US 3894871 A
Diffusion transfer photographic products and processes are disclosed for forming transparencies wherein a positive silver transfer image is maintained together with a negative silver image in a separate layer, said images being viewed together as a positive transparency. The invention is particularly applicable to forming additive color transfer images. The sum of the projected areas of the silver halide grains in the silver halide emulsion layer does not exceed about 50% of the surface area of the silver halide emulsion layer. Development of the exposed silver halide grains results in little, if any, increase in the projected area whereby the maximum negative silver density is kept low. Predominantly homogeneous grain size silver halide emulsions, preferably of particular mean diameter and grain size distribution, are utilized to obtain good sensitometry in addition to desired minimum and maximum image densities.
Description (OCR text may contain errors)
United States Patent [1 1 Land PI-IOTOGRAPHIC PRODUCTS AND PROCESSES FOR FORMING SILVER AND ADDITIVE COLOR TRANSPARENCIES Inventor:
Edwin H. Land, Cambridge, Mass. Polaroid Corporation, Cambridge,
July 27, 1973 U.S. Cl 96/3; 96/25; 96/29 R;
Int. Cl. G03c 7/00; GO3c 5/54; G030 7/04;
Field of Search 96/3, 25, 29 R, 76 R, 80
References Cited UNITED STATES PATENTS 4/1952 10/1971 lO/l97l 1l/1972 [4 1 July 15, 1975 Primary Examiner-Norman G. Torchin Assistant ExaminerRichard L. Schilling Attorney, Agent, or FirmStanley H. Mervis l l ABSTRACT Diffusion transfer photographic products and processes are disclosed for forming transparencies wherein a positive silver transfer image is maintained together with a negative silver image in a separate layer, said images being viewed together as a positive transparency. The invention is particularly applicable to forming additive color transfer images. The sum of the projected areas of. the silver halide grains in the silver halide emulsion layer does not exceed about 50% of the surface area of the silver halide emulsion layer. Development of the exposed silver halide grains results in little, if any, increase in the projected area whereby the maximum negative silver density is kept low. Predominantly homogeneous grain size silver halide emulsions, preferably of particular mean diameter and grain size distribution, are utilized to obtain good sensitometry in addition to desired minimum and maximum image densities.
65 Claims, 17 Drawing Figures SHEET A/SILVER HALIDE EMULSION LAYER IMAGE-RECENING LAYER B -ADD\TIVE COLOR SCREEN F-TRANSPARENT SUPPORT LAYER OF PROCESSING GGJPOSlTION FIG 6 P I LEXPOSED sILvER RALIDE EMuLsIoN LAYER CONTAINING LATENT RED REcoRo 30a I4- IMAeEREcEIvI-e LAYER G B '3 R ADDITIVE COLOR SCREEN STAGE \Q SUPPORT PRocEssms DEVELOPED sILvER HALIDE EMuLsIoN 6b LAYER CONTAINTNG NEGATIVE sILvER I IMAGE OF REo RECORD I R I IMAGE-RECEIVING LAYE com-mums 30b POSITIVE sILvER IMAGE 0F GREEN AND I2 6 B BLUE REcoRo STAGE c 7 FINAL ADDITIVE C 0 L0 R TRANSPARENCY ADDITIVE mLOR SCREEN J TRANSPARENT SUPPORT PATENTEUJUL 1 5 1975 3894.871 SHEET 2 PATENTED JUL 'I 5 I975 SHEET FIG. 5
ETE? um 5 ms WTW NUMBER OF GRAINS RELATIVE NUUBER OF GRAINS DIAMETER IN MICRONS (LOG SCALE) F I 6 8a DIAMETER IN MICRONS (LOG SCALE) FIG. sq
RELATIVE NUMBER OF GRAINS DIAMETER m MICRONS (LOG SCALE) FIG. 8c
Q DIAMETER IN MICRONS SCALE W" "mun. 5 was ShEET 6 N r s r..- E we; 0 M R E 8 GR AD L n n :m A t m .D 2 12 2 a 2 B 2 wwwwmww lmmaawazJ i523 zoawimz h LOG E FIG. l2
SHEET BLUE -- GREEN RED a2 20 lja'aisafa'nz LOG E FIG. l4
1 PHOTOGRAPHIC PRODUCTS AND PROCESSES FOR FORMING SILVER AND ADDITIVE COLOR TRANSPARENCIES tent but this is generally unobjectionable, particularly for projection purposes, due to the considerable difference in density between the positive and negative images.
