before the grains containing the latent image sites have been fully developed. Since more metallic silver centers can be produced by partial development of the grains than by complete development, the feeling of granularity for a given density is reduced.

  
Similarly, a decrease in granularity can also be obtained by forming dye images with incorporated couplers whose concentration is limited so as to be less than the usual value which is based on the stoichiometric reaction with the silver halides. Although the silver titers in the photographic product must initially be stronger to allow the partial development of the grains and the obtaining of maximum densities comparable to those resulting from a complete development, the silver halide which is not developed can be removed by fixing and recovered; silver consumption is therefore not increased.

  
With the technique of partial silver development and by using photographic products with microcellular support, it is possible to reduce the granularity of the silver image as described above for the dye images. For example, if an emulsion of silver halide according to the invention serves to fill the network of microcells with a support and is then exposed and partially developed, metallic silver centers are produced,

  
in proportion to the quanta of radiation received during the exhibition and the latent image sites formed. The covering power of metallic silver centers is low in comparison with what one would obtain by complete development, but one can increase this covering power by fixing the silver halide which has not been developed, by rehalogenating the silver formed in the microcells and then carrying out the physical development of this silver on a uniform layer of germs of physical development contained in the micro-cells. Since silver obtained by physical development on fine germs can provide a much higher density than silver obtained by chemical development, a much higher maximum density is easily obtained.

   In addition, the silver formed by physical development provides a uniform density in each microcell. This makes it possible to reduce the granularity, since the random arrangement of the money is replaced by the regular arrangement of the network of micro-cells.

  
f. Sensitivity as a function of the region of the spectrum When the tabular grain emulsions according to the invention are sensitized optimally in accordance

  
to the indications given above in a determined region of the spectrum, and that the sensitivity of the emulsion in

  
this region of the spectrum is compared to its sensitivity in

  
a region where the emulsion is supposed to have a natural sensitivity due to its halide composition, we

  
observed a greater difference in sensitivity than those generally observed with conventional emulsions. In

  
color photography, the poor separation of the sensitivities of bromide and silver bromoiodide in blue, green or red has long been a drawback.

  
The advantages resulting from the differences in spectral sensitivity presented by the silver bromide or silver bromoiodide emulsions according to the invention are illustrated in the case of products for color photography. The differences in increased spectral sensitivity presented by the emulsions according to the invention are not limited to the case of bromoiodide or silver bromide or to emulsions for color photography. These differences can also be observed in the case of products with a single emulsion layer. In addition, these benefits can be obtained in

  
the case of various applications made with emulsions having any halide composition recognized useful in photography. For example, silver chloride and chlorobromide are known to have a natural sensitivity in blue, weak enough to allow them to register green and red in products for color photography, without protection from light blue, but in other applications there is an advantage in increasing the difference in sensitivity between the different regions of the spectrum.

   In particular, if a silver chloride emulsion with tabular grains of high shape index is sensitized to infrared, and exposed in the region of spectral sensitivity, it can then be treated with light with less increase in the minimum density as a result of the low sensitivity of the emulsions according to the invention in the regions of the spectrum where there has been no spectral sensitization.

  
g. Color photography

  
The present invention can be used to obtain color images. In general, any product for color photography of the conventional type containing at least one layer of silver halide emulsion, can be improved simply by adding to this layer of emulsion
(or by replacing it with), a layer of emulsion with tabular grains of high shape index, in accordance with the present invention. The present invention thus finds application in color photography, either by additive or subtractive three-color synthesis.

  
With regard to the formation of color images by additive synthesis, an application of the invention consists in using the networks of elementary filters, blue, green and red in combination with a photographic product in accordance with the present invention and capable of providing a silver image. Through this network of additive primary filters, a photographic product is photographically exposed comprising a panchromatic sensitized layer and consisting of an emulsion according to the present invention, that is to say with tabular grains of high shape index. After processing and developing a silver image, examining this silver image through the network of filters allows us to observe a color image. The best way to use these images is to examine them by projection.

   Thus, both the photographic product and the filter network have the same transparent support in common.

  
By applying the invention to the product for photo-

  
  <EMI ID = 65.1>

  
other special advantages can be obtained. This type of photographic product includes a support and in general a set of three superimposed layers of silver halide emulsions for separately recording blue, green and red, providing dye images respectively yellow, magenta and blue- green. The present invention applies to any photographic product of this type and therefore relates to any photographic product of this type comprising at least one layer of tabular silver halide grain emulsion of high shape index. However, additional advantages are obtained when using tabular grain emulsions of index

  
high form consisting of bromide and silver bromoiodide. Consequently, the description which follows relates more particularly to certain embodiments in which emulsions of bromide and bromoiodide of silver are used, but if desired, it may be used instead

  
high form index silver halides of any composition. Unless expressly indicated, the products for color photography have the characteristics of the photographic products described above.

  
According to a preferred embodiment, an emulsion of silver bromide or bromoiodide with tabular grains of high shape index, sensitized for the blue minus, in accordance with the indications given above, forms at least one of the layers intended to record green light or red light in a set of three emulsion layers used to record the blue, green and red of a product for

  
  <EMI ID = 66.1>

  
tabular is arranged to receive during the exposure of the photographic product to a neutral light at 5500 [deg.] K, blue light in addition to the light that the emulsion is intended to record. The relation between the blue light and the light of the blue minus that the layer receives, can be expressed in the form of the factor &#65533; log E, where:

  

  <EMI ID = 67.1>


  
log E7 representing the logarithm of the exposure to green light or red light depending on what the tabular grain emulsion records and

  
  <EMI ID = 68.1>

  
blue that these tabular grains also receive. (In each case, E is expressed in lx.s unless otherwise indicated).

  
In practice, according to the present invention, a value of A log E of less than 0.7, and even less than 0.3, also makes it possible to obtain an acceptable reproduction of a poly-chromium subject. This is a surprising fact if one considers the high proportion of grains of the emulsion according to the invention.

  
a

  
  <EMI ID = 69.1>

  
emulsion of comparable silver halides, but not tabular or made up of grains whose index of form is lower, the color rendering is more disturbed and is unacceptable in general. It is known in the art that aberrations in color rendering from sensitized emulsions in green and in red can be reduced by using grains of smaller average diameter. However, these results are obtained at the cost of limiting the maximum photographic speed that can thus be obtained. With the emulsions of the present invention, it is not only possible to obtain an advantageous separation of the speeds in the blue and in the blue minus, but in addition, this advantage can be retained without any limitation as regards the maximum speeds that can be get in the blue minus.

  
According to a specific and preferred embodiment, a product according to the invention comprises a series of three emulsions recording blue, green and red, at least those recording blue minus are conformal bromide or silver bromoiodide emulsions to the present invention. The emulsion layer recording the blue of this series of three emulsions can also advantageously be an emulsion with tabular grains of high shape index according to

  
the present invention. According to a preferred embodiment, the tabular grains present in each of the emulsion layers of this series of three elements have a thickness of less than 0.3 μm, an average diameter at least

  
  <EMI ID = 70.1>

  
Still in a preferred form of the present invention, the products for color photography have an ISO sensitivity index at least equal to 180.

  
"The products for color photography according to the present invention do not need a yellow filter layer placed between the exposure source and the emulsion layers sensitive to green and / or red to protect these layers from exposure blue light. However, such a yellow filter layer may be present and in this case its density may be reduced to a value lower than that of any other yellow filter layer which has been used until now to protect the emulsion layers sensitive to green and red in photographic products intended to be exposed to daylight. According to a preferred embodiment, there are no emulsion layers registering blue between the layers of emulsion recording green and / or red and the source of radiation used for exposure.

   This is why, in practice, the photographic product does not contain any substances absorbing blue between the layers of emulsion sensitive to green and sensitive to red and the incident radiation from the exposure.

  
Although only one of the green or red bromide or silver bromoiodide emulsion layers should consist of tabular grains of high shape index, the color photography product contains minus three emulsions. separate to record blue, green and red respectively. The other emulsions, that is to say the emulsions other than those consisting of grains with a high shape index, can be conventional emulsions of any shape. Different types of conventional emulsions are described in Research Disclosure, publication 17643
-cited above, $ I. According to a preferred embodiment of the present invention, all the emulsion layers <EMI ID = 71.1>

  
In particular, at least one emulsion layer recording the green and at least one emulsion layer recording the green

  
'\ red are made up of tabular grains with an index of

  
high form, in accordance with the present invention. If the product comprises more than one emulsion layer for recording green and / or red, the preferred embodiment is that at least the fastest emulsion layer contains the tabular emulsion grains d 'high form index, as indicated above. Of course, all the emulsion layers registering blue, the

  
green and red of a photographic product can advantageously consist of tabular grains such as those described above, although this is not necessary for the implementation of the present invention.

  
The present invention applies perfectly to products for color photography as described above in which the speed and the contrast of the emulsion layers recording blue, green and red can be very variable. Since the emulsion layers

  
silver bromide or bromoiodide with tabular grains of high shape index, spectrally sensitized in green or red, have a relatively low speed in blue, it is possible to arrange these emulsion layers recording green and / or red at any place in a product for color photography, without taking into account the remaining emulsion layers and without taking the usual precautions to prevent the exposure of these emulsion layers to blue light.

  
The present invention applies in particular to products for color photography intended for the reproduction of colors by exposure to light from

  
day. Photographic products of this type are characterized in that they provide recordings of blue, green and red exposures, the contrasts of which are more or less harmonized and the speed of which presents limited variations when exposed to a source of

  
5500 [deg.] K (daylight). The expression "roughly harmonized contrast" means that the contrast of the blue, green and red records differ by less than 20% and preferably by less than 10% with respect to the value of the contrast of the recording in blue. Limited variations in speed of recording in blue, green and

  
red can be expressed as a variation A log E that is less than 0.3 log E. The variation in speed is the greatest of the differences between the speed of green or red recordings and the speed of blue recordings .

  
The measurements of contrast and logarithm of rapidity necessary for establishing these relationships can be determined in a photographic product according to the invention by exposing this photographic product to a color temperature of 5500 [deg.] K through a scale of density spectrally. not selective (neutral density), for example a carbon control object, then by treating the photographic product preferably under the ambient conditions corresponding to those of the use for which it is intended. The densities in blue, green and red are measured by transmitting blue light at a wavelength of
435.8 nm, green light at a wavelength of

  
546.1 nm and red light at a wavelength of
643.8 nm, as described in the American standard PH2.1-1952 published by the American National Standards Institute
(ANSI), 1430 Broadway, New York, N.Y. 10018; the characteristic curves in blue, green and red of the photographic product are thus obtained. If the photographic product has a reflective support rather than a transparent support, density measurements are carried out by reflection. From the blue, green and red characteristic curves, it is possible to determine by well known methods

  
speed and contrast. The particular techniques for measuring speed and contrast are of no specific interest from the moment when each of the blue, green and red records is the subject of identical measurements in order to allow comparisons. Many standard sensitimetric measurement techniques for color photographic products for different applications have been published by ANSI. These are for example the American Standard PH2.21-1979, PH2.47-1979 and PH2.27-1979.

  
  <EMI ID = 72.1>

  
the present invention allow the reproduction of colors when exposed to daylight and they provide particular advantages over conventional products having these characteristics. In the photographic products according to the invention, the speed in the blue

  
  <EMI ID = 73.1>

  
tabular and spectrally sensitized in green and

  
red is limited and this limited speed can be used to separate the speed in blue of the emulsion layer recording blue and the speed in blue of emulsion layers recording blue minus. Depending on the application considered, the use of tabular grains in the emulsion layers recording green and red may in itself provide sufficient separation of the responses in blue from the emulsion layers recording blue and blue minus.

  
For certain applications, it may be desirable to further increase the separation between the speed in the blue of the layers recording the blue and the layers recording the blue minus; for this, use is made of conventional separation techniques which thus add to the separation possibilities obtained using the tabular grains with a high shape index according to the present invention. For example, in a photographic product, the fastest green emulsion layer can be placed in the closest position to the radiation source and the emulsion layer is

  
the fastest blue in the position farthest from the source of exposure; in this case, the separation of the velocities in the blue of the emulsion layers

  
^ recording blue and green although they present

  
  <EMI ID = 74.1>

  
a difference of 1.0 log sound applied in separate layers and exposed separately, can be effectively reduced by the arrangement of the layers; indeed, the emulsion layer registering the green receives all the blue light during the exposure, but this layer registering the green and the other layers which are above can absorb

  
or reflect some of the blue light before it reaches the blue-recording emulsion layer. In such a case, increasing the proportion of iodide in the blue recording emulsion layer can help to increase the separation of the velocities in the blue from the blue recording layers and the blue minus which is already obtained by using the tabular grains. . When an emulsion layer recording blue is placed closer to the radiation source used for exposure than the emulsion layer recording blue minus, a yellow filter layer of limited density can be used between the layers emulsion recording blue and blue minus to increase the separation between blue and blue minus. However, it is never necessary to use any of these conventional separation techniques.

  
  <EMI ID = 75.1>

  
previous pic to get a deviation of 1 log E or something similar for the separation of speed in blue. However, this remains possible if one wishes an exceptional separation of the velocities in the blue and in the blue minus, for a determined application.

  
Thus, the present invention achieves the objectives which, in color photography, condition the exact reproduction of colors in balanced lighting conditions, while allowing a wider choice in the production of the photographic product, compared to previous possibilities.

  
Products for color photography are often described so as to highlight the different elements of color forming layers that compose them. Most often, these products contain three of these elements, superimposed each containing at least one layer of silver halide emulsion capable of recording an exposure corresponding to a separate third

  
of the spectrum and at the same time produce the complementary primary subtractive dye image. In this way, the elements recording blue, green and red respectively provide yellow, magenta and cyan dye images. The dye-forming substances need not be in the dye-forming elements; they can be supplied entirely from the processing solutions. When the dye-forming substances are incorporated into the photographic products, they can be placed in an emulsion layer or in a layer intended to collect an oxidized developer or an oxidized electron transfer agent coming from an emulsion layer adjacent to the same dye-forming element.

  
To prevent migration of oxidized developers or oxidized electron transfer agents between

  
two dye-forming elements and thus avoid the resulting color alteration, a common practice is to use reactive substances

  
with these oxidation products. Such substances can be incorporated into the emulsion layers themselves, as indicated in the patent of the United States of America

  
2,937,086, and / or in interlayers between each dye-forming element, as described in US Patent 2,336,327.

  
Each color forming element may contain only one layer, but most often it contains 2, 3 or more. with different photographic speeds. When the order of the layers does not make it possible to have this set of emulsion layers of different speeds in a single element, it is common to produce a photographic product with several (usually 2 or 3) elements recording blue, green and / or red.

