US2691585A - Prescreened posterizing photographic film - Google Patents

Description

Patented Oct. 12, 1954 PRESCREEN ED POSTERIZING PHOTOGRAPHIC FILM John A. C. Yule and Richard E. Maurer, Rochester, N. Y., assignors to Eastman Kodak Company, Rochester, N. Y., a corporation of New Jersey Application June 4, 1952, Serial No. 291,622

6 Claims. 1

This application is a continuation-in-part of our application Serial No. 168,718, filed June 17, 1950, and now abandoned, having to do with prescreened photographic material.

The object of the present invention is to provide a prescreened photographic material suitable for posterizing.

Posterizing is here used to refer to halftone reproductions in which only a few tones appear in the final print. In its simplest form a posterized print is a halftone print having a highlight tone, one or two middle tones, and a shadow tone. Modifications of this simplest form of posterizing and various other forms, having special features which are in some cases desirable and in other cases unavoidable, are all discussed later in this specification in connection with the accompanying drawing. However, the basic features of the invention can be readily understood merely with reference to the simplest form of posterizing.

In cofiled applications, Serial Nos. 291,621 and 291,623, which are also continuations-in-part of Serial No. 168,718, we discussed various prescreened photographic materials in which desensitization is distributed throughout an emulsion layer in a halftone pattern of undulating sensitivity. A sharp or two-tone pattern of sensitivit would not give the regular halftone effect. Posterizing, on the other hand, does not use either a two-tone reproduction system or a halftone system in which all intermediate tones are reproduced. In posterizing the reproduction employs halftone dots or the like but only in 3, 4, or 5 sizes and the difference between the various sizes results in a sharp break in density from one tone to the next. Thus, in spite of the fact that prescreened material according to the present invention does not have undulating sensitivity, it is able to give a type of halftone reproduction, namely, posterizing,

Posterizin has advantages over other types of prescreening and posterizing by prescreening has advantages over other posterizing systems. The advantage of posterizing over other types of prescreening lies in the fact that the stability of th prescreened material is not so critical since slight changes in the sensitivity do not appreciably affect the usefulness of posterizing film. That is, the tone reproduction curve is stylized anyway and if only three or four tones are included in the reproduction, the actual value of the individual tones is not very critical. Other systems of postenzing include the making of separate exposures for the individual tones and sometimes the use of masking arrangements, all of which are more complicated than posterizing processes using prescreened material according to the present invention.

According to the invention a film is made up with a silver halide emulsion layer on a support with desensitization distributed throughout the layer in a uniform posterizing halftone pattern with at least three and not more than eight levels of different, substantially uniform sensitivity. In practice the individual levels do not have precisely uniform sensitivity, or at least the effect is not uniform since infectious development at the edge of each level tends to introduce some contrast or gradient within the level and thus to interfere with the sharpness of the break between successive levels. To conform with actual experience, the term substantially uniform sensitivity is here used to refer to a level of sensitivity in which the variation in sensitivity is less than of the variation between the sensitivity of successive levels. The effect of this residual gradient is to introduce slight variations in tone within the fixed tones of the posterizing system, but the variations are small and have no appreciable effect on the appearance of th final posterized print.

In the posterized prescreened sheet according to the invention the areas of minimum desensitization preferably have no desensitization or, in some embodiments, are hypersensitized by preexposure. One simple form of the invention has three levels of sensitivity, the lowest level being Clayden desensitized, the highest level being hypersensitized, and the intermediate level being produced by those areas or" the film which have been neither desensitized nor hypersensitized, or in which the effects balance out each other.

The invention will be further understood from the following description when read in connection with the accompanying drawings in which:

Figs. 1-5 illustrate various forms of contact halftone screens which may be used eithe for posterizing or, according to the present invention, for producing prescreened posterizing film.

Figs. 3A to BE, inclusive, illustrate the opera tion of the screen shown in Fig. 3.

Fig, 6 illustrates schematically in section, one method of preparing prescreened posterizing film.

Figs. 7-10, inclusive, similarly illustrate various embodiment of the invention prepared as shown in Fig. 6.

Fig. 11 similarly illustrates another method of making a prescreened posterizing film.

Fig. 12 illustrates the film made according to Fig. 11.

The present invention has to do with the proat right angles.

posure since it lay behind the lines of the screen ceived the sum of these two exposures.

duction of a prescreened film for posterizing, but a number of features thereof are perhaps better understood if posterizing is first discussed in terms of contact screens. Incidentally, posterizing using contact screens has many advantages over the more .usual methods employing separate exposures or other techniques for getting the individual tones. Figs. 1-5 will, therefore, first be discussed as they relate to posterizing using contact halftone screens.