This invention is concerned with diffusion transfer 5 and U.S. Pat. No. 2,992,103 states:
photography and, more particularly, with diffusion transfer films and processes adapted to provide positive silver transfer images which may be viewed as positive transparencies without being separated from the developed negative silver image. The diffusion transfer films and processes provided by this invention are particularly adapted for use in forming additive color projection positive images.
The formation of positive images in silver by diffusion transfer processes is, of course, well known. In such processes, a photosensitive silver halide emulsion is exposed to provide a latent image in terms of exposed silver halide grains. This latent image is developed to form, in the layer containing the silver halide grains, an image in silver which image is a negative image of the subject photographed. Silver halide grains which were not exposed during the photoexposure are dissolved by a silver halide solvent and transferred by diffusion to a superposed silver receptive or image-receiving layer where the transferred silver ions are precipitated, e.g., deposited as metallic silver, to provide an image in silver which image is a positive image of the subject photographed. ln commercially available diffusion transfer films, the image-receiving layer containing the positive silver transfer image is physically separated from the developed silver halide emulsion layer containing the negative silver image.
It has been recognized that silver diffusion transfer processing may be utilized to provide images in color in accordance with the principles of additive color photography. In such application, the silver halide emulsion is exposed through an additive color screen and the resultant positive silver transfer image is viewed through an appropriately registered additive color screen. In the most useful embodiments, the same additive color screen is used in both exposure and viewing.
U.S. Pat. No. 2,6l4,926 issued Oct. 21, 1952, U.S. Pat. No. 2,707,150 issued Apr. 26, 1955, U.S. Pat. No. 2,726,l54 issued Dec. 6, 1955 and U.S. Pat. No. 2,944,894 issued July 12, 1960, all in the name of Edwin H. Land, and U.S. Pat. No. 2,992,103 issued July 1 1, l96l in the name of Edwin H. Land and Otto E. Wolff, disclose diffusion transfer additive color processes wherein a silver transfer image is viewed in registration with the additive color screen through which photoexposure was effected. The developed photosensitive layer is removed to permit viewing the resulting additive color transparency. Two of the above mentioned patents, however, note the possibility of viewing such an additive color transparency without removing the developed silver halide emulsion layer. Thus, U.S. Pat. No. 2,726,154 states:
It has been discovered in carrying out a silver halide photographic transfer process that the density of the positive image produced is much greater than the density of the negative. This intensification in the density of the positive image has been found to be of the order of 5 to 6 times and it is because of this that it is possible to allow the negative and the positive images to remain in contact with each other. Of course, under these circumstances, the highlights of the positive will be grayed to some ex- Although not as preferred, the invention contemplates indefinite maintenance of the sandwich structure for viewing purposes following the form ation of negative and positive images therein. The copending application of Edwin H. Land, Ser. No. 265,4 l 3, filed Jan. 8, 1952, and also the copending application of Edwin H. Land, Ser. No. 466,889, filed Nov. 4, 1954, disclose the formation of transfer-reversal images which possess a density of an order of 5 or 6 times greater than that possessed by -thesilver image developed in the photosensitive layer. As taught in the justmentioned applications, the high covering power of the silver of the reversal image may be utilized to avoid separation of a sandwich type structure. Although the highlights of the reversal image will be grayed to some extent under conditions wherein the photosensitive and print-receiving layers are maintained in superposed relation, i.e., in a sandwich structure, the result is generally unobjectionable, particularly when the image-bearing product is to be projected. The cited Serv No. 265,413 issued as the above-noted U.S. Pat. No. 2,726,l54. The cited Ser. No. 466,889 issued Nov. 28, 1958 as U.S. Pat. No. 2,861,885, which states:
It is apparent that the minimum density of the composite print depends, to a substantial extent, upon the maximum density of the negative since the shadows of the negative correspond to the highlights of the positive. If the above-noted ratio of positive silver covering power to negative silver covering power is realized in a composite print to be viewed by reflection, this maximum negative density can be as great as 0.3 without seriously affecting the composite image quality. A substantially higher maximum density is tolerable in the negative when the composite print is used as a transparency because the brightness of the highlights of the composite print is a function of the intensity of illumination. lt has been found that a maximum density of as high as 1.0 in the negative is permissible if the maximum density of the composite print is at least 4 times greater. Preferably, then, in a composite image of the foregoing type, the silver halide stratum, when fully developed in any conventional manner, has no greater density than approximately 0.3 if the composite print is to present a reflection image, and has no greater density than approximately 1.0 if the composite print is to serve as a transparency.
