US3660100A

US3660100A - Direct-print light-developable emulsion

Info

Publication number: US3660100A
Application number: US782789A
Authority: US
Inventors: Robert E Heeks; Donald P Sullivan
Original assignee: Xerox Corp
Current assignee: Xerox Corp
Priority date: 1967-05-25
Filing date: 1968-12-04
Publication date: 1972-05-02
Anticipated expiration: 1989-05-02
Also published as: GB1242163A; US3725073A; DE1908291A1

Abstract

A photographic emulsion of the silver chlorobromide direct-print light-developable type containing lead and thallium salts. Optimum results may be obtained by addition of gold and iodide salts and certain spectral sensitizing dyes.

Description

United States Patent Heeks et al.

1 1 May 2,1972

s4 DIRECT-PRINT LIGHT-DEVELOPABLE 3,447,927 3/1969 Bacon et a1. ..96/l08 x EMULSION OTHER PUBLICATIONS 721 t :RbertEHksPf' l- 1 men f zt ig nald P Valle, James E. Lu, sensitizing Binder- Free Silver Halide Photographic Media, col. 7573, Chemical Abstracts, Vol. 64, [73] Assignee: Xerox Corporation, Rochester, NY. 1966,

Sakai et al., Light- Developable Printing- Out Photo- Sensi- 22 F1 d. De .4, 1968 1 e c tive Materlal, Nlppon Shash1a Gakkai Kaisiu 3o 31, 164- 172 PP 782,789 (1967) (Japan), Para 64592f, Chemical Abstracts, Vol. 68, Related us. Application Data 1968- I f The 0.6. Defensive Publication Hi lson, De Pub]. Of Serial [63] $2 ;g 135 xgg No. 845,875, filed July 29, 1969, published in 8690.6.714 on December 16, 1969. [52] US Cl ..96/108 5 1 Primary Examiner-William D. Martin [58] Field of Search ..96/107, 108 Assismnt Examiner-M, R, Lusignan Attorney-James J Ralabate, Richard A. Tomlin and David C. [56] References Cited p n- UNITED STATES PATENTS 57] ABSTRACT 33 423;; A photographic emulsion of the silver chlorobromide direct- 23474 3/1964 B n 07 X print light-developable type containing lead and thallium salts. 3l78293 4/1965 96/108 Optimum results may be obtained by addition of gold and 3 5/1965 E ll 08 x iodide salts and certain spectral sensitizing dyes. 3,241,971 3/1966 Kitze ....96/108 X 3,260,605 7/1966 Sutherns ..96/108 3,287,136 1 1/1966 McBride .96/108 X 4 Claims, 1 ng Figure 3,419,396 12/1968 Chateau et al ..96 /108 I o 72 F+ 001.0 ,z I .25

I I z A D l J5 72F NO 001.0

MINUTES HELD PATENTEBmz 1922 & 72F NO GOLD |ooF GOLD 72F+GOLDZ// o I00 rc c 1 o 10 M MINUTES HELD INVIZNTOR.

ROBERT E. HEEKS DONALD P. SULLIVAN ATTORNEY DIRECT-PRINT LIGHT-DEVELOPABLE EMULSION This application is a continuation-in-part of copending applications Ser. No. 641,135 filed May 25, 1967 and Ser. No. 707,483 filed Feb. 23, 1968 and now abandoned in the US. Patent Office by Robert E. Heeks and Donald P. Sullivan.

BACKGROUND OF THE INVENTION This invention relates in general to photographic emulsions. More specifically, the invention concerns photographic emulsions of the direct-print light-developable" type.

The term direct-print light-developable refers to layers, generally deposited on a film or paper substrate, containing photosensitive materials upon which a visible image is obtained after an initial exposure to a pattern of high intensity light or other electromagnetic radiation has been utilized to form a latent imageythe visible image being obtained by additional overall exposure to generally lower intensity radiation. This additional exposure is known as latensification. The entire sheet surface is exposed including exposure of the background area and re-exposure of the area upon which the initial image was formed. Direct-print light-developable materials are distinguished from developing-out emulsions in that they do not require a wet development treatment, such'as a multi-step chemical development after initial exposure to produce a visible image. While the direct-print light-developable materials of this invention may be used for ordinary photographic purposes, they are particularly sensitive to a high intensity beam of light or other electromagnetic radiation and are, therefore, especially useful in high speed recording such as oscillographic recording and the like.