Fig. 1 illustrates a greatly enlarged section of a parallel line contact screen and Figs. 2-5 each illustrate two or four elements of a contact halftone screen having a finite number of density levels. The screen shown in Fig. l was prepared by exposure of a photographic film or plate through a parallel-line screen. The areas Ill which lay behind the lines of the screen have received little or no exposure whereas the areas H have received a uniform exposure resulting in a 'greytone' (or resulting in a color if a color process is used). Such a contact screen has two tones, neither of which is opaque. Accordingly, it can produce three tone levels in the reproduction. A very light tone inan original'subject being copied will 'expose'through both areas [6 and H. Intermediate'tones will expose through area In but not' through area II and very dense tones or shadows in the original will not expose through either area.

In Fig. 2 the contact screen was prepared in the same manner as in Fig. 1 but two successive exposures were given with'the parallel-line screen The area I2 has received no exin both positions. The areas [3 and I l have re ceived equal exposures but have received only one exposure each. The area [5 has received the sum of these two'exposures since it lay behind clear areas of the parallel-line screen during both exposures. Fig. 3 is a more general case of Fig. 2 in which the two successive exposures are different. Again area I2 has received no exposure. Area [4 has received one exposure. Area l6 has received a different exposure and area i! has re- Thus the halftone screen shown in Fig. 3 has four density levels and can produce five tones in the posterized reproduction as is illustrated in Figs. 3A to 3E. The shadows of the original do not expose film through any of the areas and hence give a minimum or clear dotshown in Fig. 3A. Less dense areas expose the film through area 42 giving a small dot shown in Fig.'3B and lighter tones in turn giving larger dots represented by 30 and 3D andfinally a solid dot represented by 3E.

Fig. 4 carries this same principle further to form an eight-tone contact screen by making three successive exposures through a parallelline screen with the orientation of the screen changing through d5" between exposures.

Fig. 5 represents a color screen such as those which have been'used for many years'in additive color processes. In the particular examples shown the areas 20, 2!, '22, and 23 are difierent colors, but the-actual form of the color screen may be practically any of those which have been proposed for additive color processes of the screen type. The use of such a screen for a posterizing contact "screen requires the exposures for the various tones to be made by difierent colored lights. A black and white original illuminated by-yellow. light, for example, will give exposure through a primary blue filter only in the very brightest-areas of theoriginal and will expose the less bright areas only through the green and red sections of the screen, the deep shadows only through the red section of the screen, and finally the deepest shadows will not cause any exposure at all.

All five of :these forms of contact screens may be used according to the present invention for the preparation of prescreened posterizing film.

In Fig. 6 a posterizing contact screen consisting of :a transparent support 25 and a stepped density halftone image 26 is placed in contact with a silver halideemulsion layer 28 which is to be prescreened. The latter emulsion layer is carried on -a transparent film layer 29 or on glass. It could be on paper or metal if the material is to be used for printing by litho processes and the like'in'known manner. The steps in the screen 26 are-shown as sharp steps and this is in general representative of the density distribution but, of course, the stepped line of demarcation does not necessarily follow any actual boundary line within the screen itself. The density may actually be screened material is represented by emulsion 28 having its sensitivity'indicated by the stepped line 3! in which the point 32 has received the maximum desensitization. The point 34 is represented by a horizontal line to indicate a uniform degree of desensitization. In actual practice this sensitivity in each level is not absolutely uniform or, in any case, is not effectively uniform in the end result. Therefore, the individual levels are shown .slightly sloped as at the point 36 in Fig. 8. These second order effects result in a variation in sensitivity within any one'level of said sensitivity but the variation is less than the difierence of sensitivity between successive levels of sensitivity. On theother hand, the effect may be partly overcome by printing through a master screen whose density levels slope so as to compensate for the effect of infectious development and thus to give effectively level sensitivity.

Also, in practice the desensitizing exposure does not result in complete elimination of sensitivity and there may be some slight desensitization even at those points which are not intended to be desensitized. That is, as illustrated in Fig. 9, the points still have some sensitivity although they have received the maximum Clayden desensitization and the points 3'5 have slightly less sensitivity than the original emulsion 28 had. It is possible, however, to prevent any effective desensitization in the areas which are to receive no desensitization, i. e., to keep the sensitivity at the points 31 at a maximum.

According to the preferred embodiment of the invention, as illustrated in Fig. 10, the desensitizationas represented by the stepped curve 42 takes care of certain levels of the sensitivity, but the points of highest sensitivity, i. e., the centers of the ultimate dots, are provided by hypersensitization represented by the broken lines 4| extending above the emulsion layer 28. This is purely a schematic illustration but it does express the situation most clearly.- This hyper- .sensitization-may,be-obtained, for example, by employing-a screen 26 which the dense areas are of one color and the other areas are of a diiferent color. The desensitizing exposure is then made with a color which is absorbed by the dense areas of the contact screen and the hypersensitizing exposure is made with a complementary color highly transmitted by these same areas of the screen. The hypersensitizing exposure is of relatively long duration anl low intensity as discussed in our cofiled applications referred to above.