Other diffusion transfer processes providing positive silver transfer images viewable without separation from the developed negative image include U.S. Pat. No. 3,536,488 issued Oct. 27, 1970 to Edwin H. Land and U.S. Pat. No. 3,615,428 issued Oct. 26, 1971 to Lucretia .I. Weed. ln U.S. Pat. No. 3,536,488, the positive silver transfer image and the developed negative image are in the same layer, the silver halide emulsion layer including a silver precipitating agent. Placement of the silver precipitating agent in the silver halide emulsion layer was effective to keep the developed negative density low by limiting the physical expansion of exposed silver halide grains upon development. in US. Pat. No. 3,615,428, two positive silver transfer images are formed, one on each side of the silver halide emulsion layers. While the positive silver transfer images formed in accordance with these techniques possess relative minimum and maximum densities having the density differences desirable for use as positive transparencies without requiring removal of the developed negative image, such processes suffer from ineffective use of the positive image silver.
The above review shows that the art has recognized on one hand the possibility of retaining the developed silver halide emulsion layer as part of the final additive color transparency, while on the other hand the art has recognized the density of the developed negative image as a deterrent making such an embodiment unattractive. In addition, as illustrated by the last-mentioned US. patents, the positive silver image frequently has not utilized the silver in the most effective form, i.e., the positive image silver was not in a particularly compact form, and this has made it very difficult to obtain transparencies with highly saturated colors, even though a maximum density of 3.0 was obtained for the positive silver image. The resultant additive color images frequently appeared to have grey" highlights due to undesirable high negative density, and degraded or unsaturated colors due to inefficient use of the positive silver, and use of higher intensity light projection to offset the grey highlights would only reduce color saturation further. On the other hand, the alternative of removing the developed silver halide emulsions to avoid these deficiencies introduces other problems, and diffusion transfer additive color processes have yet to be commercially adopted.
The most efficient use of silver is a very compact silver deposit in a thin image-receiving layer separate from the silver halide emulsion layer. It is a basic feature of the present invention that the positive silver transfer image is formed of very compactly deposited silver, and that such compact positive silver is obtained without sacrificing the desired low negative silver image density, thereby obtaining diffusion transfer additive color images having both excellent highlights and excellent color resolution and saturation.
The present invention is concerned with providing silver diffusion transfer films and processes which provide high quality positive transfer images viewable by transmitted light without requiring separation therefrom of the developed negative silver image. The diffusion transfer films and processes of this invention are uniquely suited for use, in combination with an appropriate optical screen, in providing superior additive color transparencies, and the invention will be described in more particularity in connection with the additive color application thereof.
Accordingly, it is a principal object of the present invention to provide novel diffusion transfer films and process useful in the formation of transfer images, especially additive color transparencies including a positive silver transfer image.
A further object of this invention is to provide novel photosensitive elements which include a silver halide emulsion layer and a silver receptive layer, the silver halide emulsion layer containing silver halide grains of a quantity and character uniquely useful in providing low covering power developed negative images and high covering power positive silver transfer images,
thus permitting said images to be retained together for viewing as a positive image.
Yet another object of this invention is to provide novel diffusion transfer additive color photosensitive elements wherein the silver halide emulsion is predomi' nantly homogeneous in grain size and which provides a characteristic curve, i.e., photographic response independent of the grain size, said grain size characteristics being uniquely adapted to provide highly efiective utilization of silver and to satisfy the minimum and maximum density and other requirements of a high quality color image of the type where the positive and negative images are in separate layers and are maintained together as part of a permanent laminate.
Another object of this invention is to provide diffusion transfer additive color transparency films wherein the grain size characteristics of the silver halide emulsion are related in a unique manner to the dimensions of the color screen filter elements.
Still another object of this invention is to provide diffusion transfer additive color transparencies possessing large dynamic ranges.