  
According to the present invention, at least one emulsion layer recording green or red containing tabular grains of bromide or silver bromoiodide according to the invention, is arranged in the photographic product so as to receive a higher proportion blue light during the exposure. This higher proportion of blue light reaching the tabular grain emulsion layer of high shape index is the consequence of a lower absorption of blue light by the superimposed yellow filter or is simply the result of the elimination of this yellow filter. The higher proportion of blue light reaching the tabular emulsion layer may also result from a new arrangement of the dye-forming element which is closer to the source of exposure.

   For example, elements recording red and green and containing tabular grain emulsions of high shape index recording green and red, may be arranged closer to the source of exposure relative to the element recording the blue.

  
Products for color photography according to

  
the present invention may have any form compatible with the conditions defined above. Any of the six structures presented in "Spectral Studies of the Photography Process" (table 27-a, p. 211), Focal ress, New York, can be used. In order to provide a simple example, it is possible during the preparation of a conventional silver halide product for color photography, to add to it one or more layers of tabular grain emulsion of high shape index.

  
sensitized to the blue minus and arranged to receive the exposure before the other emulsion layers. However, in most cases, it is preferable to replace one or more layers of conventional emulsions recording the blue minus with corresponding layers made up of tabular grains with a high shape index, possibly by modifying the order of the layers.

  
To illustrate the invention, a number of preferred structures are shown below.

  
Structure I
  <EMI ID = 76.1>
 Structure II

  

  <EMI ID = 77.1>


  
Structure III

  

  <EMI ID = 78.1>


  
Structure IV
  <EMI ID = 79.1>
 Structure V

  

  <EMI ID = 80.1>


  
Structure VI
  <EMI ID = 81.1>
 Structure VII

  

  <EMI ID = 82.1>


  
B, V and R respectively designate elements recording blue, green and red of any conventional type.

  
T, in prefix, means that the emulsion layer (s) contain tabular grains of silver bromide or bronoiodide with a high shape index, as described above.

  
F, in prefix, means that the layer has a higher speed than that of at least one other element which, in the structure, registers light in the same part of the spectrum.

  
L, in prefix, means that the emulsion layer is slower, that is to say has a lower speed than that of at least one other element of the structure recording light in the same part of the spectrum.

  
I, denotes an interlayer containing a substance to react with the oxidation products, but practically without a yellow filter.

  
Each fast or slow element may have a different photographic speed than that of another element of the structure which records light in the same third of the spectrum, due to its arrangement in the structure, its inherent properties or the association of these two factors.

  
  <EMI ID = 83.1>

  
place of the support: In general, the support is the farthest from the source of exposure, that is to say that on the diagrams it would be under the emulsion layers. If the support is &#65533; colorless and transmits non-scattered light, i.e. if it is transparent, it can be placed between the exposure source and the layers. Of a

  
more generally, the support may be disposed between the source of exposure and any color forming layer recording radiation for which the support is transparent.

  
In structure I, we see that there is no yellow filter. However, in accordance with practice, in products which contain a yellow filter, the blue-forming color forming element is arranged closest to the source of exposure. In its form the

  
simpler, each element of this product consists of a single layer of silver halide emulsion. In the variants, each color forming element

  
may contain 2 or 3 different layers of silver halide emulsion or more. In the case of sets of 3 emulsion layers, we compare the layers which in each element have the strongest speed, we preferably harmonize the contrasts and in this case, the photographic speeds of the layers recording the green and the

  
red differ from the speed of the blue recording layer by a difference of less than 0.3 log E. When, in each element, there are 2 or 3 layers of emulsion of different speed or more, there is also preferably 2 or 3 sets of three or more emulsion layers

  
  <EMI ID = 84.1>

  
whose speed presents this relationship.

  
yellow filter below the blue recording element increases the photographic speed of this element.

  
The absence of a yellow filter in the interlayer

  
is not absolutely necessary in the case of structure I. Between the elements registering the blue and the green, one can have a yellow filter, at a rate of a quantity lower than the normal normal, without deviating from the framework of the present invention. In addition, the interlayer separating the elements recording the green and the red can likewise also contain a yellow filter at a rate of a quantity which can go up to that used in a conventional manner. When using such quantities of yellow filter, it is not necessary to limit the composition of the element recording the red to the tabular grains of bromide and silver bromoiodide, as indicated above, but any conventional form of grain can be used compatible with the required considerations of contrast and speed.

  
We will not return to the common aspects of

  
  <EMI ID = 85.1>

  
only the aspects specific to each of the structures. In structure II, blue, green and red are each recorded by two distinct elements instead of incorporating the slow and fast emulsion layers recording respectively blue, red or green in the same elements. It is only necessary that the emulsion layers of the fastest elements are
-constituted tabular grains of silver bromide or silver bromoiodide according to the invention. The slower elements registering green and red, because of their slower speed, and the presence of the layer registering the faster blue which is above, benefit from appropriate protection against exposure to blue light, without the need to use a yellow filter.

   The use of bromide or silver bromoiodide emulsions with tabular grains of high shape index in the emulsion layers of low speed elements registering green and red is not excluded.

  
If one places the fastest element recording red on the slowest element recording green, one can increase the speed according to the indications of the patent of the United States of America 4,184,876 as well as to the publications of German application 2 704 797, 2 622 923, 2 622 924 and 2 704 826.

  
Structure III presents with structure I the following difrences:

  
The element recording blue is furthest from the source of exposure. As a result, the green recording element is closest to the exposure source and the red recording element is closer to the exposure source than in the conventional case. This arrangement is very advantageous for obtaining images

  
in very sharp colors with excellent color quality. Since the element recording green, which contributes the most to the visual impression of the color image, being the closest to the source of exposure, it can provide an image of great clarity, since it does not there is no interposed layer to diffuse the light. The element recording red, which comes second for the contribution to the visual impression of a color image, receives a light which

  
passed through only the element recording green and which consequently was not broadcast by a recording element
- entering blue. Of course, the element registering the blue undergoes, by comparison with the structure I, a loss of sharpness, but this drawback is not significant with regard to the advantages resulting from the arrangement of the elements registering the green and the red, because the the contribution of the blue recording element to the visual impression of the color image is by far the least important.

  
Structure IV derives from structure III; it includes distinct, fast and slow elements for registering red, made up of tabular grain emulsions with a high shape index.

  
The structure V differs from the structure IV by the presence of an additional element recording the blue, placed on the elements of low speed recording.

  
green, red and blue. The fastest blue recorder includes a tabular-grain silver bromide or bromoiodide emulsion of high shape index. In this case, the fastest blue recording element helps absorb the blue light and therefore

  
to attenuate the transmission of blue light on the elements recording green and red of lower speed. A structural modification consists in not using emulsion with tabular grains of high deformity index in the elements registering green and red, of lower speed.

  
Structure VI differs from structure IV in the presence of a tabular grain element recording blue, between the elements recording green and red and the source of exposure. As indicated above, the blue-grained tabular grain element may consist of one or more blue-grained tabular grain layers and when there are thus more layers of blue-registering emulsion, their speed may be different. To counteract the effects

  
from the less favorable position occupied by the element registering the red, the structure VI provides a second rapid element registering the red which is arranged between the tabular grain element registering the blue and the source of exposure. Because of its favorable arrangement, this second rapid element registering the red is faster than the first rapid element registering the red, this insofar as these two elements consist of identical emulsions. Of course, this first and second rapid elements recording the red can be made up of identical or different emulsions, the relative speeds of which can be adjusted by known techniques.

   Instead of using two rapid elements recording red, the second rapid element recording red can be replaced by a second rapid element recording green.

  
Structure VII is identical to structure VI, except that it provides both a second rapid element recording red and a second rapid element recording green, both comprising tabular grains and arranged between the source of exposure and the tabular grain element recording blue.

  
Many other interesting structures are also

  
  <EMI ID = 86.1>

  
only examples. In each of these structures, the position of the elements registering green and red can be exchanged: the position of the fastest elements registering red and green can be exchanged in the different structures and / or it can be the same for the slowest elements registering green and red.

  
In general, for color photography, the emulsions are arranged so as to form a set of superimposed layers containing substances forming dyes incorporated, for example couplers, but this is not essential.

  
Three chromogenic components each containing a silver halide emulsion recording light corresponding to a third of the spectrum and a coupler forming the complementary primary subtractive dye, can be arranged together in a single layer of a product for color photography. Examples of such products are given in U.S. Patents 2,698,794 and 2,843,489.

  
It is thanks to the relatively large separation between the sensitivities in the blue and the blue minus of the elements recording the green and the red and consisting of emulsions of, bromide or silver bromoiodide with tabular grains, that we can decrease the quantity of yellow filter or even eliminate these yellow filters and / or - arrange the layers according to new structures. To measure

  
the relative response of the elements registering green and red to blue light in a product for color photography, one technique consists in exposing a sample of such a product through a density scale with neutral light, it is that is to say with a

  
  <EMI ID = 87.1>

  
process the sample. We then expose a second sample in the same way, but by inserting a Wratten 98 Filter which transmits light only between 400 and
490 nm; the product is then treated in the same way as above. Using the transmission densities in blue, green and red, determined according to 93

  
American standard'PH2.1-1952 mentioned above, we can draw three characteristic curves. for each

  
  <EMI ID = 88.1>

  
in the blue of the element (s) recording the blue and The velocities in the blue respectively of the element (s) recording the green and the red, can be determined from the following relationships:

  

  <EMI ID = 89.1>


  
in which

  
BW98 is the speed in the blue of the element recording the blue exposed through a Wratten 98 filter;

  
VW98 is the speed in the blue of the element recording the green, exposed through the Wratten 98 filter;

  
  <EMI ID = 90.1>

  
registering the red, exposed through the Wratten 98 filter;

  
BN is the speed in the blue of the element registering blue, exposed to neutral light;

  
VN is the speed in the green of the element registering the green exposed to neutral light and;

  
  <EMI ID = 91.1>

  
recording red, exposed to neutral light.

  
In the present description, it is accepted that the densities in blue, green and red are due only to the elements recording respectively blue, green and red, which amounts to neglecting the parasitic spectral absorptions of the yellow dyes. magenta and blue green. In fact, the importance of these parasitic absorptions is rarely sufficient to have a practical effect on the results obtained in the applications mentioned.

  
The products for color photography according to the invention have

  
  <EMI ID = 92.1> <EMI ID = 93.1>

  
  <EMI ID = 94.1>

  
less than eight times, and according to the optimal embodiment at least ten times, the speed in the blue of the elements recording the green and / or the red consisting of emulsions with tabular grains of high index of shape in accordance with the above indications.

  
The separation between the sensitivities in blue and in the blue minus of a product for color photography according to the invention can be measured in another way. This other measure is to compare the speed in

  
the green of an element recording green, or the speed in red of an element recording red, with the speed in blue of these same elements. The exposure and treatment techniques described above are still used, with the difference that instead of exposure to neutral light, exposure to blue minus is used by inserting a Wratten 9 filter which transmits light only beyond 490 nm. We thus determine the different

  
  <EMI ID = 95.1>

  

  <EMI ID = 96.1>


  
  <EMI ID = 97.1>

  
VW9 is the speed in the green of the element registering the exposed green through the Wratten 9 filter and,

  
  <EMI ID = 98.1>

  
registering the red exposed through the Wratten 9 filter.

  
As before, the parasitic spectral absorption produced by the dyes, which is rarely significant, is neglected.

  
Elements registering red and green and containing tabular bromide or silver bromoiodide emulsions as indicated above, have velocities in blue and region respectively

  
7 of the spectrum where they are spectrally sensitized which are at least in a ratio 10 (1.0 log E) and preferably,

  
at least in a 20 (1.3 log E) ratio. This expresses the difference between the speeds in blue and the speeds in blue minus.

  
If we compare the relations A and B and the relations C and D for the same product, the results are not identical, even if the elements recording the green and the red are identical, except as regards the length of wave of their spectral awareness. Indeed, in most cases, the element recording the red receives a light which has already passed through the element recording the green. However, if you prepare a second identical product

  
in the first, but in which we have exchanged the position of the elements recording the green and the red, we note that for the element recording the red of the second product,

  
the values of relations B and D are practically identical to the values of relations A and C for the elements recorded in the green of the first product. In other words, the choice of spectral sensitization in green as opposed to spectral sensitization in red has no determining effect on the quantitative values obtained with the above relationships. This is why it is customary not to distinguish speed in green and in red when compared to speed in blue, but to designate them generically under the name of speed in blue minus.

  
h. Diffusion at wide angles.

  
The tabular grain emulsions of high form index according to the invention are advantageous because, compared to non-tabular emulsions or of lower form index, they make it possible to reduce the diffusion of

  
light at wide angles.

  
We can give a quantitative demonstration of this. According to what is shown in FIG. 4, a sample of emulsion 1, in accordance with the present description is applied to a transparent support 3 at the rate of 1.08 g of silver per square meter. Preferably, the emulsion and its support are immersed in a liquid (not shown in the figure) having an appropriate refractive index to reduce the effects of Fresnel reflection on the surface of the support and the emulsion. The emulsion layer is exposed in a direction perpendicular to the plane of the substrate.

  
  <EMI ID = 99.1>

  
shot from the source, the light follows a path materialized by the dotted line 7, forming an optical axis, which meets the emulsion at point A. The light which passes through the support and the emulsion can be detected at a constant distance from the emulsion on a hemispherical detection surface 9. At a point B-which is at the intersection of the extension of the optical path and the detection surface, a maximum of light intensity is detected.

  
An arbitrary point C is chosen on the detection surface. The dotted line connecting A and C forms

  
an angle &#65533; with the emulsion layer. By moving point C on the detection surface, you can vary the angle [cent] from 0 to 90 [deg.]. By measuring the intensity of scattered light as a function of &#65533;, it is possible, because of the symmetry of the scattering of light around the optical axis 7, to determine the cumulative light distribution as a function of &#65533;65533;. For this distribution, see DePalma and Gasper, "Determining the Optical Properties of Photographic Emulsions by the Monte Carlo Method", Photographic Science and Engineering, Vol. 16, N [deg.] 3, May-June 1971, pp. 181-191.

  
After determining the cumulative light distribution based on &#65533; for values from 0 to 90 [deg.] in the case of emulsion 1 according to the invention, the same process is repeated in the case of a conventional emulsion consisting of grains whose average volume is the same and applied with the same silver title on another part of the support 3. We compare the cumulative light distributions

  
  <EMI ID = 100.1>

  
for values of &#65533; going up to -70 [deg.] and in some

  
1 <EMI ID = 101.1>

  
that the amount of scattered light is lower with the emulsions according to the invention. In Figure 4, the angle

  
  <EMI ID = 102.1>

  
evaluated by reference to angle 6. So the emulsions

  
with tabular grains of high shape index according to the invention, exhibit less diffusion at large angles. Since the wide-angle scattering contributes significantly to reducing the sharpness of the image, the emulsions according to the invention make it possible in each case to increase this sharpness.