Direct-print light-developable type materials are today in general use for recording of oscillographic traces. However, the direct-print light-developable materials of the prior art are not entirely satisfactory for this recording purpose for a number of reasons. For example, it is generally true that the prior art materials which exhibit high sensitivity to an exposure of short duration to a high intensity beam tend to exhibit a correspondingly low degree of background stability. The latensification procedure for these materials is highly critical and accordingly is exceedingly difficult to carry out. The latent image may be entirely lost during the latensification operation where the background darkens too rapidly. In other cases, a phenomenon known as reversal may occur, this being a darkening of the background which proceeds at a more rapid rate than darkening of the image trace. In these situations, the trace may be temporarily or permanently obscured. Conversely, materials which exhibit a high degree of background stability tend to exhibit a correspondingly low degree of sensitivity to the recording beam and, thus, the minimum exposure time is far above that desired for use in modern recording instruments.

Attempts have been made to sensitize direct-print lightdevelopable materials in order to improve their imagingcharacteristics. For example, as described by Hunt in US. Pat. Nos. 3,033,678 and 3,033,682, stannous or plumbous salts may be added to silver halide direct-print light-developable emulsions to increase their sensitivity. While the materials of the above Hunt patents show improvement in terms of higher writing speed and stability, they are not entirely satisfactory for a number of reasons. For example, although their background stability is superior to other prior direct-print light-developable elements and will retain a readable record over a period of several hours under the illumination of normal room lighting, background density will in time develop to a point where the image record is not sufiiciently clear. The image density also has an undesirable tendency to regress. These materials also tend to be unstable when exposed to ultraviolet radiation or other radiation more intense than room lighting, as would be used in making copies by conventional methods, e.g., azo or electrophotographic copies.

Sensitized direct-print light-developable materials have also been described by Byme in US. Pat. No. 3,123,474 which described sensitizing a silver halide emulsion with a combination of a plumbous salt and a thiourea. While this material shows increased sensitivity, it is not as stable when exposed for prolonged times to daylight or normal room lighting as would be desirable. Upon prolonged exposure to ordinary illumination, the background darkens and the image regresses depending upon the viewing conditions which reduces the ratio between background and image density.

On occasion it is desirable to make a truly permanent record of an oscillographic trace. In this circumstance, it is desirable to develop the usual direct-print light-developable emulsioneither before or after latensification by chemical developing-out techniques. Many conventional direct-print light-developable emulsions require special developing materials and procedures. Preferably, a direct-print lightdevelopable emulsion used in an oscillographic recorder should be developable by the usual direct-print lightdevelopable techniques or alternatively, by means of a chemical developing-out out process.

Difi'erent direct-print light-developable materials produce image traces and background areas of varying colors. Typically, the image trace may be developed as a brown or bluegrey line. The background areas which would preferably remain white often acquire a uniform color, typically, pink, brown, or blue. Preferably, there should be good contrast between the trace and backgrounds both in density and in color to permit convenient and accurate examination and evaluation of the trace.

Thus,'there is a continuing need for improved direct-print light-developable materials having improved sensitivity, stability and appearance characteristics.

Although parent applications U.S. Ser. No. 641,135 filed May 25, 1967, and Ser. No. 707,483 filed Feb. 23, 1968 discloses improved direct-print light-developable emulsions, it has been found that the background areas of these emulsions may be further stabilized and the quality-of the final image may be further improved as described herein.

SUMMARY OF THE INVENTION It is, therefore, an object of this invention to provide a direct-print light-developable emulsion overcoming the above-noted disadvantages.

Another object of this invention is to provide a novel light sensitive composition.

Another object of this invention is to providea more sensitive dry process for the reproduction of images.

Still another object of this invention is to provide a photosensitive emulsion which may be developed either by direct-print light-developable techniques or by chemical developing-out techniques. I

Still another object of this invention is to provide animproved direct-print light-developable emulsion which possesses both a high degree of photosensitivity and a high degree of background stability under general latensification.

A still further object of this invention is to provide a directprint light-developable emulsion resistant to light degradation after imaging.

A further object of this invention is to provide a direct-print light-developable emulsion which is capable of producing images having improved contrast and color characteristics.

THe above objects and others are accomplished in accordance with this invention by providing a photographic direct-print light-developable emulsion comprising a photosensitive silver halide and gelatin containing lead and thallium salts as background stabilizers. Preferably, the emulsions are sensitizedwith gold and iodide salts.