Fig. 11 shows another simple method of manufacturing prescreened material according to a preferred form of the invention. In Fig. 11 a color screen comprising a transparent support 55] and a large number of tiny filter areas 5|, 52, and 53 is placed in contact with a sensitive silver halide emulsion layer 6| carried on a support 52. This color screen may be of any of the types used in additive color photography such as starch grain screens, ruled screens, etc., or may be made in the same manner as these screens but of different colors. The emulsion 6! receives a desensitizing exposure from a high intensity short duration flash lamp 55 through a color filter 5'1. The color of the filter 5'! is such that the light is transmitted fully by filter dots or spots 5|, for example, to desensitize the emulsion 6| adjacent to these filter spots. In the embodiment shown the light from the filter 51 is not transmitted by either of the areas 52 or 53. For example, the filter 51 may pe a primary blue filter and the areas 52 and 53 may be respectively primary green and primary red or black. Alternatively, the filter 51 may be such that it transmits light which is fully transmitted by one type of filter dot and partly transmitted by another type so that two levels of desensitization are produced in the emulsion layer 51 A second exposure is given before, after, or simultaneously with the flash exposure, by a tungsten lamp 54 through a color filter 55. In the examples shown the filter 55 is such that the light is transmitted only by areas 52 and is absorbed fully by areas 5| and 53. This is a hypersensitizing exposure. The effect of the two exposures is illustrated in Fig. 12 in which the desensitization behind filter dots 5| is represented by the lines 63 and the hypersensitization is represented by the broken lines 64 arranged schematically to extend above the emulsion surface r' 55 which is taken to represent normal sensitivity of the emulsion 6|. The areas behind the filters 53 have not received light from either the desensitizing lamp 56 or the hypersensitizing lamp 54 and hence the sensitivity is unchanged in these areas. Actually color filters are not perfectly sharp cutting in their absorption, but the e1nulsion behind the areas 53 does not change much in sensitivity since the desensitizing and hypersensitizing exposures tend to counteract one another.

We claim:

1. A photographic material for posterizing comprising a support and a uniformly thick photosensitive silver halide emulsion layer on the support with Clayden desensitization distributed throughout the layer in a uniform posterizing halftone pattern with at least three and not more than eight levels of different substantially uniform sensitivities, varying in any one level less than one third the difference between the sensitivities of successive levels.

2. A material according to claim 1 in which the areas of minimum desensitization have substantially no desensitization.

3. A photographic material for posterizing comprising a support and a uniformly thick photosensitive silver halide emulsion layer on the support with Clayden desensitization and hypersensitization distributed throughout the layer in a uniform posterizing halitone pattern with three levels of different substantially uniform sensitivities, the areas of maximum sensitivity being hypersensitized and the areas of minimum sensitivity being Clayden desensitized and the areas of intermediate sensitivity being neither hypersensitized nor desensitized.

4. A photographic material for posterizing comprising a support and a uniformly thick photosensitive silver halide emulsionlayer on the support with Clayden desensitization distributed throughout the layer in a uniform posterizing halftone pattern with at least three and not more than eight levels of different substantially uniform sensitivities.

5. A material according to claim 4 in which the areas of minimum desensitization have substantially no desensitization.

6. A material according to claim 4 in which the areas of maximum sensitivity are hypersensitized by exposure.

References Cited in the file of this patent UNITED STATES PATENTS- Number Name Date 2,102,021 Murray Dec. 14, 1937 FOREIGN PATENTS Number Country Date 250,770 Great Britain Apr. 22, 1926 937,885 France Mar. 22, 1948

Claims (1)

1. A PHOTOGRAPHIC MATERIAL FOR POSTERIZING COMPRISING A SUPPORT AND A UNIFORMLY THICK PHOTOSENSITIVE SILVER HALIDE EMULSION LAYER ON THE SUPPORT WITH CLAYDEN DESENSITIZATION DISTRIBUTED THROUGHOUT THE LAYER IN A UNIFORM POSTERIZING HALFTONE PATTERN WITH AT LEAST THREE AND NOT MORE THAN EIGHT LEVELS OF DIFFERENT SUBSTANTIALLY UNIFORM SENSITIVITIES, VARYING IN ANY ONE LEVEL LESS THEN ONE THIRD THE DIFFERNECE BETWEEN THE SENSITIVITIES OF SUCCESSIVE LEVELS.

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