  
In the present description, the term "capture angle" designates the value of the angle-6 for which half of the light which reaches the detection surface is contained in a surface subtended by a cone formed by the rotation of the AC line around the polar axis at

  
  <EMI ID = 103.1>

  
the detection surface is in the remaining part of the surface.

  
The following theoretical considerations

  
about the decrease in wide angle scattering, cannot limit the present invention.

  
It is assumed that the main faces of a flat crystal, with a high shape index, as well as the orientation of the grains in the layer are responsible for the improvements.

  
sharpness. It has been observed that the tabular grains present in the silver halide emulsion layers are approximately aligned in parallel with the flat surface of the support on which the layer is applied. Thus, the light, whose direction is perpendicular to the photographic product, has an incidence also perpendicular to one of the main faces of the crystals. The small thickness of the tabular grains, as well as their orientation in the layer, allows the emulsions according to the invention to be applied in thinner layers than the conventional emulsions, which also contributes to improving the sharpness. However, the emulsion layers of a product according to the invention see their sharpness improved, even when their thickness is comparable to that of conventional emulsion layers.

  
According to a specific embodiment of the invention which is also preferred, the emulsion layers consist of tabular grains with a high shape index, the minimum average diameter of which is at least 1.0 μm and better still

  
  <EMI ID = 104.1>

  
grains, we can obtain both better speed and better sharpness. Although the maximum average grain diameters may vary according to the granularity tolerable in a given application, the maximum average diameters

  
i mum of the grains with a high shape index of the emulsions according to the invention are in all cases less than 30 m, preferably less than 15 m and according to one embodiment.

  
  <EMI ID = 105.1>

  
Although it is possible to reduce the wide-angle scattering with monolayers of tabular grain emulsion with a high shape index according to the invention, this does not necessarily mean that the wide-angle scattering is reduced in the layers of a product for color photography. In certain assemblies of layers for color photography, the emulsions with tabular grains of high shape index according to the invention can indeed cause a deterioration in the sharpness of the underlying layers.

  
  <EMI ID = 106.1>

  
emulsion recording blue is closest

  
of the radiation source used for the exposure, then

  
that the underlying emulsion layer recording the green consists of an emulsion with tabular grains according to the invention. This emulsion layer registering green is itself above the emulsion layer registering red. If the blue emulsion layer contains grains with an average diameter included

  
  <EMI ID = 107.1>

  
many non-tabular emulsions, we will observe a maximum diffusion of light passing through this layer before reaching the emulsion layers registering green and red. Unfortunately if the light

  
has already been broadcast before reaching the emulsion layer

  
tabular grains with a high shape index forming the layer registering green, the tabular grains can diffuse more than a conventional emulsion

  
which crosses them to reach the layer registering the red. Consequently, the choice of emulsions and the arrangement of the layers entail in this particular case a significant degradation of the sharpness of the emulsion layer registering the red, degradation which is greater than that which would be observed if this product does not understand: no emulsion according to the invention.

  
In order to obtain the best advantages of the present invention with regard to sharpness, in an emulsion layer which is located under a tabular grain layer of high shape index according to the invention, it is desirable that this tabular grain layer is arranged so that it receives practically non-scattered light. In other words, in the photographic products according to the present invention, improvements in sharpness are obtained in the emulsion layers arranged under the tabular grain emulsion layers only if these tabular grain emulsion layers are not themselves arranged under a layer whose turbidity causes a diffusion of light. For example, if an emulsion layer

  
  <EMI ID = 108.1>

  
high, according to the invention is arranged on an emulsion layer recording red and under a layer of Lippmann emulsion and / or an emulsion layer recording blue with tabular grains with high form index according to the invention, the Sharpness of the red registering layer will be improved by the presence of the tabular grain emulsion layers above it. If the angle of capture of the layer or layers lying on the emulsion layer recording the green which includes tabular grains of high shape index

  
is less than about 10 [deg.], an improvement in the sharpness of the layer registering red can be obtained. It is of course unimportant that the recording layer

  
the red is itself a layer with tabular grains of high shape index according to the invention, at least as regards the effects of the layers lying above on the sharpness of the layer registering the red.

  
In a product for color photography containing several color forming elements arranged one above the other, it is preferable that at least the emulsion layer intended to be closest to the radiation source used for the exposure , or consists of an emulsion with tabular grains of high shape index, in order to take advantage of the advantages provided by this invention as regards image clarity. According to a particularly preferred embodiment of the present invention, each emulsion layer which is arranged so as to be closest to a radiation source with respect to another emulsion layer, consists of an emulsion with tabular grains with a high form index.

   The layered arrangements II to above described above constitute examples of products for color photography which, according to the present invention, make it possible to obtain a significant improvement in the sharpness of the underlying emulsion layers.

  
Color photography products have been used to illustrate the benefits that tabular grain emulsions with high form index provide.

  
'&#65533;

  
to get regarding sharpness; but, it is also possible to improve the sharpness of the products with several layers for black and white photography, that is to say products forming silver images. In a common way, we separate the emulsions forming black and white images into fast layers and slow layers. If the tabular grain emulsions according to the present invention are used in the layers closest to the sources of exposure, the sharpness of the underlying emulsion layers is improved.

  
The following examples illustrate the invention.

  
In each of the following examples, the contents of the reactor are vigorously stirred during the entire time in which the silver salts and the halides are introduced. Percentages are expressed by mass unless otherwise indicated. The term M expresses the molar concentration. All solutions are aqueous unless otherwise indicated.

  
To determine the average diameter of the tabular grains and the percentage of the projected surface they occupy,

  
tabular grains having a diameter of less than 0.6 un) have been included, unless otherwise indicated; however, tabular grains of small diameters are insufficient to significantly alter the numbers shown.

Comparative example 1

  
This example shows the non-selective epitaxial deposition of silver chloride on tabular grains of silver bromoiodide at 6 mol% of iodide not spectrally sensitized before this deposition.

  
Emulsion 1 A: Preparation of tabular grains carrying bromoiodide

  
  <EMI ID = 109.1>

  
To 6.0 liters of a 1.5% by mass gelatin solution containing 0.12 M of potassium bromide, brought to 55 [deg.] C, a mixture is added, with stirring and by the double jet method. solution 2, OM of potassium bromide containing 0.12 M of potassium iodide and a solution 2, OM of silver nitrate. The addition takes place in 8 min, maintaining the pBr at 0.92; 5.3% of the total amount of silver nitrate used is consumed. Then, the bromide and silver salt solutions are introduced simultaneously, maintaining the pBr at 0.92 and with an accelerated flow rate (multiplied by 6 between the start and the end of the addition, that is to say say at a rate 6 times faster at the end than at the start of the addition). It takes 41 mins. 94.7% of the total amount of silver nitrate used is consumed. We

  
used a total of 3.0 moles of silver nitrate.

  
  <EMI ID = 110.1>

  
it is described in US Patent 2,614,929. The emulsion is stored under pAg conditions of 7.6 measured at 40 [deg.] C. The tabular grains of silver bromoiodide obtained (at 6 mol% of iodide) have

  
  <EMI ID = 111.1>

  
average shape index of 33: 1. Tabular grains represent 85% of the projected surface.

  
Emulsion 1 B: Epitaxial growth of silver chloride on the faces

  
main crystals of the grains carrying AgBrI.

  
40 g (0.04 mole) of the agBrI emulsion 1A with tabular grains, previously prepared, are used. The pAg is adjusted to 7.2, at 40 [deg.] C, with

  
  <EMI ID = 112.1>

  
silver nitrate, maintaining the pAg at 7.5. This results in an epitaxial deposition of silver chloride representing 5% by mole of the totality of the silver halide. For the sake of brevity, this type of emulsion will be called a "5% mole silver chloride emulsion" and similar terminology will be used for other emulsions.

  
FIG. 5 represents an electron micrograph of a carbon replica of the emulsion. It can be seen that the silver chloride is deposited on the main crystal faces. Although for some grains, as can be seen, the epitaxial deposition is preferably made near the edges of the main crystal faces, the deposition took place, in general, in a more or less disordered manner, on the crystal faces main. It should be noted that the carrier AgBrI grains (at 6 mol% of iodide) were not spectrally sensitized before the formation of silver chloride.

  
EXAMPLE 2

  
This example shows the deposition of silver chloride along the edges of spectrally sensitized tabular grains of silver bromide.

  
Emulsion 2A: Tabular carrier grains of silver bromide.

  
A 2.0 liters of a 1.5% by mass gelatin solution which is

  
  <EMI ID = 113.1>

  
by double jet, a 0.30M solution of sodium bromide and a 0.05M solution of silver nitrate. The addition takes place in 5 min while maintaining the pBr at 1.14; 0.4% of the total amount of silver nitrate used is consumed. The bromide and silver nitrate solutions are then added simultaneously, maintaining the pBr at 1.14, with an accelerated flow.
(multiplied by 3 between the beginning and the end of the addition). The operation is done in 4 minutes; 0.66% of the total amount of silver nitrate used is consumed.

  
Then added, in one, a 1.5M solution of sodium bromide and a 1.5M solution of silver nitrate with an accelerated flow rate (multiplied by 14.3 between the start and the end) and maintaining the pBr at 1.14; 66.2% of the total amount of silver nitrate used is consumed. Then, the solutions are added no longer with an accelerated flow, but at a constant flow, this for 6.6 min; 32.8% of the total amount of silver nitrate used is consumed. A total of about 3.03 moles of silver salt was used.

  
The emulsion is cooled to 40 ° C., washed by coagulation as

  
it is described in US Patent 2,614,929, it is stored at a pAg of 8.0 measured at 40 [deg.] C.

  
The tabular silver bromide grains obtained have a diameter

  
  <EMI ID = 114.1>

  
56: 1 average; the tabular grains represent 85% of the total projected surface.

  
Emulsion 2B: Epitaxial growth of silver chloride on the faces

  
main crystals of carrier fears (control).

  
The previously prepared silver bromide grain emulsion is centrifuged and then resuspended in a solution.

  
  <EMI ID = 115.1>

  
silver chloride in 40 g (0.04 mole) of the emulsion by adding, in 4.1 min, by double jet, a 0.55M NaCl solution and a solution

  
  <EMI ID = 116.1>

  
The emulsion is spectrally sensitized with 1.0 millimole (per mole of silver) of dye A, the triethylamine salt of anhydro-5-chloro-9-ethyl-5'-phenyl-3,3 hydroxide '-bis (3-sulfopropyl) oxacarbocyanine.

  
Emulsion 2C: Epitaxial growth of silver chloride in a way

  
selective on the grain edges.

  
The emulsion is prepared as indicated in the preceding paragraph B, with the difference that the spectral sensitization with 1.0 millimole of dye A per mole of silver takes place before the addition of the sodium chloride and nitrate solutions. silver.

  
As shown in FIG. 6, the emulsion 2B, sensitized spectrally after the formation of silver chloride, presents deposits of silver chloride distributed in a disordered manner on the surface of the crystals.

  
  <EMI ID = 117.1> the formation of silver chloride, which has deposits of silver chloride located almost exclusively along the grain edges. In general, the few small grains which appear on the main crystal faces of the tabular grains do not result from an epitaxial deposit, but are distinct grains.

  
The emulsions 2B and 2C are applied to a polyester support at the rate of 1.61 g of silver per square meter and 3.58 g of gelatin per <EMI ID = 118.1>

  
expose the layers obtained for 1 / 10th of a second to a tungsten Light source of 600 W and 2850 [deg.] K behind a density scale of 0 to 6.0 (ranges of 0.30). They are treated for different durations ranging from 1 min to 20 min, at 20 [deg.] C, in a developer with N-methyl-p-aminophenol sulfate and hydroquinone. The sensitometric results are shown in Table II.

TABLE II

  

  <EMI ID = 119.1>


  
EXAMPLE 3 -

  
This example shows that the addition of small quantities of iodide to a tabular grain emulsion of silver bromide allows an epitaxial deposition of silver chloride on the tops of tabular carrier crystals not spectrally sensitized.

  
Control emulsion 3A: Epitaxial growth of distributed silver chloride

  
in a disorderly manner on the main crystal faces of carrier grains.

  
The emulsion 2A is used, containing tabular grains carrying silver bromide, described in paragraph A of example 2. It is centered

  
  <EMI ID = 120.1>

  
sodium chloride. 2.5% by mole of silver chloride is then precipitated in 40 g (0.04 mole) of the emulsion by addition, by double jet, of a 0.55M solution of sodium chloride and of a solution. 0.50M silver nitrate. The addition takes place for 4.1 min, at 40 [deg.] C, maintaining the pAg at 7.5. The emulsion is spectrally sensitized with

  
1.0 millimole of dye A per mole of silver.

Emulsion 3B: Epitaxial growth of silver chloride located selectively on the tops of the bearing grains.

  
400 g (0.4 mole) of the emulsion 2A with grains carrying silver bromide are used, to which 0.5% by mole of iodide is added by introduction, over 10 min, of a 4.0 solution x 10 -2 M potassium iodide with a flow rate of 5.0 ml / min. The emulsion is centrifuged and resuspended with a 1.85 x 10 -2 M solution of sodium chloride. 2.5 mole% of silver chloride is then precipitated in 40 g (0.04 mole) of the emulsion by addition, by double jet:, of a 0.55M NaCl solution and of a 0.50M silver nitrate solution. The addition takes place

  
  <EMI ID = 121.1>

  
The emulsion is spectrally sensitized with 1.0 millimole of dye A per mole of silver.

  
Control emulsion 3C: Grains containing iodide, but without chloride

  
silver.

  
The emulsion 3C is prepared and spectrally sensitized as previously indicated for the emulsion 3B; there is, however, no epitaxial deposition of silver chloride.

  
The emulsion 3A, spectrally sensitized after the formation of silver chloride, exhibits a disordered deposit of silver chloride on

  
i

  
the entire surface of the main crystal faces (see Figure 8).

  
Emulsion 3B, to which 0.5% was added. in moles of potassium iodide before the formation of silver chloride, shows a deposit

  
of silver chloride located almost exclusively on the tops of the grains (see Figure 9).

  
The small grains appearing on the main crystal faces are distinct and do not result from epitaxial growth on these faces.