Good results have-been obtained with from about 0.001 to 1.0 mole percent gold salt, about 0.01 to 25 mole percent iodide salt, about 0.05 to 15 percent lead salt and about 0.001 to 0.2 mole percent thallium salt based on the silver.

For the best balance of high contrast, high photosensitivity, low background density and good background color, the amounts of sensitizing agents and background stabilizers in per mole of silver in the layer) are: about 0.01 to 0.1 mole percent gold salt, about 0.4 to 10 mole percent iodide salt, about 0.5 to 3.0 mole percent lead salt and about 0.005 to 0.02 mole percent thallium salt. Optimum results have been obtained where the amounts of sensitizing and stabilizing agents in the direct-print light-developable emulsion (in mole percent per mole of silver in the layer) are: 1.4 mole percent iodide, 0.03 mole percent gold salt, 1.8 percent lead salt and 0.012 mole percent thallium salt. The sensitizers and stabilizers may be added in any suitable form. Typically they are added as water soluble salts. However this is not necessary. For example, some ingredients'may be added as solids or in other solvents such as acids, bases, or water miscible organic solvents.

As is further pointed out in the examples below, if no lead or thallium salts are added, background stability and A D are poor. The use of thallium alone provides some improvement. The use of lead alone also provides some improvement. However, there is a high degree of stabilizing provided by the combination of the lead and thallium salts which exceeds the additive effect of the lead and thallium salts. It appears that there is a synergistic effect produced by the use of both the lead salt and the thallium salt. It has also been found that addition of the gold enhances the final net density provided the material is kept in the dark for a short time between'the exposure and latensification steps. The amount of time the material should be kept in the dark after exposure to provide the best net density is a function of the temperature. Although the mechanism is not fully understood, the gold sensitized emulsions of this invention apparently undergo a reaction after having been exposed imagewise. Apparently higher temperatures increase the rate of this reaction. For example, it has been found that holding these emulsions in the dark at 7O" F. for about minutes provides an improvement in final image density equivalent to that obtained when the material is held in the dark for 15 seconds at a temperature of 100 F. It is possible that other direct-print light-developable materials will be discovered which respond similarly to being held in the dark.

It has been found that the greatly enhanced spectral response may be obtained with the above described emulsions through the incorporation of very small amounts of certain dyes. Excellent results have been obtained with from about 5 to about 60 mg. dye/g. mole of silver in the emulsion. Dyes which have been found to produce excellent results are 3- ethyl-thiazoline-3-ethylrhodanine-2'-methyl-dimethine merocyanine; 3-ethylthiazoline-3-(p-sulfophenyl)-rhodanine-2'- methylidimethine merocyanine; 3,4-diethylthiazoline-3-carboxyethyl rhodanine-2{-methyldimethine merocyanine; 1:3:3- trimethylindoline-dimethine merocyanine; 3-ethylthiozoline- 3-ethyl-2-thio-2:4 (3:5) oxaazoledione-dimethine merocyanine; 2,4,5',7'-tetraiodofluorescein disodium salt and mixtures thereof.

The emulsion layer may comprise any suitable conventional silver halide and binder. Best results have been obtained with silver chlorobromide emulsions; these are, therefore preferred. Gelatin is the conventional binder for photographic emulsions. However, any other suitable binder, such as polyvinylalcohol, may be used if desired. Ordinarily, this layer should be no greater than about 25 microns in thickness. For optimum results, the layers should contain about three grams of elemental silver per square meter of coated emulsion. The emulsion layer may contain additional ingredients to enhance, synergize or otherwise modify its characteristics, as desired. Typical silver bromide emulsions are described in Photographic Emulsions, E. .I. Wall, American Photographic Publishing Company, Boston (1929) and in US. Pat. No. 3,249,440.

The gold, iodide, thallium and lead ions incorporated in the direct-print light-developable emulsion may be introduced in the form of any suitable salt thereof. Chloroauric acid, potassium iodide, lead nitrate, and thallium nitrate have been found to be especially suitable forms. These soluble chloride, potassium, and nitrate ions do not interfere with the imaging process. Any other suitable salts may be used if desired.