Yet another object of this invention is to provide diffusion transfer additive color films and processes utilizing substituted-halide mixed silver halide emulsions having grain size distributions and characteristics adapted to provide superior additive color transparen- A further object of this invention is to provide diffusion transfer processes wherein a silver halide emulsion layer containing silver halide grains of a particular character and silver coverage is developed to provide a negative image having a maximum transmission density not greater than about 0.3. and the development of said silver halide is utilized to provide a positive transfer image in a separate layer, said negative and positive images being viewable together as a positive image without separation or an intermediate masking layer.
Other objects of the invention will in part be obvious and will in part appear hereinafter.
The invention accordingly comprises the products possessing the features, properties and relation of elements, and the processes including the steps and relation of the steps with respect to each other, which are exemplified in the following detailed disclosure, and the scope of the application of which will be indicated in the claims.
For a fuller understanding of the nature and objects of this invention, reference should be had to the following detailed description taken in connection with the accompanying drawings wherein:
FIG. 1 is a diagrammatic enlarged cross-sectional view of a diffusion transfer additive color photosensitive element embodying the present invention during the three illustrated stages of the formation of an additive color transparency by silver diffusion transfer processing, i.e., photoexposure, processing and final image;
FIG. 2 reproduces an optical photomicrograph at l,000 magnification of a transmission view through an unexposed diffusion transfer additive color film embodying the present invention;
FIG. 3 reproduces an optical photograph at l,OOO magnification of a transmission view through the diffusion transfer additive color film shown in FIG. 2 following exposure (maximum) to red light and diffusion transfer processing;
FIG. 4 reproduces an electron micrograph at l0,000 magnification of a portion of the diffusion transfer additive color film shown in FIG. 2 following maximum exposure to green light and an intermediate level exposure to blue and red light and diffusion transfer processing;
FIG. 5 reproduces an electron micrograph at I0,000X magnification of replicas of undeveloped silver iodobromide grains ofa silver halide emulsion used in a commercial silver diffusion transfer process;
FIG. 6 reproduces an electron micrograph at 10,000X magnification of replicas of undeveloped silver iodobromide grains of another silver halide emulsion used in another commercial silver diffusion transfer process;
FIG. 7 reproduces an electron micrograph at 10,000X magnification of replicas of undeveloped silver iodochlorobromide grains of a predominantly homogenous grain size substituted-halide silver halide emulsion particularly useful in films embodying the present invention, the preparation of which emulsion is described in Example 1;
FIG. 8a reproduces a graph of the grain sizefrequency distribution of the substituted-halide silver halide emulsion of FIG. 7;
FIGS. 8b, 8c and 8d reproduce graphs of the grain size-frequency distribution of other silver halide emulsions useful in certain embodiments of this invention;
FIG. 9 reproduces an electron micrograph at l0,000X magnification of a transmission view through an unprocessed silver halide layer containing the predominantly homogeneous silver halide emulsion of FIG. 7 coated at a silver coverage found to be particularly useful in the practice of this invention;
FIG. I0 reproduces an electron micrograph at I0,000 magnification of a transmission view through the silver halide layer shown in FIG. 9 following expo sure (maximum) and development;
FIG. 11 reproduces a graph of the projected area of a monolayer of silver halide grains as a function of their diameter at a constant silver coverage; and
FIGS. 12, 13 and 14 reproduce characteristic curves of the red, green and blue densities of the neutral column of additive color transparencies obtained in accordance with certain of the examples.
As noted above, the present invention is concerned with diffusion transfer processes and is directed towards providing photographic films and processes to provide a diffusion transfer positive silver image of high maximum density and a negative silver image of low maximum density, said images being viewable together as a high quality positive transparency notwithstanding the fact that they are carried by a common support. Suitable relationships between the maximum transmission densities of each of the positive and negative images, e.g., densities of 3.0 and 0.3 respectively, have been recognized in the previously cited US. patents, and positive transparencies having satisfactory density relationships have in fact been obtained in specific situations. It has not been possible heretofore, however, to obtain the desired positive and negative maximum densities using separate silver receptive and silver halide emulsion layers carried on the same support without making a sacrifice in or of some color resolution or separation, color saturation, efficiency of silver utilization, dynamic range, contrast and/or other desired sensitometric property. The compromises in sensitometric performance which occur in the practice of the prior art proposals are substantially reduced if not eliminated by the present invention.