  
The emulsions 3A, 3B and 3C are applied to a support, they are exposed and they are treated as described in Example 2. The sensitometric results are shown in Table III below:

TABLE I II

  

  <EMI ID = 122.1>


  
EXAMPLE 4 -

  
This example shows the epitaxial deposit of silver chloride located almost exclusively on the tops of tabular grains of silver bromide spectrally sensitized.

  
') Emulsion 4A: Preparation of tabular carrier bromide grains

  
silver.

  
To 3.0 liters of a solution at 1.5% by mass of gelatin which is also 0.067M in sodium bromide, brought to 80 [deg.] C, a solution is added, with stirring and by double jet 0.1M sodium bromide and

  
  <EMI ID = 123.1>

  
maintaining a pBr of 1.17; 0.22% of the total amount of silver nitrate used is consumed. in

  
A solution 3. OM of sodium bromide and a solution 3. OM of silver nitrate is then added simultaneously for 31 minutes using an accelerated flow rate (multiplied by 24.8 between the start and the end) and maintaining a pBr of 1.17; 91.0% of the total amount of silver nitrate used is consumed. The addition of the sodium bromide solution is stopped and that of silver nitrate is continued until a pAg of 7.75 is obtained; 6.8% of the total amount of silver nitrate used is consumed. A total of about 6.85 moles of silver nitrate were used.

  
The emulsion is cooled to 40 [deg.] C, it is washed by coagulation by the process described in US Patent 2,614,929. It is stored at a pAg of 8.5 measured at 40 [deg. .]VS.

  
The emulsion obtained contains tabular grains of bromide of ar-

  
  <EMI ID = 124.1>

  
and the average form index of 26: 1. Tabular grains represent
96% of the total projected area.

  
Emulsion 4B: Epitaxial growth of AgCl localized selectively on

  
the tops of the bearing grains.

  
40.0 g (0.04 mole) of the emulsion 4A with tabular carrier grains of AgBr, previously prepared, are used. The pAg is adjusted to 7.2 at

  
  <EMI ID = 125.1>

  
5 min at 40 [deg.] C. 1.0 ml of a 0.5M NaCl solution is then added.

  
5.0% by mole of AgCl is precipitated in the emulsion by addition, by double jet, of a 0.52M NaCl solution and of a 0.50M solution

  
  <EMI ID = 126.1>

  
7.2.

  
FIG. 10 represents an electron micrograph of a carbon replica of the silver bromide emulsion with epitaxial deposition of silver chloride.

  
EXAMPLE 5 -

  
This example shows the epitaxial deposition of silver chloride selectively located on the tops of tabular grains of AgBrI.

  
Emulsion SA: Preparation of tabular carrier bromoiodide grains

  
silver 6% by mole of iodide.

  
A 6.0 liters of a 1.5% by mass gelatin solution which is

  
in addition 0.12 M in potassium bromide, brought to 55 [deg.] C, adding, with stirring and by double jet, a 1.12 M solution of KBr which is also 0.06 M in KI and a solution 1, OM of AgNO. The addition takes place in 8 min; 5.0% of the total amount of silver nitrate used is consumed. In

  
  <EMI ID = 127.1>

  
Then an emulsion 2, OM of KBr which is also 0.12M in KI and a solution 2, OM of AgN03 are added simultaneously. The addition takes place in
30 min, maintaining a pBr of 0.92, at 70 [deg.] C, with an accelerated flow
(multiplied by 4 between the start and the end of the operation); we consume
95.0% of the total amount of silver nitrate used. A total of about 3.16 moles of silver salt was used.

  
The emulsion is cooled to 35 [deg.] C, washed by coagulation as described in US Patent 2,614,929. It is stored at a pAg of 8.2 measured at 35 [deg. ]VS.

  
The emulsion obtained contains tabular bromoiodide grains

  
  <EMI ID = 128.1>

  
tabular grains represent 85% of the total projected area. Emulsion 5B: Epitaxial growth of AgCl selectively localized on

  
the tops of the bearing grains.

  
40 g (0.04 mole) of the emulsion 5A previously prepared which contains tabular carrier grains of AgBrI are used. We adjust the pAg

  
  <EMI ID = 129.1>

  
1.0 ml of a 0.54M NaCl solution. The emulsion is spectrally sensitized with 1.0 millimole of dye A per mole of silver. 5.0% by mole of AgCl is precipitated in this emulsion by addition, by double

  
¯7 <EMI ID = 130.1>

  
The addition takes place in 7.8 min, at 40 [deg.] C, maintaining the pAg at 7.5.

  
FIGS. 11A and 11B represent micrographs of secondary electrons of the emulsion 5B, showing the epitaxial deposition of 5% by mole of AgCl on the vertices of the tabular crystal of AgBrI (at 6% by mole of iodide).

  
EXAMPLE 6 -

  
This example shows the epitaxial deposit of silver bromide selectively located on the tops of the tabular grains of silver bromoiodide spectrally sensitized.

  
Emulsion 6A: Preparation of tabular carrier grains of AgBrI at 12%

  
by mole of iodide.

  
To 9.0 liters of a solution at 1.5% by mass of gelatin which is also 0.14M in potassium bromide, brought to 55 [deg.] C, is added, in

  
  <EMI ID = 131.1>

  
consumes 0.4% of the total amount of silver nitrate used. Then a 2.05M KBr solution is added by double jet, over 15 s.

  
  <EMI ID = 132.1>

  
the total amount of silver nitrate used. The halide and silver nitrate solutions are then introduced simultaneously for 7.5 minutes, keeping the pBr at 0.92, which consumes 2.3% of the total amount of silver nitrate used. Then, simultaneously, in 41 minutes, the halide and silver nitrate solutions are kept, maintaining the pBr at 0.92 and using an accelerated flow (multiplied by 6.6 between the start and the end of the 'addition). We consume

  
96.9% of the total amount of silver nitrate used.

  
The emulsion is cooled to 35 [deg.] C, washed by coagulation as described in US Patent 2,614,929 and stored at pAg 8.2 measured at 40 [deg.] vs.

  
The emulsion obtained contains tabular bromoiodide grains

  
  <EMI ID = 133.1>

  
tabular grains represent 75% of the total projected area. Emulsion 6B: Epitaxial growth of AgBr selectively located on

  
the tops of the bearing grains.

  
2 56.8 g (0.06 mole) of the emulsion 6A prepared previously is used.

  
  <EMI ID = 134.1>

  
of dye A per mole of silver and the emulsion is maintained for 5 min at 40 [deg.] C. 4.2 mol% of AgBr are then precipitated in the emulsion by the addition, by double jet, of a 0.2M NaBr solution which contains

  
  <EMI ID = 135.1>

  
for 10 min.

  
Study of the grains after stopping development.

  
A 6B emulsion, previously prepared, containing tabular grains of AgBrI with epitaxial deposit of AgBr, chemically sensitized, is applied to a cellulose ester support, at 1.07 g of silver per square meter and 2.15 g of gelatin per square meter.

  
The layer is exposed to obtain a maximum density for 1/100 th of a second to a tungsten light source of 600 W and
3000 [deg.] K. It is treated for 75 s, at 20 [deg.] C, in the developer A below:

  
Developer A

  

  <EMI ID = 136.1>


  
pH measured at 9.4

  
After development, the layer is placed, for 1 min, in a

  
  <EMI ID = 137.1>

  
tilled.

  
FIG. 12 represents an electron micrograph of a gelatin imprint of partially developed grains. The blackest areas represent the developed silver. The location of the developed silver shows that the latent image forms almost exclusively on the tops of the tabular grains or in their vicinity.

  
EXAMPLE 7 -

  
This example studies the sensitivity and minimum density of an emulsion, after its preparation and after storage, as a function of the epitaxy. This example also makes it possible to locate the latent image centers by examining partially developed grains.

  
Emulsion 7A: Tabular carrier grain emulsion of AgBrl (6% in

  
mole of iodide) chemically and spectrally sensitized.

  
The emulsion 1A containing tabular carrier grains of AgBrI (at 6% by mole of iodide) is chemically sensitized by addition of

  
5 mg of Na2S203 '5H20 per mole of silver and 5 mg of KAuCl4 per mole of silver and maintaining the emulsion for 10 min at 60 [deg.] C. The emulsion is then sensitized spectrally with 1.5 millimole of dye A per mole of silver.

  
The emulsion is applied to a polyester support at the rate of 1.61 g of silver per square meter and 3.58 g of gelatin per square meter. The emulsion is coated with a layer of gelatin (0.54 g / m <2>).

  
  <EMI ID = 138.1>

  
spectrally.

  
0.04 mole of emulsion 1A containing tabular carrier grains of AgBrI containing 6% by mole of iodide is used. The pAg is adjusted to 7.2 at

  
  <EMI ID = 139.1>

  
0.006M KI. 1.0 ml of a 0.80M NaCl solution is then added.

  
The emulsion is spectrally sensitized with 1.5 millimole of dye A per mole of silver. 1.25% by mole of AgCl is then precipitated in the emulsion by the addition, by double jet, of a 0.54M solution of NaCl and of a 0.50M solution of AgN03 for 2 min, maintaining the pAg at 7.5 to 40 [deg.] C.

  
7C emulsion: AgBrI emulsion with epitaxial deposition of AgCl, sensitized

  
chemically and spectrally.

  
We use the emulsion lA which contains tabular carrier grains

  
  <EMI ID = 140.1>

  
from KI. 1.0 ml of a 0.74M sodium chloride solution is added. The emulsion is sensitized spectrally with 1.5 millimole of dye A per mole of silver and it is maintained for 30 min at 40 [deg.] CA twice, the emulsion is centrifuged, then it is put back in the form of sus -

  
  <EMI ID = 141.1>

  
1.25% by mole of AgCl is precipitated in 40 g (0.04 mole) of the emulsion by the addition, by double jet, of a 0.54M solution of NaCl and of a 0.50M solution of AgN03 . The addition takes place for 2.1 min, maintaining the pAg at 7.5 to 40 [deg.] C.

  
  <EMI ID = 142.1>

  
added 15 s after the addition of NaCl and AgN03 solutions

  
started.

  
Figure 13 is an electron micrograph of this emulsion. We can see an epitaxial deposit located selectively on the tops of the grains.

  
7D emulsion: AgBrI emulsion with epitaxial deposition of AgCl, sensitized

  
chemically and spectrally.

  
The 7D emulsion is prepared as indicated for the previous 7C emulsion, with the difference, however, that during deposition: epitaxial of AgCl on the grains carrying AgBrI spectrally sensitized, the emulsion is chemically sensitized by addition 1.0 mg of KAuCl4 per mole of silver and 1.0 mg of Na2S203.5H20 per mole of silver.

  
The preceding emulsions are applied to a support, they are exposed and they are treated as in Example 2. The sensitometric results are given in Table IV below:

TABLE IV

  

  <EMI ID = 143.1>


  
  <EMI ID = 144.1>

  
As shown in Table IV, the emulsions 7B, 7C and 7D which contain tabular grains of silver bromoiodide carrying an epitaxial deposit of silver chloride and which are spectrally sensitized are, whether they are chemically sensitized or not, clearly faster (about 1.2 LogE) than the 7A emulsion formed from grains carrying silver bromoiodide sensitized chemically and spectrally. It should also be noted that much less chemical sensitizer was used for emulsions 7C and 7D than for emulsion 7A.

  
&#65533;

  
Emulsion layers 7A and 7C are kept for one week at
49 [deg.] C and under 50% relative humidity. They are then exposed for 1 / 10th of a second to a tungsten light source of 600 W and 2850 [deg.] K behind a density scale of 0 to 6.0 (ranges of 0.30). Is the

  
  <EMI ID = 145.1>

  
p-aminophenol and hydroquinone. The sensitometric results show that the 7C emulsion (AgBrI grains / epitaxial deposit of AgCl) is more

  
  <EMI ID = 146.1>

  

  <EMI ID = 147.1>


  
Study of the grains after stopping development.

  
The emulsion is applied to a cellulose ester support, at a rate of 1.61 g of silver per square meter and 3.58 g of gelatin per square meter.

  
  <EMI ID = 148.1>

  
7C with tabular grains of AgBrI having an epitaxial deposit of AgCl. These layers are coated with a gelatin overlay (0.54 g / m <2>).

  
The emulsion layer 7A is exposed to obtain a maximum density, for 1 / 10O of a second, at a tungsten light source of 600 W

  
  <EMI ID = 149.1>

  
the composition of which is indicated below.

  
The emulsion layer 7C is exposed to obtain a maximum density, for 1 / 10th of a second, at a tungsten light source of 600 W and 2850 [deg.] K behind a neutral filter of density 2.0, then it is treaty

  
  <EMI ID = 150.1>

  
Developer B
  <EMI ID = 151.1>
 After development, the layers are placed for 30 s in a stop bath of acetic acid (0.5%), then washed with distilled water for two minutes.

  
FIG. 3 represents an electron micrograph of a gelatin imprint of partially developed grains of the emulsion 7A. The location of the developed silver (blackest areas) shows that the formation of the latent image takes place mainly, in some way, along the edges of the tabular grains. FIG. 2 represents the partially developed grains of the 7C emulsion and shows that the latent image is formed almost exclusively in the vicinity of the submets of the tabular grains.

  
EXAMPLE 8 -

  
This example makes it possible to compare the photographic response of an emulsion with tabular grains of silver bromoiodide when this emulsion is spectrally sensitized before the epitaxial deposition of silver chloride and that of the same emulsion but spectrally sensitized after the epitaxial deposition of silver chloride.

  
Emulsion 8A: Selective epitaxial growth of AgCl on the tops of

  
carrier grains (spectrally sensitized before precipitation of silver chloride).

  
Emulsion 1A is used which contains tabular carrier grains

  
  <EMI ID = 152.1>

  
simultaneous tion of an O, IOM emulsion of AgN03 and a 0.006M solution of ICI. 1.0 ml of 0.7411 NaCl solution is added.

  
The emulsion is spectrally sensitized by the addition of 1.5 millimole of dye A per mole of silver and it is maintained for 30 min at 40 [deg.] C. Twice, the emulsion is centrifuged and resuspended

  
  <EMI ID = 153.1>

  
0.50M AgN03. The addition takes place for 2 a, maintaining the pAg at 7.5 to 40 [deg.] C.

  
Fifteen seconds after the start of the introduction of the

  
  <EMI ID = 154.1>

  
Emulsion 8B: Epitaxial growth of AgCl in any way on

  
the main faces of the carrier grains (spectrally sensitized after the precipitation of silver chloride).

  
The emulsion 8B is prepared as indicated for the emulsion 8A, but the spectral sensitization with 1.5 millimole of dye A per mole of silver takes place after the deposition of AgCl.