, meshhausen and Grier Mark VII Sensitometer at 100 DESCRIPTION OF PREFERRED EMBODIMENTS The following examples further specifically define the present invention with respect to particular sensitizing and stabilizing agents and the amounts thereof used in the directprint light-developable emulsion of this invention. All percentages used are mole percent per mole of silver unless otherwise indicated. The examples below utilizing sensitizing and stabilizing agents within the ranges described above are intended to illustrate various preferred embodiments of the direct-print light-developable emulsion of the present invention.

A gelatino-silver halide emulsion for use in the following examples with different amounts of different sensitizers and stabilizers is prepared as follows: An aqueous solution of silver nitrate is added to an aqueous gelatin solution of potassium chloride in such a manner that there is about mole percent of chloride ion based on the silver nitrate. The resulting mixture is ripened for about 30 minutes at about 140 F. Potassium bromide in an amount sufficient to provide about mole percent based on silver is then added. All of the lead and thallium nitrate desired in the emulsion may be added along with the silver nitrate or all of the thallium nitrate alone may be added with the silver nitrate or part of the thallium nitrate and part of the lead nitrate may be added with the silver nitrate, the remainder of the lead nitrate and thallium nitrate being added to the remelt as described below. However, about 0.07 mole percent lead is present in the mix in all of the examples. Ripening is continued for an additional 10 minutes.

The emulsion is then chilled, set, shredded and washed to remove excess salts. This material is then melted at about 100 F., the sensitizers to be tested and the remainder of the lead nitrate, and/or thallium nitrate where applicable, are then added as is described in the specific examples below; the mixture is cooled to about 95 F. and coated onto a substrate to obtain a layer having about 3 grams of elemental silver per square meter.

Although all of the lead nitrate may be added to the mix as described above, it is preferred to add about 0.07 percent to the mix and about 1.76 percent to the remelt for optimum net density. It is preferred to add all of the thallium nitrate to the mix.

The direct-print light-developable emulsions thus produced in each example are tested by exposing in an Edgerton, Germicroseconds exposure time. The test strips are exposed through a [7 step wedge. After exposure, the test strips are latensified for about 10 minutes under General Electric Cool White Fluorescent lamps at an illuminational level of about 50 foot candles. Preferably, the exposed strips are kept in the dark for about 2 minutes before the latensification exposure.

EXAMPLE I A silver halide emulsion prepared as described above is tested without the addition of any of the sensitizers of this invention. No images are obtained.

EXAMPLE II to about F. and held at this temperature for about 5 minutes. A 1 percent solution of potassium iodide is added at this time until the desired amount of sensitizer has been added. The melt is then cooled to about 95 F. and coated as described above. Samples having different amounts of potassium iodide are tested as described above and the results are tabulated in Table I. As can be seen from Table I, a print-out effect was produced with this emulsion with fair images resulting. The best images are obtained with about 0.4 to about 10 mole percent potassium iodide. An optimum image is obtained with about 1.4 mole percent potassium iodide.

EXAMPLE II] An emulsion is prepared and tested as described above with the addition of from about 0.008 to about 0.4 mole percent chloroauric acid as the only sensitizer. The solution of chloroauric acid in water is added to the molten emulsion just before coating. The resulting test strips are tested as described above and the results shown in Table I. As can be seen from Table I, very little sensitization is obtained with the gold salt alone.

EXAMPLE IV In this experiment test emulsions are prepared using the 0ptimum amount of the iodide salt sensitizer (1.4 mole percent potassium iodide) with varying amounts of chloroauric acid. The test strips are prepared and tested as described above. As can be seen from the tabulation in Table I, the addition of the gold salt to the optimized iodide salt shows a dramatic increase in sensitivity and overall image quality. Clearly, the increase in quality is greater than additive since the gold had substantially no effect alone. Best results are obtained with from about 0.1 to about 1.0 mole percent gold salt. Optimum results are obtained with about 0.03 mole percent of the gold EXAMPLE VI EXAMPLE VII In this experiment test strips are prepared using the optimum amounts of the iodide salt (1.4 mole percent potassium iodide) lead salt (1.8 mole percent lead nitrate) and gold salt (0.03 mole percent chloroauric acid). Varying amounts of the thallium salt are added to the mix. As can be seen from Table I, there is a significant decrease in background. It is preferred that from about 0.005 mole percent to about 0.02 mole percent of the thallium salts be used. Optimum'results are obtained when about 1.8 mole percent lead salt and about 0.012

mole percent thallium salts are used. The test strips are held at salt. about 70 F. for 5 minutes.