It has already been stated that the present invention is especially useful in providing additive color transparencies by silver diffusion transfer processes. It is also true that the unique features of the present invention are most readily illustrated, understood and appreciated in the context of an additive color transparency. Accordingly, the more detailed description and discussion of the invention, particularly with respect to the preferred embodiments thereof, will be in connection with the provision of additive color transparencies which include, as part of an integral film structure, a transparent support, an optical screen such as an additive color screen, a negative silver image and, in a layer separate from the layer containing the negative silver image, a positive silver transfer image. In such additive color transparencies, a particularly useful additive color screen comprises sets of minute color filter elements, the individual filter elements of a given set transmitting light of a predetermined range of wavelengths of visible light, preferably one of the so-called primary color wavelength ranges. Particularly useful additive color screens thus comprise red, green and blue color filter elements, i.e., color filter elements which transmit, respectively, red, green and blue light, each filter element absorbing visible light outside its transmitted red, green or blue wavelength range. These color filter elements are arranged in an interspersed, juxtaposed arrangement to provide a regular repeating pattern well known in the art and customarily referred to simply as an additive color screen. In a particularly useful embodiment, the screen is formed of interspersed red, green and blue lines. The finer the filter elements or lines, the less likely the additive color screen will be resolved by the viewers eyes i.e., the less likely the viewer will see", i.e., be aware of, the additive color screen when the additive color image is enlarged many times in viewing as a color transparency.
The diffusion transfer positive images with which this invention is concerned comprise a positive transfer image and a negative silver image, the two images being in separate layers on a common, transparent support and viewed as a single, positive image. There is no masking layer between the positive and negative images. Such positive images may be referred to for convenience as integral positive-negative images, and more particularly as integral positive-negative transparencies". In such composite images, the maximum density of the negative silver image by definition determines the lowest possible minimum density which the integral positive-negative image can exhibit. Accordingly, the density of the negative silver image in areas of maximum exposure should be kept as low as possible.
The usual camera speed silver halide emulsions have a relatively wide distribution of grain sizes, a fact readily apparent from visual examination of the electron micrographs reproduced in FIGS. 5 and 6. Large silver halide grains are traditionally desirable in camera speed silver halide emulsions because of their usually higher speed. The covering power ofa given quantity of silver halide is reduced as the size of the individual silver halide particles (grains) increases, and this fact would argue for the desirability of large grains in silver halide emulsions which are to be retained with a positive silver transfer image. Notwithstanding this, large grain silver halide emulsions may lead to undesirably high graininess and other undesirable sensitometric results when utilized in integral negative-positive transparency film. indeed, if the silver halide grains are large and an additive color screen is formed of extremely fine filter elements, i.e., the silver halide grains are large relative to the filter element width, an undesirably large number of silver halide grains are likely to be positioned at the border of two different filter elements and thus be exposable by either of two different wavelength ranges of light. This results in reduced color separation and saturation. Small silver halide grains avoid the latter problems but result in much greater covering power of the negative silver image for the same given quantity of silver. Furthermore, if the photographic speed of the small grains is much less than that of the large grains, and this is a very common situation, the small grains will be inefficiently utilized in the process by being transferred to add inappropriate positive density instead of contributing to the sensitometric response of the film, with the result that dynamic range, latitude, film speed and contrast are adversely affected.
The present invention is directed to providing integral negative-positive diffusion transfer transparencies, particularly additive color transparencies, which make efficient use of minimum quantities of silver to obtain high quality images having desired minimum and maximum densities and exhibiting extended dynamic ranges and improved color quality. The number of silver halide grains available to record information is maximized while the total projected area of the silver halide grains is minimized.
In accordance with this invention. it has been found that such objectives may be satisfied and improved in tegral negativepositive images, particularly additive color transparencies, composed of a positive silver transfer image and an unmasked negative silver image in separate layers and viewable together as a positive image, may be obtained by using a silver halide emulsion which has a predominantly homogeneous grain size distribution. This emulsion is coated at low silver coverages and is one whose characteristic curve, or photographic response, is independent of grain size, thereby providing desired longer dynamic range. It has further been found that there is an important and hitherto unrecognized relationship between the silver coverage, the projected area of the silver halide grains, and of the projected area of the silver grains or particles forming the negative silver image, with the grain size distribution of the silver halide emulsion, which relationship is uniquely satisfied by the use of a predominantly homogeneous grain size silver halide emulsion. In additive color embodiments, it has been found that there is a further important and also hitherto unrecognized relationship between the grain size characteris tics of the silver halide emulsion and the minimum di* mension (width) of the individual optical filter elements which is effective to improve color resolution. Given these relationships. it has further been found that there is a silver halide grain mean diameter and size frequency distribution which is most effective for obtaining a given combination of negative and positive transmission maximum densities from a given quantity of silver halide, and for such a combination with a given additive color screen.