  
The electron micrographs of the emulsion 8A, sensitized spectrally before the precipitation of AgCl, show that the deposition of AgCl took place exclusively near the tops of the tabular crystals of AgBrI.

  
In the emulsion 8B, which was sensitized spectrally after the

  
  <EMI ID = 155.1>

  
than on the main crystal faces of the grains.

  
The SA and 8B emulsions are applied in layers on a cellulose triacetate support at a rate of 1.61 g of silver per square meter and

  
  <EMI ID = 156.1>

  
in Example 2. The sensitometric results show that, for the same minimum density (0.10), the emulsion 8A is faster by 0.70 logE than the emulsion 8B.

  
EXAMPLE 9 -

  
This example shows the photographic response of an AgBrI grain emulsion having an epitaxial deposit of AgCl, sensitized before the precipitation of silver chloride.

  
Emulsion 9A: Epitaxial growth of AgCl made selectively on

  
peaks of bearing grains.

  
40 g (0.04 mole) of LA emulsion containing tabular carrier grains of AgBrI (6 mol% of iodide) are used. We adjust the pAg to

  
  <EMI ID = 157.1>

  
The emulsion is sensitized spectrally with 1.87 millimole of dye C per mole of silver. Dye C is the triethylamine salt

  
  <EMI ID = 158.1>

  
1.25% by mole of AgCl in this emulsion by addition, by double jet, of a 0.54M solution of NaCl and of a 0.50M solution of AgNO3. This addition takes place for 2 min while maintaining the pAg at 7.5 to 40 [deg.] C. Emulsion 9B: Epitaxial deposit of AgCl sensitized to gold, localized

  
selectively on the tops of the bearing grains.

  
The emulsion 9B is prepared as indicated for the emulsion 9A. However, 15 s after the start of the addition of the

  
  <EMI ID = 159.1>

Emulsion 9C: Epitaxial deposit of AgCl, sensitized to sulfur selectively on the tops of the bearing grains.

  
The 9C emulsion is prepared as indicated for the 9A emulsion. However, fifteen seconds after the start of the addition of the solutions

  
  <EMI ID = 160.1>

  
9D emulsion: epitaxial deposition of AgCl sensitized to selenium, localized

  
selectively on the summits of carrier fears.

  
The 9D emulsion is prepared as indicated for the 9A emulsion. However, fifteen seconds after the start of the addition of the solutions

  
  <EMI ID = 161.1>

  
per mole of money.

  
The emulsions 9A to 9D are applied to a cellulose triacetate film support at the rate of 1.15 g of silver per square meter and 3.50 g of gelatin per square meter.

  
In addition, a sample of the emulsion 1A with tabular carrier grains of AgBrI is spectrally sensitized with 1.87 mg of dye C per mole of silver. A layer of this emulsion is applied to a support, as above.

  
Another sample of emulsion 1A is chemically sensitized

  
  <EMI ID = 162.1>

  
mole of silver and it is maintained for 10 min at 60 [deg.] C, then this emulsion is spectrally sensitized with 1.87 mg of dye C per mole of silver. The emulsion obtained is then applied to a support, as above.

  
  <EMI ID = 163.1> <EMI ID = 164.1>. lator with N-methyl-p-aminophenol sulfate and hydroquinone.

  
The sensitometric results show that the emulsions 9A to 9D comprising grains of AgBrI with epitaxial deposition / are much faster (more than 2.0 logE) and have a higher maximum density than the emulsions with tabular carrier grains of AgBrI spectrally sensitized with or without chemical sensitization (see Table VI below).

  

  <EMI ID = 165.1>


  

  <EMI ID = 166.1>
 

  
EXAMPLE 10 -

  
This example concerns the epitaxial deposition of AgBr on the vertices of spectrally sensitized AgBrI tabular crystals.

  
Emulsion 10A: Epitaxial growth of AgBr selectively located on

  
  <EMI ID = 167.1>

  
by mole of iodide.

  
We use emulsion 1A containing tabular carrier grains

  
  <EMI ID = 168.1>

  
1.5 millimoles of dye A per mole of silver.

  
After spectral sensitization, the emulsion is centrifuged and resuspended in distilled water, this twice.

  
0.6 mole% of AgBr is then precipitated in 40 g (0.04 mole) of this emulsion by the addition, by double jet, of a 0.2M solution of

  
  <EMI ID = 169.1>

  
1.5 min, maintaining the pAg at 7.5 to 40 [deg.] C. Fifteen seconds after the start of the addition of the NaBr and AgNO solutions, 1.0 mg of Na2S203, 5H20 and 1.0 mg of KAuCl4 are added per mole of silver.

  
FIG. 4 represents an electron micrograph of a carbon replica of the grain emulsion of AgBrI with epitaxial deposit of AgBr.

  
A sample of the grain emulsion 1A is chemically sensitized

  
  <EMI ID = 170.1>

  
at 60 [deg.] C. It is then sensitized spectrally with 1.5 millimole of dye A per mole of silver.

  
The emulsion lA with carrier grains and the emulsion 10A with grains carrying an epitaxial deposit are applied to a support, they are exposed and

  
they are treated as in Example 2.

  
The sensitometric results show that the deposition 10A emulsion

  
of epitaxial chemical sensitizer, sensitized by a quantity / much less than that used for the emulsion lA and at a lower temperature, is approximately

  
  <EMI ID = 171.1>

  
equal minimum density (0.10).

  
EXAMPLE 11 -

  
This example shows the epitaxial deposition of AgCl on tabular grains of AgBr spectrally sensitized by an over-association.

  
'} dye sensitizer.

  
Emulsion 11 A: Tabular carrier grains of AgBr.

  
This emulsion is prepared by the process used for the emulsion

  
  <EMI ID = 172.1>

  
average shape of 43: 1 and represent 90% of the total projected surface of the silver bromide grains.

  
Emulsion 11 B: Epitaxial growth of AgCl selectively localized

  
on the top of tabular carrier grains of AgBr spectrally sensitized by a combination of dyes.

  
40 g (0.04 mole) of the 11 A emulsion are used and the pAg is adjusted

  
  <EMI ID = 173.1>

  
1.0 mg of a 0.61M NaCl solution. The emulsion is spectrally sensitized with 1.5 millimole of dye B per mole of silver.

  
1.25% er is precipitated. mole of AgCl in this emulsion by addition, by double jet, of a 0.54M NaCl solution and a 0.50M solution of AgN03. The addition takes place for 2 min, maintaining the pAg at 7.5 at <EMI ID = 174.1>

  
Sensitometric results.

  
The following layers are formed:

  
Layer 1:

  
The emulsion 11 A is spectrally sensitized with 1.5 millimole of dye B per mole of silver and with 0.15 millimole of dye D per mole of silver. Dye D is 2- (p-diethylaminostyryl) benzothiazole. The emulsion is coated on a polyester support at a rate of 1.73 g of silver per square meter and 3.58 g of gelatin per square meter. Apply an overlay of gelatin (0.54 g / m)

  
Layer 2:

  
  <EMI ID = 175.1>

  
on a support as described for layer 1.

  
Layer 3:

  
The emulsion 11 B (AgBr / AgCl epitaxy) sensitized spectrally with the dye B, is further sensitized with 0.15 millimole of dye D per mole of silver after the deposition of silver chloride. Then applied to a support as described for layer 1. The layers are exposed and treated as in Example 2. The sensitometric results are given in Table VII.

  
7
  <EMI ID = 176.1>
  <EMI ID = 177.1>
 As we could see, the emulsion 11 B (with grains of AgBr with epitaxial deposition of AgCl) which was spectrally sensitized before the deposition of AgCl, is faster (by 131 log units of the rapidity ) than the emulsion 11 A with spectrally sensitized carrier grains. The emulsion 11 B is even faster (by 63 logarithmic units of the speed) than the emulsion 11 A chemically and spectrally sensitized. EXAMPLE 12 -

  
This example concerns the preparation of an AgBrI grain emulsion with an epitaxial deposit of AgCl, by adding a fine grain emulsion of AgCl to a tabular grain emulsion of AgBrI.

  
12 A emulsion: AgCl fine grain emulsion.

  
A 3.0 liters of a 3.3% by mass gelatin solution which is

  
  <EMI ID = 178.1>

  
a 4, OM solution of sodium chloride and a 4, OM solution of silver nitrate. The addition takes place at 35 [deg.] C for 0.4 min, at pAg of 6.9. 0.24 mole of silver chloride emulsion is thus prepared.

  
Emulsion 12 B: AgBrI grain emulsion with epitaxial deposit of AgCl,

  
containing 2.5% by mole of AgCl.

  
30 g of emulsion 1 A, containing

  
tabular grains of AgBrI at 6% by mole of iodide, with 1.1 millimole of dye A per mole of silver and it is maintained for 15 min at 40 [deg.] C.

  
  <EMI ID = 179.1>

  
of the silver chloride emulsion 12A prepared previously and the mixture is stirred for 30 min at 40 [deg.] C.

  
Electronic micrographs, including that of FIG. 15, show that the epitaxial deposition of silver chloride takes place, selectively, on the tops of the tabular crystals of AgBrI.

  
EXAMPLE 13 -

  
This example shows that the epitaxial deposition of silver chloride can take place selectively on the tops of the tabular carrier grains of silver bromoiodide in the absence of an adsorbed site regulator when the carrier grains contain a quantity sufficient iodide.

  
Emulsion 13 A: Tabular carrier grains of silver bromoiodide

  
12% by mole of iodide.

  
This emulsion, prepared by a process of precipitation by

  
  <EMI ID = 180.1>

  
average shape of 40: 1 and represent more than 85% of the total projected surface of the grains present. The grains contain 12 mol% of iodide, the iodide being introduced uniformly during precipitation by double jet. the emulsion is spectrally sensitized with

  
0.6 millimole of dye A per mole of silver.

  
Emulsion 13 B.

  
The emulsion 13B is prepared as indicated for the emulsion 13A; however, before spectral sensitization, the emulsion is chemically sensitized with 3.4 mg of sodium thiosulfate pentahydrate per mole of silver and 1.7 mg of potassium tetrachloroaurate per mole of silver, for 10 min at 65 [deg .]VS.

  
13 C emulsion: Spectral sensitization after selective epitaxial deposition

  
on the peaks.

  
The 13A emulsion which contains tabular grains of silver bromoiodide at 12 mol% iodide is used and the pAg is adjusted to 7.2

  
  <EMI ID = 181.1>

  
sodium. 2.5 mol% of silver chloride are then precipitated in
40 g (0.04 mole) of the emulsion with tabular carrier grains by addition, by double jet, for 4 min, of a 0.55M solution of sodium chloride and a 0.5M solution of nitrate silver, maintaining the pAg at 7.5 to 40 [deg.] C. The emulsion is sensitized spectrally with 0.6 millimole of dye A per mole of silver.

  
The 13C emulsion, spectrally sensitized after the precipitation of silver chloride, comprises a deposit of silver chloride located almost exclusively on the tops of the tabular crystals of silver bromoiodide. Figure 16 shows an electron micrograph of a carbon replica of the 13C emulsion.

  
The emulsions 13A, 13B and 13C are applied to a support, they are exposed and they are treated as in example 2. The results

  
7 triques are given in Table VIII.

  
  <EMI ID = 182.1>

  

  <EMI ID = 183.1>


  
EXAMPLE 14 -

  
This example shows that the epitaxial growth of silver chloride on spectrally sensitized tabular silver bramoiodide grains may not take place on all vertices.

  
Emulsion 14 A.: Epitaxial growth of localized silver chloride

  
selectively on the tops of the bearing grains.

  
The emulsion 14A is prepared by the method described for the emulsion 1A of Example 1. After the precipitation, the pAg of the emulsion is adjusted.

  
  <EMI ID = 184.1>

  
mole per liter in chloride ion.

  
The emulsion is sensitized spectrally with 1.5 millimole of dye A per mole of silver and it is maintained for 30 min at 40 [deg.] C. 1.2 mol% of silver chloride are then precipitated in 9.5 liters.

  
(3.9 mole) of the carrier grain emulsion by addition, by double jet, of a 2.19M solution of sodium chloride and a 2, OM solution of silver nitrate. The addition takes place for 4 min while maintaining the pAg

  
at 7.2 to 40 [deg.] C.

  
The electron micrographs of the emulsion 14A (including that of FIG. 17) show that the epitaxial growth of silver chloride on the tabular grains of silver bromoiodide at 6 mol% of spectrally sensitized iodide is generally localized on less of 6 vertices of each hexagonal tabular crystal.

  
') EXAMPLE 15 -

  
This example shows the epitaxial deposition of silver chloride selectively at sites in the central annular region of

  
  <EMI ID = 185.1>

  
iodide.

  
15 A emulsion: Preparation of tabular carrier bromoiodide grains

  
  <EMI ID = 186.1>

  
double jet, a 1.12M solution of potassium bromide which is 0.12M in potassium iodide and a 1.0 OM solution of silver nitrate; the addition takes place in 1 min at pBr of 0.92; 0.6% of the total amount of silver nitrate used is consumed. The temperature is then raised to 7 0 [deg.] C in 7 min. A solution 2, OM of potassium bromide which is 0.24M in potassium iodide is added simultaneously and a solution 2, OM of silver nitrate, maintaining a constant pBr, with an accelerated flow (flow multiplied by 2.75 between start and end); the addition is made for 17.6 min and 29.2% of the total amount of silver nitrate used is consumed.

  
  <EMI ID = 187.1>

  
  <EMI ID = 188.1>

  
silver trate, maintaining the pBr at 0.92; 11.7% of the total amount of silver nitrate used is consumed.

  
  <EMI ID = 189.1>

  
potassium bromide which is 0.24M in potassium iodide and a 2, OM solution of silver nitrate, maintaining the pBr at 0.92 to 55 [deg.] C; we

  
  <EMI ID = 190.1>

A total of about 3.4 moles of silver salt was used.

  
The emulsion is cooled to 35 [deg.] C, washed by coagulation according to the method described in US Patent 2,614,929, and stored at pAg 8.4 measured at 35 [deg. ]VS.

  
The emulsion obtained contains tabular bromoiodide grains

  
  <EMI ID = 191.1>

  
and an average thickness of 0.13 µm. The grains which have a thickness <EMI ID = 192.1>

  
average shape of 13.8: 1 and represent 80% of the total projected surface of the grains.

  
Emulsion 15 B: Localized silver chloride epitaxial growth

  
selectively on sites in the annular region

  
carrier grains.

  
40 g (0.04 mol) of the previously prepared 15A emulsion which contains tabular carrier grains of silver bromoiodide are used.