TABLE I Sensitizer-stabilizer (mole percent per mole silver) Thallium Iodide Gold Lead salt salt Image Example salt salt (remelt) (in mix) Dam. Dmin AD rating II O 0. 39 0. 32 0. 07 1 0. 47 0. 36 0. 11 1 0. 52 0. 38 0. 14 2 0. 54 0. 40 0. 14 3 0. 54 0. 38 0. 16 4 0. 53 0. 36 0. 17 3 III 0 0 VI 1. 4 0.03 0.67 0. 35 0.30 8 l. 4 0.03 0.72 0.35 0.35 9 1. 4 0.03 0.71 0. 34 0.35 l) 1. 4 0.03 0. G5 0. 32 0.31 8 1. 4 0.03 0. 62 0. 30 0.30 7

VII 1. 4 0.03 1.8 0. 69 0.35 0. 34 fl 1. 4 0.03 1. 8 0. 69 0. 34 0.35 10 1.4 0.03 1.8 0.67 0.34 0. 33 10 1. 4 0.03 1.8 0.67 0. 33 0.34 10 1. 4 0.03 1. 8 0.034 0.55 0. 31 0.31 7

EXAMPLE V Additional test strips are prepared and tested using the optimum amounts of the iodide salt 1.4 mole percent potassium iodide) and gold salt (0.03 mole percent chloroauric acid) with varying amounts of a lead nitrate. The lead salt is added to the remelt. The strips are prepared and tested as described above. As can be seen from Table I, there is an increase in image quality with the addition of the lead salt. The improvement is mainly in image density. It is preferred that from about 0.5 to about 3.0 mole percent lead salt be used. Optimum results are obtained with about 1.8 mole percent lead salt.

As indicated by the above table, outstanding results are obtained with a direct-print light-developable emulsion sensitized with the sensitizing agents of the present invention. In the above table, columns 1 through 4 indicated the mole percents of gold salt, iodide salt, lead salt and thallium salt, respectively. The fifth and sixth columns give the maximum and minimum image density. The seventh column gives the density difference which indicates contrast between the image and background. The eighth column gives an arbitrary image quality rating. This rating varies from 0 to 10 with 0 indicating no image and 10 indicating an image of highest quality. Desirable characteristics which are considered in establishing a rating for a given image include rate of intensification high image density, low background density, image sharpness, apparent contrast between image and background and background color and uniformity. As can be seen from the table, the combination of optimal amounts of iodide salt and gold salt gives high image density and excellent contrast between image and background. The further addition of lead salt and thallium, salt further improves overall image quality, especially in the areas of net image density, sharpness and background suppression.

EXAMPLE IX A set of teststrips are prepared as described above, using sensitizers in amounts determined to be optimum. Thus, the emulsion includes about 1.4 mole percent potassium iodide, about 0.03 mole percent chloroauric acid, about 1.8 mole percent lead nitrate and about 0.012 mole percent thallium nitrate. Varying concentrations of several dyes are added to samples of the above emulsions. Each dye containing sample is coated onto a paper backing, exposed in an Edgerton, Germeshhausen and Grier Mark VII sensitometer at 100 microseconds exposure time. Separate samples are tested with filters giving individual exposure to white, blue, green and red light. After latensification, the density difference between image and background is measured witha densitometer. The results, compared to a control emulsion containing no dye,-are tabulated in Tablell below.

TABLE II trntion Al) sls- Dye mole Ag) White Blue Green Red None (control) .36 .32 00 3,4-diethylthiazo]inc-3,

carboxyethyl rhotlaniue l). 4 .37 31 13 00 2-metl1yldim0thine merocyanine 37. 6 39 31 26 00 3-ethylthiaz0line-3- ethylrhodauine 6.2 40 .32 12 00 2'methyldimethine erocyanine 24. 8 .40 33 28 00 3 -et hylt hiazoline-(Hthyl-Q- [bio-231325) 4.7 .39 .31 .02 00 0xaazoledlone-dimethine merocyanine 18.8 .39 .32 13 00 113:3-trimethylindoline- (limethine merocyanine 5. 8 .38 32 01 00 23. 2 38 32 17 00 2',4,5,7-tetraiod0fiu0rescein, disodium salt 15.6 39 31 00 62. 4 36 .25 05 00 As can be seen from Table II, the dyes extend the sensitivity of the emulsion into the green region of the spectrum.