More particularly, it has been found that such integral positive-negative transparencies having highly satisfactory relationships between the maximum transmis sion densities of each of the positive and negative silver images may be obtained with a more desirable combination of sensitometric properties by using a silver halide emulsion the silver halide grains of which are predominantly homogeneous in diameter, said emulsion being coated in a quantity and manner such that the sum of the projected areas of said silver halide grains is not more than about 50% of the surface area of the silver halide emulsion layer. in additive color embodiments, the mean diameter of the silver halide grains should be about one-fifth to one-tenth the width of the color filter elements. In general, the silver halide grains should have a mean diameter within the range of about 0.7 to 1.5 microns. Where the additive color screen is a very fine screen, as in the Super 8 movie image size, the silver halide grain mean diameter will preferably be within the range of about 0.7 to 1.0 micron, and most preferably a mean diameter of about 0.9 micron, with at least of the silver halide grains having a diameter within i30% of said mean diameter. Where the image format is larger, as in the case of 35 mm or 3% X 4% transparencies, a coarser screen may be satisfactory and the mean diameter of the silver halide grains may be larger, e.g., within the range of about 1.2 to 1.4 microns. (Silver halide emulsions satisfying the above criteria would be recognized by those skilled in the art as being narrow in grain size distribution; indeed, such silver halide emulsions would be significantly narrower in grain size distribution than any commercially utilized camera speed silver halide emulsion.) The silver halide emulsion preferably is coated as a single grain layer" or monolayer" of silver halide, grains, i.e., the silver halide emulsion is substantially free of overlapping silver halide grains, although the silver halide emulsion layer itself may be thicker than the silver halide grains. The silver halide grains in the coated emulsion layer advantageously are relatively uniformly distributed and are free of clusters of grains which would have a diameter approaching the width of a color filter element. The silver halide emulsion is preferably coated at a silver to gelatin ratio of about 1:1 to [11.5 by weight.
Individual silver halide grains have, of course, finite dimensions and one frequently describes silver halide emulsions, inter alia, in terms of the means diameter" of the silver halide grains thereof. The silver halide grains of the silver halide emulsions used in this invention are regular in crystal habit, i.e., they are generally polyhedra of three-fold symmetry, such as spheres, cubes, octahedra, and nearly spherical, rounded-off octahedra such as plates or platelets. Three-fold symmetry is used here to mean symmetry about three mutually perpendicular axes.
The projected area" of an individual silver halide grain or developed silver grain is the area of the maximum plane section which may be drawn through the grain parallel with the surface of the layer in which said grain is disposed. The projected area of the grain thus corresponds to the area of the shadow which would be cast if one projected a light through the layer containing said grain, and it is a measure of the area over which the grain will block transmission of light through said layer. The sum of the projected areas of all the silver halide grains in a given silver halide emulsion layer will be the sum of the projected areas of the individual grains minus any overlapping projected area of overlapping grains. In accordance with this invention, as noted above, the sum of the projected areas of the silver halide grains of the silver halide emulsion layer should not be more than about 50% of the surface area of said silver halide emulsion layer. Furthermore, the sum of the projected areas of the fully exposed and developed silver grains (providing the maximum density of the negative silver image) should not exceed about 60% of the surface area of the corresponding portion of the silver halide emulsion layer. If the sum of the projected areas of the developed negative silver grains in a fully exposed area is about 60%. that portion of the negative image will transmit about 40% of the light projected thereon and will have an optical transmission density of approximately 0.4. If the sum of the projected areas of the developed negative silver grains in a fully exposed area is about 50%, that portion of the negative image will transmit approximately 50% of the light projected thereon and have an optical transmission density of approximately 0.3. in the preferred embodiments of this invention, the exposed silver halide grains are developed under conditions which limit their growth during development to not more than about l% in projected area.