  
12% by mole of iodide. The pAg is adjusted to 7.2 at 40 [deg.] C by adding a

  
  <EMI ID = 193.1>

  
0.7 4M sodium chloride. 5% by mole of silver chloride is precipitated in the emulsion by addition, by double jet, of a 1.04 M solution of sodium chloride and a 1.0 OM solution of silver nitrate. This addition takes place in 1 minute while maintaining the pAg at 7.5 to 40 [deg.] C.

  
epitaxial

  
15 C emulsion: Selectively localized silver chloride / growth

  
on some sites of the annular regions of the bearing grains.

  
The 15C emulsion is prepared by the process described for the emulsion
15B previous. However, a 0.55 M solution of sodium chloride and a 0.5 M solution of silver nitrate are added for 7.8 min while maintaining the pAg at 7.5 to 40 [deg.] C.

  
FIG. 18 represents an electron micrograph of a carbon replica of the emulsion 15B which contains tabular grains of silver bromoiodide with an epitaxial deposit of silver chloride. An internal concentric hexagonal or triangular ring of silver bromide formed during precipitation of the tabular silver bromoiodide crystals on which the silver chloride was selectively deposited. It should be noted that the epitaxial deposition of silver chloride can take place on the silver bromide ring in the form of small crystallites and that the tabular crystals at 12 mol% of iodide are not spectrally sensitized.

  
Similar results are observed with the 15C emulsion; however, in this case, the slower rate of epitaxial deposition of silver chloride results in a smaller number of grains carrying an epitaxial deposition, the individual growths being consequently more numerous.

  
EXAMPLE 16 -

  
This example shows the epitaxial deposition of silver chloride on a

  
  <EMI ID = 194.1>

  
silver bromoiodide. The carrier grains are not spectrally sensitized before the precipitation of silver chloride.

  
16 A emulsion: Tabular carrier grains of silver bromoiodide

  
  <EMI ID = 195.1>

  
  <EMI ID = 196.1>

  
To 6.0 liters of a 1.5% by mass gelatin solution which is 0.12M in potassium bromide, a 1.12M solution of potassium bromide which is 0 is added, with stirring and by double jet. , 12M in potassium iodide and a solution 1, OM of silver nitrate. The addition takes place for 1 min at 55 [deg.] C, at a pBr of 0.92; we consume 0.5% of the quantity

  
  <EMI ID = 197.1>

  
7 a. Then a solution 2, OM of potassium bromide is added which is also 0.24M in potassium iodide and a solution 2, OM of silver nitrate. The addition takes place for 30 min while keeping the pBr constant, with an accelerated flow (multiplied by 4 between the start and the end); 82.9% of the total amount of silver nitrate used is consumed. The temperature is lowered to 55 [deg.] C. Then a solution 2, OM of potassium bromide and a solution 2, OM of silver nitrate are added. The addition takes place for 3.75 min, keeping the pBr at 0.92; 16.6% of the total amount of silver nitrate used is consumed. A total of about 3.6 moles of silver salt was used,

  
The emulsion is cooled to 35 [deg.] C and washed by the coagulation process described in US Patent 2,614,929. The emulsion is stored at pAg 8.4 measured at 35 [deg .]VS.

  
The emulsion obtained contains tabular bromoiodide grains

  
  <EMI ID = 198.1>

  
with an average shape of 24: 1 and representing 80% of the total projected surface of the grains.

  
Emulsion 16 B: Epitaxial growth of silver chloride in the

  
peripheral zone.

  
The emulsion 16A with tabular carrier grains of bromo- is dispersed.

  
  <EMI ID = 199.1>

  
distilled, centrifuged and resuspended in distilled water to obtain an emulsion containing one mole of silver per kilogram of emulsion. 2.5% by mole of silver chloride is precipitated in 0.04 mole of the emulsion 16A by addition, by double jet, of a 0.25M solution of sodium chloride and of a 0.25M solution of silver nitrate. The addition takes place for 0.8 min while maintaining the pAg at 6.75 to 40 [deg.] C. The emulsion is spectrally sensitized with 1.0 millimole of dye A per mole of silver.

  
The electron micrographs of the 16A emulsion (including that shown in FIG. 19) show that the epitaxial deposition of silver chloride is localized along the edges of the tabular carrier grains of silver bromoiodide at 12 mol% of iodide , which are not spectrally sensitized. The growth of silver chloride takes place selectively in the peripheral regions of the crystal carrying silver bromoiodide.

  
16 C emulsion: Sensitization of 16A emulsion.

  
Part of the emulsion 16A is used, to which 3.0 mg of sodium thiosulfate pentahydrate per mole of silver and 1.5 mg of potassium tetrachloroaurate per mole of silver are added. We heat the mixture

  
  <EMI ID = 200.1>

  
millimole of dye A per mole of silver.

  
The emulsions 16B and 16C are applied to a cellulose triacetate support at the rate of 1.61 g of silver per square meter and 3.58 g of gelatin per square meter. The emulsion layers are exposed and treated as described in Example 2. The sensitometric results show that, for an equal minimum density (0.15), the emulsion 16B is 0.16 log E plus faster than 16C emulsion. It should be noted that the 16B emulsion is not treated with any chemical sensitizer, whether it is sodium thiosulfate pentahydrate or potassium tetrachloroaurate.

  
EXAMPLE 17 -

  
This example shows the deposition of silver chloride selectively on a central part formed of silver bromide of a tabular crystal of silver bromoiodide. The silver chloride deposit is internally sensitized with iridium. The emulsion is not spectrally sensitized before the precipitation of the silver chloride. Emula ion 17A: Tabular grains of silver bromoiodide comprising a

  
central region of silver bromide.

  
This emulsion is prepared by a double jet precipitation technique. This emulsion comprises grains having a central region formed of silver bromide (6.7 mol% of the total grain) surrounded by an annular region formed of silver bromoiodide containing 12 mol% of iodide. The emulsion includes grains with an average diameter

  
  <EMI ID = 201.1>

  
average shape index of 24: 1 and represent 80% of the total projected surface of the grains.

  
Emulsion 17 B.

  
This emulsion is prepared by spectrally sensitizing part of the emulsion 17A with 0.6 millimole of dye A per mole of silver. Emulsion 17C: Localized silver chloride epitaxial growth

  
selectively on a central region of the carrier grains.

  
Part of the 17A emulsion is dispersed in distilled water, centrifuged and resuspended in a solution

  
  <EMI ID = 202.1>

  
silver chloride in 40 g (0.04 mole) of the tabular carrier grain emulsion by the addition, by a double jet, of a 0.55M solution of sodium chloride and of an O, SM solution of nitrate d 'silver. The addition takes place for 17.6 min while maintaining the pAg at 7.5 to 40 [deg.] C. The emulsion is then sensitized spectrally by the addition of 0.6 millimole of dye A per mole of silver.

  
Emulsion 17 D

  
The 17D emulsion is prepared by a process analogous to that used for the 17C emulsion; however, fifteen seconds after the start of the addition of the sodium chloride and silver nitrate solutions,

  
an iridium sensitizer is added.

  
The emulsions 17B, 17C and
17D at the rate of 1.61 g of silver per square meter and 3.58 g of gelatin

  
per square meter. A layer of gelatin is applied to the emulsions
(0.54 g / m <2>). The layers are exposed for 1/10 s to a tungsten light source of 600 W and 2850 [deg.] K behind a density scale of 0 to 6.0. The layers are treated for 6 min at 20 [deg.] C, in a developer
(A) with N-methyl-p-aminophenol sulfate and ascorbic acid or else in a developer (B) containing N-methyl-p-aminophenol sulfate, ascorbic acid and 10 g / l of sodium sulfite. The addition of sodium sulfite allows surface and internal development; therefore, developer (B) is an internal developer in the

  
meaning usually used in photography (also called "Total Developer"). Developer A is a surface developer. The percentage of silver developed by X-ray fluorescence is determined.

  
the curves giving the percentage of silver developed as a function of exposure. The results are shown in Table IX.

  

  <EMI ID = 203.1>


  

  <EMI ID = 204.1>
 

  
The emulsion layers 17B and 17D are also exposed for 0.5 s to a tungsten light source of 600 W and 28500K behind a

  
  <EMI ID = 205.1>

  
internal) of the type described in U.S. Patent 3,826,654. Another set of layers is exposed and processed

  
  <EMI ID = 206.1>

  
deal with the total developer. The results are shown in Table X.

  

  <EMI ID = 207.1>


  

  <EMI ID = 208.1>
 

  
As shown in Table X, the 17D emulsion is 1.05 log E faster than the control emulsion 17B. When the emulsion layer
17B witness is whitened, most of the latent image is eliminated. However, when the emulsion layer 17D is bleached, very little latent image is eliminated. This indicates that the latent image is much less whitened due to the presence of an epitaxial phase of silver chloride on the surface of the grains.

  
FIG. 20 is an electronic electrography of the emulsion 17C showing the epitaxial deposition of silver chloride on the central region formed by silver bromide of the tabular grains of silver bromoiodide. Figure 21 shows a secondary electron micrograph of the 17C emulsion, showing the epitaxial deposition of silver chloride in the central region.

  
EXAMPLE 18 -

  
This example shows the epitaxial deposition of silver thiocyanate on the tabular grains of a silver bromoiodide emulsion.

  
  <EMI ID = 209.1>

  
selectively on the grain edges.

  
40 g (0.04 mole) of emulsion 1A containing tabular carrier grains of silver bromoiodide are used.

  
6% by mole of iodide. The pAg is adjusted to 7.2 at 40 [deg.] C by the simultaneous addition of a 0.1M solution of silver nitrate and a 0.006M solution of potassium iodide.

  
1.0 ml of a 0.13M sodium thiocyanate solution is then added. 5% by mol of silver thiocyanate is precipitated in the emulsion by the addition, by double jet, of a 0.25M solution of sodium thiocyanate and of a 0.25M solution

  
silver nitrate. The addition takes place in 16 min while maintaining the pAg at 7.5 to 40 [deg.] C.

  
EMULSION 18B. Epitaxial growth of silver thiocyanate

  
by selective deposition on the tops of the grains.

  
The emulsion 18B is prepared by the method described for the emulsion 18A; however, before the addition, by double jet, of the solutions of sodium thiocyanate and silver nitrate, the emulsion is spectrally sensitized with

  
1.1 millimole of dye A per mole of silver.

  
Electronic micrographs of 18A emulsions and

  
18B show that the emulsion 18A, which was not spectrally sensitized before the addition of the silver nitrate and sodium thiocyanate solutions, comprises

  
grains having an epitaxial deposit of silver thiocyanate selectively located on the edges of tabular grains of silver bromoiodide. Figure 22 is an electron micrograph of the emulsion 18A.

  
The 18B emulsion, spectrally sensitized before epitaxial growth, shows that the deposition of silver thiocyanate took place almost exclusively on the tops of the tabular carrier grains. Figure 23 is an electron micrograph of the emulsion 18B.

  
EXAMPLE 19 -

  
This example shows the chemical sensitization of a tabular grain emulsion of silver bromoiodide comprising an epitaxial deposit of silver thiocyanate located selectively on the vertices.

  
EMULSION 19A. Epitaxial deposit of chemically sensitized silver thiocyanate, located on the tops of the grains.

  
Emulsion 1A is used comprising tabular carrier grains of silver bromoiodide at 6 mol% of iodide.

  
The pAg is adjusted to 7.2 at 40 [deg.] C by simultaneous addition of a 0.1M solution of silver nitrate and a 0.006M solution of potassium iodide. The emulsion is centrifuged and resuspended in distilled water. At 40 g

  
(0.04 mole) of the emulsion, 1.0 mole of a 0.13M solution of sodium thiocyanate is added.

  
The emulsion is spectrally sensitized with 1.1 millimole of dye A per mole of silver; we then precipitate 2.5%

  
by mole of silver thiocyanate in the emulsion by addition, by double jet, of a 0.25M solution of sodium thiocyanate and of a 0.25M solution of silver nitrate. The addi-

  
  <EMI ID = 210.1>

  
The emulsion is chemically sensitized by adding, one minute after the start of the addition of the thio- solutions.

  
  <EMI ID = 211.1>

  
sodium fate pentahydrate per mole of silver and 1.0 mg of potassium tetrachloroaurate per mole of silver.

  
The emulsion 19A previously prepared is applied to a support, exposed and treated as in Example 2.

  
The emulsion 1A is chemically sensitized with 7.5 mg of sodium thiosulfate pentahydrate per mole of silver and 2.5 mg of potassium tetrachloroaurate per mole of silver

  
  <EMI ID = 212.1>

  
with 1.1 millimole of dye A per mole of silver. We layer it on a support and experience it as an emulsion

  
  <EMI ID = 213.1>

  
The sensitometric results show that the silver bromoiodide emulsion with epitaxial deposition of silver thiocyanate is 0.34 log E faster than the tabular carrier emulsion of silver bromoiodide, for the same minimum density of 0.10.

  
EXAMPLE 20'-

  
This example shows the epitaxial deposition of silver thiocyanate on a tabular grain emulsion of silver chloride.

  
EMULSION 20A control: Tabular chloride-bearing grains

  
silver.

  
Two liters of a 0.625% by mass solution of synthetic polymer, containing 0.35% by mass of adenine and which is also 0.5M in calcium chloride and

  
  <EMI ID = 214.1>

  
The synthetic polymer is the copolymer of 3-thiapentyl methacrylate, acrylic acid and sodium 2-methacryloyloxyethyl1-sulfonate (molar ratio 1: 2: 7). We add

  
to this solution, by shaking and by double jet, a solution 2, OM of calcium chloride and a solution 2, OM of silver nitrate; the addition takes place in one minute and 0.08% of the total amount of silver nitrate used is consumed.

  
The silver chloride / nitrate solutions, with controlled pCl, are then added simultaneously with an accelerated flow.
(multiplied by 2.3 between the beginning and the end), in 15 minutes, which consumes 28.3% of the total amount of silver nitrate used. The silver chloride and nitrate solutions are then added for another 26.4 minutes, which consumes 71.1% of the total amount of silver nitrate used. During the first third of the precipitation, 30.0 ml of a 0.2M sodium hydroxide solution is added slowly to maintain the pH at 2.6 to 55 [deg.] C. A total of about 2.6 moles of silver nitrate was used. The emulsion is cooled to room temperature and dispersed in a solution

  
  <EMI ID = 215.1>

  
we decant. The solid phase is resuspended in a 3% by mass gelatin solution and the pAg is adjusted to 7.5 to 40 [deg.] C with a sodium chloride solution.

  
The emulsion obtained contains tabular grains of

  
  <EMI ID = 216.1>

  
an average shape index of 15: 1 and represent 80% of the total projected surface of the grains.