A set of experiments are run using test strips prepared and exposed as described above. In the first experiment the sensitizers are added in the preferred amounts with the exception that the gold is omitted from the emulsion. The test strips are held in the dark at about room temperature for varying times. The results are shown in the Figure.

In the second set of experiments the above test is repeated except that the strips are held at a temperature of about 100 F. for varying times.

In the third set of experiments the first set of experiments are repeated except that gold is present in the optimum quantity.

1n the fourth set of experiments the gold sensitized emulsion is heated to a temperature of 100 F. and held for varying times.

The A D readings obtained are plotted against the time held for the four test runs described above in the Figure.

As can be seen from the Figure the gold sensitized emulsion provides increased A D when held for a time in the dark. It can also be seen that the time required to obtain a relatively improved A D can be reduced by the application of heat.

Although specific components and proportions have been described in the above examples, other suitable materials, as listed above, may be used with similar results. In addition, other materials may be added to the emulsion layer to synergize, enhance, or otherwise modify its properties. For example, sensitizing dyes, various finishing aids such as form aldehyde, surfactants, surface active agents such as saponin, humectants such as glycerin, stabilizers such as 7-hydroxy-5- methyl-1,3,4-triazoindolizine, matting agents, etc., may be used in the emulsion layer.

Other modifications and ramifications of the present invention will occur to those skilled in the art upon a reading of the present disclosure. These are intended to be included within the scope of this invention.

What is claimed is:

l. A photosensitive direct-print light-developable material comprising a photosensitive layer consisting essentially of a silver halide, a binder, about 0.05 to about 15 mole percent of a bivalent lead salt, about 0.001 to about 0.2 mole percent of a monovalent thallium salt, about 0.01 to about 25 mole percent of an iodide salt and from about 0.001 to about 1.0 mole percent of a trivalent gold salt all mole percents based on moles of silver.

2. A photosensitive direct-print light-developable material comprising a photosensitive layer consisting essentially of a silver halide, a binder, about 0.01 to about 0.1 mole percent of a trivalent gold salt, about 0.4 to about 10 mole percent iodide salt, about 0.5 to about 3.0 mole percent of a bivalent lead salt and about 0.005 to about 0.02 mole percent of a monovalent thallium salt, all mole percents based on moles of silver.

3. A photosensitive direct-print light-developable material comprising a photosensitive layer consisting essentially of a silver halide, a binder, about 1.4 mole percent of an iodide salt, about 0.03 mole percent of a trivalent gold salt, about 1.8 mole percent of a bivalent lead salt and about 0.005 to about 0.017 mole percent of a monovalent thallium salt all mole percents based on moles of silver.

4. The direct-print light-developable material of claim 1 wherein there is included a sensitizing amount of a dye selected from the group consisting of 3-ethylthiazoline-3- ethylrhodaine-2'-methyldimethine merocyanine; 3-

' ethylthiazoline-3-(p-sulfophenyl)-rhodanine-2'-

Claims

2. A photosensitive direct-print light-developable material comprising a photosensitive layer consisting essentially of a silver halide, a binder, about 0.01 to about 0.1 mole percent of a trivalent gold salt, about 0.4 to about 10 mole percent iodide salt, about 0.5 to about 3.0 mole percent of a bivalent lead salt and about 0.005 to about 0.02 mole percent of a monovalent thallium salt, all mole percents based on moles of silver.
3. A photosensitive direct-print light-developable material comprising a photosensitive layer consisting essentially of a silver halide, a binder, about 1.4 mole percent of an iodide salt, about 0.03 mole percent of a trivalent gold salt, about 1.8 mole percent of a bivalent lead salt and about 0.005 to about 0.017 mole percent of a monovalent thallium salt all mole percents based on moles of silver.
4. The direct-print light-developable material of claim 1 wherein there is included a sensitizing amount of a dye selected from the group consisting of 3-ethylthiazoline-3-ethylrhOdaine-2''-methyldimethine merocyanine; 3-ethylthiazoline-3-(p-sulfophenyl)-rhodanine-2''-methylidimethine merocyanine; 3,4-diethylthiazoline-3-carboxyethyl rhodanine-2''-methyldimethine merocyanine; 1:3:3-trimethylindoline-dimethine merocyanine; 3-ethylthiazoline-3-ethyl-2-thio-2:4(3:5) oxaazoledione-dimethine merocyanine; 2'' ,4'' ,5'' ,7''-tetraiodofluorescein disodium salt and mixtures thereof.