The delta (A) or difference between the maximum density of the positive silver transfer image and the maximum density of the negative silver image prefera bly is at least 2.4 to 2.7 density units (transmission). It should be understood, however, that the maximum densities of the individual red, green and blue color records may vary slightly, e.g., within about 0.1 to 0.3 density units, particularly if the image silver is not neutral in tone. Satisfactory additive color transparencies will still be obtained notwithstanding such a variation provided at least two of the three color records exhibit a delta in excess of 2.0 if the minimum density if below 0.3, particularly if the maximum density is about 10 or more times the minimum density.
As noted above, in the preferred embodiments the silver halide emulsion has a mean grain diameter within the range of about 0.7 to 1.0 microns, preferably a mean diameter of about 0.9 micron. Assuming a silver halide grain of diameter 0.9 micron is a sphere, such a grain would have a projected area of 0.64 square micron. A silver halide sphere 0.87 micron in diameter would have a projected area of 0.6 square micron. It will therefore be seen that one may express the grain size characteristics of a silver halide emulsion in terms of the mean projected area of the silver halide grains. In such terms, the mean projected area of the silver halide grains of the predominantly homogeneous emulsion used in the preferred embodiments of this invention is about 0.6 square micron, and at least 90% of the silver halide grains of said emulsion should have a projected area within the range of approximately 0.5 to 1.7 times said the projected area.
The silver halide emulsions used in this invention have been described as being predominantly homogeneous in grain size, and preferable grain size distribu tions have been noted. Silver halide emulsions of narrow grain size distribution are not, per se, novel, and techniques for obtaining such silver halide emulsions are well known. Such techniques include physical separation and removal of grains smaller and/or larger than desired. Silver halide emulsion manufacturing procedures also are known which are adapted to produce narrow grain size distribution emulsions. It should be understood, however, that the silver halide emulsions must not only be predominantly homogeneous in grain size distribution, but the emulsion must also be one whose characteristic curve or photographic response is substantially independent of grain size distribution. In emulsions of wide grain size distribution, the characteristic curve is the result of the individual responses of a plurality of grain size families. Indeed, when one separates a particular grain size family of grains, the resulting silver halide emulsion is frequently a high contrast emulsion. The present invention, however, utilizes silver halide emulsions which are predominantly homogeneous in grain size (and therefore have similar solubility characteristics) and have a photographic response substantially independent of grain size. This latter characteristic may be considered to contemplate a mixture of silver halide grains of about the same diameter but which vary in their sensitivity, i.e., in their response in the diffusion transfer process. (A particularly useful silver halide emulsion satisfying the above criteria is a substituted-halide mixed silver halide emulsion; such emulsions will be described in more detail hereinafter.) Indeed, it has been found that the use of such predominantly homogeneous grain size silver halide emulsions has given markedly improved additive color transparcncies having satisfactory maximum densities of the positive and negative images so that these images may be maintained together without sacrifice of desired sensitometry. Such homogeneous grain size silver halide emulsions maximize the ability of the silver halide layer to record information during photoexposure without increasing the total projected area ofa given silver halide coverage.
Techniques for removing silver halide grains below and/or above a predetermined size or size range from a silver halide emulsion, e.g., by centrifugal separation, are known in the art and may be utilized in obtaining silver halide emulsions which are predominantly homogeneous in grain size. Silver halide emulsions of the type contemplated for use in the present invention may also be prepared by blending several silver halide emulsions or emulsion fractions each having substantially the same grain size but sensitized to different levels or speeds.
It has been stated above that a desirable maximum transmission density of the positive silver transfer image is about 3.0. It has been determined, e.g., by vacuum deposition of silver substantially uniformly on a transparent support in a stratum 0.1 to 0.15 micron thick, that l00 mg. per square foot of high covering power silver is sufficient to provide a transmission density of 3.0. It has further been determined that if l00 mg. of silver per square foot is provided in the form of silver halide spheres approximately 0.87 micron in diameter and coated in a layer 1 grain thick, (i.e., the silver halide layer is substantially free of overlapping silver halide grains), the silver halide grains will have a total projected area of 50% or less of the surface area of the silver halide emulsion layer. If this silver halide layer is given a full or maximum density exposure and the exposed silver halide grains developed to provide silver grains or particles which have substantially the same projected area as the silver halides had, the fully exposed and developed silver halide emulsion layer will have a maximum transmission density of 0.3. To the ex-