  
EMULSION 20B. Epitaxial growth of silver thiocyanate

  
selectively on the grain edges.

  
Precipitate 5 mol% of silver thiocyanate in

  
40 g (0.04 mole) of the emulsion 20A prepared previously by adding, by double jet, for 7.8 min, a 0.5M solution of sodium thiocyanate and a 0.5 M solution of nitrate silver.

  
The electron micrographs of emulsion 20B show that the silver thiocyanate is deposited almost exclusively on the edges of the tabular crystals of silver chloride. Figure 24 is an electron micrograph

  
of this emulsion. Tabular chloride crystals

  
  <EMI ID = 217.1>

  
but the silver thiocyanate is deposited indifferently on the two types of edges.

  
EXAMPLE 21 -

  
This example shows the control of the deposit of silver bromide on certain sites of the spectrally sensitized tabular grains of silver bromide. The additional silver bromide is deposited predominantly on the tops and there are some growths of silver bromide along the edges.

  
EXAMPLE 21A - Controlled growth of silver bromide on

  
certain sites of grains carrying silver bromide.

  
40 g (0.04M) of the emulsion 4A described in Example 4 are used; the pAg is adjusted to 7.2 at 40 [deg.] C with a 0.1M solution of silver nitrate. The emulsion is sensitized spectrally with 2.4 millimoles of dye E per mole of silver and the emulsion is maintained for 5 min at 40 [deg.] C.

  
Color E is the triethylamine hydroxide salt

  
  <EMI ID = 218.1>

  
(3-sulfobutyl) -benzimidazclocarbocyanine. 6.25 mol% of silver bromide are then precipitated in the emulsion by the addition, by double jet, of a 0.2M solution of sodium bromide and of a 0.2M solution of silver nitrate; the addition is done for 15.7 min while maintaining the pAg at 7.2

  
at 40 [deg.] C.

  
Figure 25 shows an electron micrograph of a carbon replica of the emulsion. A deposit of silver bromide is observed along the edges of the tabular grains; however, silver bromide was deposited mainly on the tops of the tabular grains. The small grains that appear on the main faces of the tabular grains are distinct from it and do not result from epitaxial growth.

  
EXAMPLE 22 -

  
This example shows the controlled deposition of silver bromoiodide on spectrally sensitized tabular grains of silver bromoiodide. The additional silver bromoiodide is chemically sensitized during its deposition and is localized selectively on the tops of the bearing grains.

  
EMULSION 22A. Tabular carrier grains of bromoiodide

  
silver 6% by mole of iodide.

  
The emulsion 1A is used which contains tabular carrier grains of silver bromoiodide at 6 mol% of iodide and is chemically sensitized with 4 mg of sodium thiosulfate pentahydrate per mol of silver and 4 mg of potassium tetrachloroaurate per mole of silver for 10 min

  
  <EMI ID = 219.1>

  
mole of dye A per mole of silver.

  
EMULSION 22B. Epitaxial growth of silver bromoiodide

  
selectively on the tops of the grains.

  
The emulsion 22A, spectrally sensitized with 1.2 millimole of dye A per mole of silver, is centrifuged and then it is resuspended in distilled water.

  
2.5% by mole of silver bromoiodide containing 6% by mole of iodide in 40 g (0.04 mole) of the emulsion are then precipitated. The precipitation is carried out by addition, by double jet, of a 0.188 M solution of potassium bromide and

  
0.012M in potassium iodide and a 0.2M solution of silver nitrate. This addition takes place for 9.9 min.

  
maintaining the pAg at 7.5 to 40 [deg.] C. Fifteen seconds after

  
at the start of precipitation, 1.0 mg of sodium thiosulfate pentahydrate per mole of silver and 1.0 mg of potassium tetrachloroaurate are added per mole of silver. Once the precipitation is complete, the emulsion is heated for

  
  <EMI ID = 220.1>

  
The electron micrographs of the emulsion 22B, including that of FIG. 26, show that the silver bromoiodide is deposited on the tops of the grains carrying silver bromoiodide.

  
The emulsions 22A and 22B are applied to a cellulose triacetate support at a rate of 1.61 g of silver per square meter and 3.58 g of gelatin per square meter, they are exposed and then treated as in the example. 2. The sensitometric results show that, for the same minimum density of 0.2, the emulsion 22B is 0.62 log E faster than the emulsion 22A.

  
EXAMPLE 23 -

  
This example relates to a silver halide emulsion comprising tabular grains having an average shape index slightly greater than 8: 1 and which contain 2.44% by mole of silver chloride deposited preferentially on the tops and the edges of the grains. tabular. EMULSION 23A. Tabular carrier grains of bromoiodide

  
of silver with an average shape index of 8.1: 1.

A) PREPARATION OF A TABULAR GRAIN EMULSION COMPRISING

A heart of silver bromide.

  
  <EMI ID = 221.1>

  
bone gelatin which is also 0.142M potassium bromide, which is stirred well, a solution of 1.15M is added

  
potassium bromide and an OM solution of silver nitrate. The addition takes place by double jet, at constant flow rate for 2 min and at a pBr of 0.85; we consume 1.75% of

  
the total amount of silver nitrate used. After waiting 30 s, the pBr of the emulsion is adjusted to 1.22 at 65 [deg.] C

  
by the addition in 7.33 min of a 2, OM solution of silver nitrate at a constant flow rate; 6.42% of the total amount of silver nitrate used is consumed. A 2.29M solution of potassium bromide and a 2, OM solution of silver nitrate are then added by double jet. The addition takes place

  
in 26 min, at a pBr of 1.22 to 65 [deg.] C, with an accelerated flow
(multiplied by 5.6 between the start and the end); we consume
37.6% of the total amount of silver nitrate used. The pBr of the emulsion is adjusted to approximately 2.32 to 65 [deg.] C by the addition, in 6.25 min. Of a solution 2, OM of silver nitrate, at constant flow rate, which consumes 6.85% of the total amount of silver nitrate used. A 2.29M solution of potassium bromide and a 2, OM solution of silver nitrate are added by double jet using a constant flow rate, the addition taking place in 54.1 min at a pBr of 2.32

  
at 65 [deg.] C; 47.4% of the total amount of silver nitrate used is consumed. A total of about 9.13 moles of silver nitrate was used to prepare this emulsion.

  
After precipitation, the emulsion is cooled to 40 ° C. and 1.65 liters of a 15.3% by mass solution of phthalylated gelatin are added. The emulsion is washed twice by the coagulation process described in the patent of the United States of America

  
2,614,929. 1.55 liters of a solution are then added.

  
13.3% by mass of bone gelatin and the pH of the emulsion is adjusted to 5.5, the pAg to 8.3 to 40 [deg.] C.

  
The emulsion obtained comprises tabular grains of

  
  <EMI ID = 222.1>

  
11.2: 1.

B) ADDITION OF A SILVER BROMIDE HULL.

  
To 2.5 liters of a 0.4M aqueous solution of potassium nitrate containing 1479 g (1.5 mole) of the heart emulsion, which is stirred well, a solution is added by double jet.

  
  <EMI ID = 223.1>

  
silver. The addition is done with a constant flow, during
135 min at pAg from 8.2 to 65 [deg.] C; 5.06 moles of silver nitrate are consumed. After precipitation, the emulsion is cooled

  
  <EMI ID = 224.1>

  
mass of phthalylated gelatin. The emulsion is washed 3 times by the coagulation process described in US Pat. No. 2,614,929. 1.0 liter of a 14.5% by mass solution of bone gelatin is then added and we adjust the pH

  
of the emulsion at 5.5 and the pAg at 8.3 at 40 [deg.] C.

  
The emulsion obtained contains tabular grains of

  
  <EMI ID = 225.1>

  
Tabular grains represent more than 80% of the projected surface of the grains.

  
EMULSION 23B. Use of soluble iodide (0.5% by mole)

  
as a site regulator.

  
To 40.0 g (0.04 mole) of the emulsion 23A, 0.5% by mole of iodide is added at 40 [deg.] C, over 10 minutes, by introducing a 0.04M iodide solution. potassium at constant flow. The emulsion is centrifuged and resuspended in

  
a solution of 1.8 x 10- <2> M of sodium chloride to have

  
a total weight of 40.0 g. 2.44 mol% of silver chloride are precipitated from the emulsion by the addition, by double jet, of a 0.55M solution of sodium chloride and of a 0.50M solution of silver nitrate. The addition takes place at a constant flow rate, for 3.9 min, maintaining the pAg at 7.5 at

  
40 [deg.] C. The epitaxial deposit of silver chloride is located almost exclusively on the tops of the tabular grains. 23C EMULSION. Use of a spectral sensitizer like

  
site regulator.

  
  <EMI ID = 226.1>

  
silver nitrate. 1.0 ml of a 0.61M solution of sodium chloride is then added. The emulsion is sensitized spectrally with 0.84 millimole per mole of silver of anhydro-hydroxide-5,5 ', 6,6'-tetrachloro-1,1'-diethyl-3,3'-di (3sulfobutyl ) -benzimidazolocarbocyanine and the emulsion is maintained for 16 min at 40 [deg.] C. 2.44 mole% of silver chloride is then precipitated from the emulsion by the addition, by double jet, of a 0.55M solution of sodium chloride and a 0.50M solution of silver nitrate. The addition takes place at a constant flow rate, for 3.9 min, while maintaining the pAg at 7.5 to 40 [deg.] C. The epitaxial deposit of silver chloride is located on the tops and along the edges of the tabular grains of silver bromide.

  
EMULSION 23B. Control emulsion not containing a regulator

  
of site.

  
When the epitaxial deposition process is reproduced in the absence of iodide and spectral sensitizing dye, the silver chloride deposits in any way

  
on the faces of the tabular bearing grains.

  
EXAMPLE 24 -

  
This example shows that it is possible to use tabular bearing grains of high shape index to favor the epitaxial deposition of silver salt in a selective manner on alternate sites formed by ridges. These tabular bearing grains have main dodecagonal crystal faces (projected surfaces) bounded by six edges belonging to a first group of crystal planes.

  
  <EMI ID = 227.1>

  
with six edges belonging to a second group of crystalline planes (which are believed to be planes of crystal index-

  
  <EMI ID = 228.1>

  
24A EMULSIONS. Tabular bearing grains having a surface

  
projected dodecagonal.

  
3.0 liters of an aqueous solution containing 0.625% by mass of copolymer of 3-thiopentyl methacrylate, of acrylic acid and of 2-methacryloyloxyethyl-1- is prepared at pH 2.6 to 55 [deg.] C sodium sulfonate (molar ratio 1: 2: 7) 0.021M adenine, 0.0126M sodium bromide and 0.50M calcium chloride.

  
An aqueous solution 2, OM is added by double jet.

  
of calcium chloride and an aqueous solution 2, OM of silver nitrate, at constant flow rate, for 2 min; 3.98% of the total amount of silver nitrate used is consumed.

  
Add the halide and silver salt solutions

  
  <EMI ID = 229.1>

  
(multiplied by 2.3 between the start and the end); we consume
49.7% of the total amount of silver nitrate used. The halide and silver salt solutions are then added, for 10 minutes, at a constant rate, which consumes
46.4% of the total amount of silver nitrate used.

  
The pH was constantly maintained at about 2.6. Approximately 2.26 moles of silver nitrate were used to prepare the emulsion.

  
The emulsion obtained contains tabular grains of silver chlorobromide (99.6 / 0.4 in mole) which have a surface

  
  <EMI ID = 230.1>

  
the tabular grains represent more than 85% of the projected surface.

  
EMULSION 24B. Silver bromide deposit preferentially located on tabular grains of a silver chlorobromide emulsion.

  
To 2615 g (1.13 mole) of emulsion 24A with tabular grains of unwashed silver chlorobromide, an aqueous solution is added, by simple jet, for 5 min at 55 [deg.] C, at constant flow rate. 0.128M sodium bromide. We add about

  
30% by mole of bromide. Silver bromide is preferentially deposited on the edges of crystallographic index

  
  <EMI ID = 231.1>

  
The emulsion 24B is cooled to 20 [deg.] C, diluted in about 14.0 liters of distilled water, stirred and then allowed to stand. The supernatant liquid is decanted and the emulsion is redispersed in 330 g of an aqueous solution to
10% bone gelatin. The pH is adjusted to 5.5 and the pAg to 7.5

  
at 40 [deg.] C.

  
The 24B emulsion is spectrally sensitized with 0.5 millimole of triethylamine salt of anhydro5-chloro-9-ethyl-5'-phenyl-3 '- (3-sulfobutyl) -3- (3-sulfopropyl hydroxide) ) oxacarbocyanine per mole of silver.

  
The emulsion is chemically sensitized with 10 mg of sodium thiosulfate pentahydrate per mole of silver, 1600 mg of sodium thiocyanate per mole of silver and 5 mg of potassium tetrachloroaurate per mole of silver. The emulsion is maintained at 55 [deg.] C for 5 min.

  
24C EMULSION. Localized silver bromide deposit of a

  
arbitrarily on tabular grains of a silver chlorobromide emulsion.

  
A part of the emulsion 24A is washed as described for the emulsion 24B. The washed emulsion is then spectrally sensitized with 0.5 millimole of triethylamine salt

  
  <EMI ID = 232.1>

  
(3-sulfobutyl) -3- (3-sulfopropyl) oxacarbocyanine per mole of silver. A sufficient quantity of a sodium bromide solution is then rapidly added to the emulsion to obtain the addition of 3 moles of bromide per hundred moles of halide present in the emulsion 24A. The emulsion is then chemically sensitized as described for emulsion 24B.

  
The electron micrographs of this 24C emulsion show that the silver bromide has somehow deposited on the surfaces of the grains.

  
The emulsions 24B and 24C are applied to a cellulose triacetate support at a rate of 2.15 g of silver per square meter and 5.38 g of gelatin per square meter. The layers are exposed for 1 / 50th of a second to a tungsten light source of 600 W and 5500 [deg.] K behind a scale of continuous densities from 0 to 4.0. We process the emulsion layers

  
  <EMI ID = 233.1>

  
with N-methyl-p-aminophenol sulfate and ascorbic acid.

  
The sensitometric results show that the emulsion
24B, in which the epitaxial deposit of silver bromide is located on the edges of crystallographic index

  
  <EMI ID = 234.1>

  
faster than the control emulsion 24C which contains tabular carrier grains of silver halide on which the silver bromide is deposited in any way.

  
The photographic speed of the 24B emulsion can be increased by carrying out chemical and spectral sensitizations in the presence of a relatively low concentration (0.1% by mole) of soluble iodide. Two additional emulsions similar to emulsion 24B are prepared, with the difference that 0.6 ml of spectral sensitizer, 7.5 mg of sodium thiosulfate pentahydrate, 1600 mg of sodium thiocyanate are used per mole of silver. and 3.5 mg potassium tetrachloroaurate; we maintain

  
  <EMI ID = 235.1>

  
In addition, one of these two emulsions is added

  
0.1% by mole of sodium iodide before spectral sensitization. The photographic speed of these emulsions is evaluated as described above. The product containing the emulsion treated with iodide is 0.38 log E faster than that comprising the untreated emulsion

  
by iodide.

  
EXAMPLE 25 -

  
This example shows that the emulsions according to the invention have a higher covering power and a speed

  
faster fixing than comparable emulsions containing non-tabular carrier grains.

  
25A EMULSION. Bromoiodide-bearing grain emulsion

  
non-tabular money.

  
This emulsion is prepared by a conventional precipitation method by double jet at pH 4.5 and pAg from 5.1 to 79 [deg.] C.

  
The precipitation takes place by the process described on request

  
European Patent 19,917 published December 10, 1980. By X-ray diffraction, it has been determined that the molar ratio of bromide to iodide is 77:23 and that the iodide is uniformly distributed in the grains. The grains have a

  
  <EMI ID = 236.1>

  
EMULSION 25B. Epitaxial deposition of silver chloride on

  
non-tabular grains of the 25A emulsion.

  
2.5 mol% (relative to the total amount of halide) of silver chloride is deposited by epitaxy on the octahedral grains of the emulsion 25A. The process is the

  
&#65533;

  
next: 0.075 mol of the 25A emulsion is placed in a reactor and its final weight is brought to 50.0 g by addition of distilled water. 1.25 ml of a 0.735M sodium chloride solution is then added. 2.5% by mole of silver chloride is then precipitated by addition, by double jet, of a 0.55M solution of sodium chloride and of a 0.5M solution of silver nitrate. The addition takes place at constant flow,

  
  <EMI ID = 237.1>

  
mainly on the tops of the bearing grains. 25C EMULSION. Tabular carrier grain emulsion of

  
silver bromoiodide.

  
We choose an emulsion with tabular grains of silver bromoiodide, of high shape index, of medium volume

  
  <EMI ID = 238.1>

  
X-ray diffraction, we determine the molar ratio of bromide to iodide which is 80:20, the iodide being uniform

  
evenly distributed in the grains. tabular

  
tabular

  
The emulsion contains grains / whose average diameter is

  
  <EMI ID = 239.1>

  
These represent more than 90% of the total projected surface of the silver halide grains.

  
25D EMULSION. Epitaxial deposition of silver chloride on

  
25C emulsion tabular grains.

  
The process used for this epitaxial deposition is the same as that used for the emulsion 25B, the emulsion 25C having replaced the emulsion 25B in the reactor. The epitaxial deposition takes place mainly on the vertices and the edges of the tabular bearing grains.

  
The control emulsion 25B is applied to a polyester support at the rate of 2.83 g of silver per square meter and 10 g of gelatin per square meter. The layer is exposed for

  
0.5 s to a 600 W tungsten light source and
3000 [deg.] K behind a density scale from 0 to 6.0 (ranges of 0.30), then it is treated for 20 min, at 20 [deg.] C, in a developer with N-methyl sulfate -p-aminophenol and hydroquinone.

  
25D emulsion is applied to a polyester support

  
at the rate of 2.89 g of silver per square meter and 10 g of gelatin per square meter. It is exposed and treated in the same way as the 25B emulsion.

  
The 25D emulsion has a higher covering power than that of the 25B control emulsion, for grain volumes

  
and analog film titles. The 25D emulsion has a minimum density of 0.16 and a maximum density of

  
1.25. For the control emulsion 25B, the minimum density is

  
0.10 and the maximum density is 0.54. X-ray fluorescence analysis shows that at the maximum density,
97.2% of the silver developed in the case of the control emulsion and 100% in the case of the tabular grain emulsion.

  
Separate, untreated samples, layers

  
  <EMI ID = 240.1>

  
times, in a sodium thiosulfate fixing bath
(Kodak F-5), then they are washed for 30 minutes. The silver remaining in the layers is examined by X-ray fluorescence.

  
As shown in Table XI below, with the tabular grain emulsion layers carrying an epitaxial deposit, the removal of silver by fixing takes place more quickly than with the octahedral grain emulsion layers carrying a epitaxial deposition.

TABLE XI

  

  <EMI ID = 241.1>
 

CLAIMS

  
1 - Emulsion comprising tabular halide grains

  
silver and a dispersing medium, characterized by the following:

  
(1) at least 50% of the total projected area of

  
silver halide grains consists of tabular grains, the thickness of which is less

  
  <EMI ID = 242.1>

  
and the average shape index is at least 8: 1, the shape index being the ratio of the diameter

  
the thickness of the grain and the diameter of the grain being the diameter of a circle whose surface is equal to the projected surface of the grain,

  
(2) the tabular grains of silver halides are

  
bounded by parallel main faces

  
  <EMI ID = 243.1>

  
(3) tabular silver halide grains have

  
awareness sites whose orientation in relation to the grain is predetermined.


    

Claims (1)

2 - Emulsion according to claim 1, characterized in that the awareness-raising sites are oriented in a predetermined manner both between themselves and in relation to the grain. 3 - Emulsion according to either of Claims 1 or 2, characterized in that the sensitization sites are in the form of an epitaxial deposit of silver salt on the grains. 4 - Emulsion according to one of claims 1 to 3, characterized in that the tabular grains have an average shape index of at least 12: 1. 5 - Emulsion according to one of Claims 1 to 3, characterized in that the tabular grains have an average shape index of at least 20: 1. 6 - Emulsion according to one of Claims 1 to 5, characterized in that the tabular grains have an average thickness of less than 0.3 m. 7 - Emulsion according to one of Claims 1 to 6, characterized in that the dispersion medium is a peptizer. 8 - Emulsion according to Claim 7, characterized in that the peptizer is gelatin or a derivative of gelatin. 9 - Emulsion according to one of Claims 1 to 8, characterized in that the tabular grains represent at least 70% of the total projected surface of the silver halide grains. 10 - Emulsion according to one of Claims 1 to 9, characterized in that the tabular grains represent at least 90% of the total projected surface of the silver halide grains. 11 - Emulsion according to one of claims 1 to 10, characterized in that the tabular grains consist of bromide. 12 - Emulsion according to Claim 11, characterized in which the tabular grains further include iodide. 13 - Emulsion according to one of Claims 1 to 10, characterized in that the tabular grains comprise silver chloride. 14 - Emulsion according to one of Claims 3 to 13, characterized in that the silver salt is a silver halide. 15 - Emulsion according to Claim 14, characterized in that the silver salt comprises bromide. 6 - Emulsion according to claim 14, characterized in that the silver salt comprises chloride. 7 - Emulsion according to one of Claims 3 to 13, characterized in that the silver salt is silver thiocyanate. 18 - Emulsion according to one of Claims 1 to 17, characterized in that a site regulator is adsorbed on the tabular silver halide grains. 19 - Emulsion according to Claim 18, characterized in that the site regulator is a spectral sensitizing dye. 20 - Emulsion according to Claim 19, characterized in that the spectral sensitizing dye is adsorbed on the tabular grains in the form of aggregates. <EMI ID = 244.1> characterized in that at least the silver salt or the tabular grains contain a sensitivity modifier compound. 22 - Emulsion according to one of Claims 3 to 21, characterized in that the epitaxial deposit of silver salt is located on less than half of the surface of the main crystal faces. 23 - Emulsion according to Claim 22, characterized in that the epitaxial deposit of silver salt is located on less than 25% of the surface of the main crystal faces. 24 - Emulsion according to Claim 23, characterized in that the epitaxial deposit of silver salt is located on less than 10% of the surface of the main crystal faces. 25 - Emulsion according to one of Claims 3 to 24, characterized in that the location of the silver salt deposit is practically limited to the edges of the tabular grains. 26 - Emulsion according to one of Claims 3 to 24, characterized in that the location of the silver salt deposit is practically limited to one or more vertices of the tabular grains. 27 - Emulsion according to Claim 3, characterized by the following points: the tabular grains whose thickness is less  <EMI ID = 245.1> and the average aspect ratio is at least 12: 1, occupy at least 70% of the total projected area of the silver halide grains; the tabular grains are limited by opposite main crystalline hexagonal or triangular faces; at least one silver salt, chosen from silver halides and silver thiocyanate, is epitaxially deposited almost exclusively on specific surface sites of the tabular grains; a spectral sensitizing dye forming aggregates is adsorbed at least on the parts of the main faces free of epitaxial deposition of silver halide or silver thiocyanate. 28 - Emulsion according to Claim 27, characterized in that the quantity of spectral sensitizing dye is sufficient to produce a monomolecular layer over at least 15% of the surface of the tabular grains.  <EMI ID = 246.1> in that the quantity of spectral sensitizing dye is sufficient to produce a monomolecular layer on at least 70% of the surface of the tabular grains.  <EMI ID = 247.1> characterized in that the spectral sensitizing dye is a cyanine or a merocyanine forming aggregates.  <EMI ID = 248.1> in that the aggregate-forming cyanine contains at least one nucleus chosen from the group consisting of quinolinium, benzoxazolium, benzothiazolium, benzoselenazolium, benzimidazolium, naphthoxazolium nuclei, -7 naphthothiazolium and naphthoselenazolium. 32 - Emulsion according to claim 31, characterized " in that the spectral sensitizing dye is chosen from the group consisting of: anhydro-9-ethyl-3,3'-bis (3-sulfopro-  <EMI ID = 249.1> bis (3-sulfobutyl) thiacarbocyanine, anhydro-5,5 ', 6,6'-tetrachloro-1,1'diethyl-3,3'-bis (3-sulfobutyl) benzimidazolocarcocyanine hydroxide,  <EMI ID = 250.1> 33 - Emulsion according to Claim 3, characterized in that : the tabular grains of silver bromoiodide of which  <EMI ID = 251.1> less than 12: 1, occupy at least 70% of the total projected area of the silver halide grains, the tabular grains of silver bromoiodide are limited by opposite parallel main faces  <EMI ID = 252.1> silver genides and silver thiocyanate, is deposited epitaxially exclusively on specific surface sites of the tabular grains of silver bromoiodide. '7 34 - Emulsion according to Claim 33, characterized in that at least one epitaxial silver halide with a sensitivity-modifying compound incorporated is located exclusively on specific surface sites of the tabular grains of silver bromoiodide. 35 - Emulsion according to Claim 34, characterized in that the sensitivity-modifying compound generates / electron traps in the epitaxial silver halide. 36 - Emulsion according to either of Claims 34 or 35, characterized in that the sensitivity modifying compound is a noble metal from group VIII of the Periodic Table of the Elements. 37 - Emulsion according to one of Claims 33 to 36, characterized in that it is chemically sensitized with at least one sulfur, selenium or gold compound. 38 - Emulsion according to one of Claims 3 to 10, characterized in that the tabular grains of silver halide are grains of silver bromoiodide bounded by opposite main principal faces il 113, and in that an epitaxial deposit of silver thiocyanate is practically located exclusively on the edges or the tops of the tabular grains of silver bromoiodide.  <EMI ID = 253.1> characterized in that the tabular silver halide grains are silver bromoiodide grains bounded by opposite parallel main faces  <EMI ID = 254.1> of silver is practically located exclusively on the edges or the tops of the tabular grains of silver bromoiodide.  <EMI ID = 255.1> characterized in that: the tabular silver halide grains are silver bromoiodide grains limited by  <EMI ID = 256.1> an epitaxial deposit of silver bromide is practically located exclusively on the edges or tops of the tabular grains of silver bromoiodide, and a spectral sensitizing dye forming aggregates is adsorbed at least on the parts of the main faces free of epitaxial deposition of silver bromide. 41 - Emulsion according to one of Claims 3 to 10, characterized by the following points: tabular silver halide grains are silver bromoiodide grains bounded by  <EMI ID = 257.1> the iodide content in the central region of these grains is less than 5 mol%, and it is at least 8 mol% in a peripheral annular region, the central region of the main faces constitutes a preferential site for raising awareness, the epitaxial deposition of silver chloride is practically localized exclusively on preferential sites of sensitization of the tabular grains of silver bromoiodide. 42 - Emulsion according to Claim 41, characterized in that the annular region contains at least 12 mole% of iodide. 43 - Emulsion according to either of Claims 41 or 42, characterized in that a single crystal of silver chloride is deposited epitaxially on each main face. 44 - Emulsion according to one of Claims 41 to 43, characterized in that at least one dopant is incorporated 1 with silver chloride. 45 - Emulsion according to Claim 44, characterized in that the dopant is a noble metal from group VIII of the Periodic Table of the Elements. 46 - Emulsion according to claim 44, characterized in that the dopant is at least one compound of sulfur, selenium or gold. 47 - Emulsion according to one of Claims 3 to 10, characterized by the following points: ) the tabular silver halide grains are silver bromide grains bounded by faces  <EMI ID = 258.1> an epitaxial deposit of silver chloride is practically localized exclusively on the edges or  <EMI ID = 259.1> a spectral sensitizing dye forming aggregates is adsorbed at least on the parts of the main faces free of epitaxial deposition of silver chloride.  <EMI ID = 260.1> characterized in that: tabular silver halide grains are silver bromide grains bounded by faces  <EMI ID = 261.1> an epitaxial deposit of silver thiocyanate is practically localized exclusively on the edges of the tabular grains of silver bromide. 49 - Emulsion according to one of Claims 3 to 10, characterized in that: tabular silver halide grains are silver bromide grains bounded by faces  <EMI ID = 262.1> an epitaxial deposit of silver thiocyanate is practically localized on the edges or tops of the grains, and a spectral sensitizing dye forming aggregates is adsorbed at least on the parts of the main faces free of epitaxial deposit of silver thiocyanate 50 - Emulsion according to one of Claims 3 to 10, characterized in that the tabular silver halide grains are silver chloride grains bounded by opposite parallel main faces  <EMI ID = 263.1> of silver is practically located exclusively on the edges of the tabular grains of silver chloride. 51 - Emulsion according to one of Claims 3 to 10, characterized in that: the tabular silver halide grains are limited by opposite parallel main faces {111}, the silver halide constituting the tabular grains also forms non-tabular growths on one or more vertices of the tabular grains, and a spectral sensitizing dye forming aggregates is adsorbed on the main faces of the tabular grains. 52 - Emulsion according to Claim 51, characterized in that a dopant is incorporated into the tabular grains and / or the non-tabular growths.