Title:
Photographic products and processes for forming silver and additive color transparencies
United States Patent 3894871
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.


Inventors:
LAND EDWIN H
Application Number:
05/383196
Publication Date:
07/15/1975
Filing Date:
07/27/1973
Assignee:
Polaroid Corporation (Cambridge, MA)
Primary Class:
Other Classes:
430/227, 430/229, 430/230, 430/234, 430/246, 430/511, 430/567
International Classes:
G03C8/06; G03C8/08; G03C8/30; (IPC1-7): G03C7/00; G03C1/48; G03C1/84; G03C5/54; G03C7/04
Field of Search:
96/3,25,29R,76R,80
View Patent Images:
Primary Examiner:
Torchin, Norman G.
Assistant Examiner:
Schilling, Richard L.
Attorney, Agent or Firm:
Mervis, Stanley H.
Claims:
What is claimed is

1. A photosensitive element for forming a color transparency by diffusion transfer processing to provide a developed negative silver image and a positive silver transfer image, said negative silver image and said positive silver transfer image being viewable as a positive image without separation, said photosensitive element comprising (a) a transparent support carrying a light-transmitting screen composed of minute optical elements, (b) a layer containing a photosensitive silver halide emulsion and (c) a silver receptive layer including a silver precipitating agent, said silver receptive layer being positioned between said light-transmitting screen and said silver halide emulsion layer; the silver halide grains of said silver halide emulsion being predominantly homogeneous in crystal diameter and habit, the sum of the projected areas of said silver halide grains being not more than about 50% of the surface area of said silver halide emulsion layer.

2. A photosensitive element as defined in claim 1 wherein said silver halide emulsion layer is substantially free of overlapping silver halide grains.

3. A photosensitive element as defined in claim 1 wherein said light-transmitting screen is an additive color screen composed of sets of interspersed minute color filter elements, the color filter elements of each set transmitting the same predetermined wavelength range of visible light.

4. A photosensitive element as defined in claim 3 wherein said additive color screen is composed of red, green and blue filter elements.

5. A photosensitive element as defined in claim 3 wherein said silver halide grains have a mean diameter of about one-fifth to one-tenth the width of said color filter elements, at least 90% of said silver halide grains having a diameter within ± 30% of the mean grain diameter.

6. A photosensitive element as defined in claim 1 wherein said silver halide grains have a mean diameter within the range of about 0.70 to 1.5 micron.

7. A photosensitive element as defined in claim 1 wherein said silver halide grains have a mean diameter of about 0.9 micron.

8. A photosensitive element as defined in claim 1 wherein said silver halide emulsion layer contains about 90 to 125 mg./ft.2 of silver.

9. A photosensitive element as defined in claim 1 wherein said minute optical elements are lenticules.

10. A photosensitive element as defined in claim 4 wherein said red, green and blue filter elements are in the form of lines, and said additive color screen contains approximately 550 lines/color/inch.

11. A photosensitive element as defined in claim 4 wherein said red, green and blue filter elements are in the form of lines, and said additive color screen contains approximately 750 lines/color/inch.

12. A photosensitive element as defined in claim 4 wherein said red, green and blue filter elements are in the form of lines, and said additive color screen contains approximately 1000 lines/color/inch.

13. A photosensitive element as defined in claim 1 wherein said silver halide emulsion is a substituted-halide silver halide emulsion prepared by replacing part of the chloride anions of a silver chloride emulsion with bromide anions or with bromide and iodide anions.

14. A photosensitive element as defined in claim 13 wherein said substituted-halide silver halide emulsion is a silver iodochlorobromide emulsion.

15. A photosensitive element as defined in claim 13 wherein said substituted-halide silver halide emulsion is a silver chlorobromide emulsion.

16. A photosensitive element as defined in claim 1 wherein said silver halide emulsion is a silver iodobromide emulsion.

17. A photosensitive element as defined in claim 1 wherein said silver halide grains have a mean projected area of about 0.6 square micron.

18. A photosensitive element as defined in claim 17 wherein at least 90% of said silver halide grains have a projected area between 0.5 and 1.5 times said mean projected area.

19. A photosensitive element for forming a positive transparency by diffusion transfer processing to provide a developed negative silver image and a positive silver transfer image, said negative silver image and said positive silver transfer image being viewable as a positive image without separation, said photosensitive element comprising (a) a transparent support carrying (b) a layer containing a photosensitive silver halide emulsion and (c) a silver receptive layer including a silver precipitating agent; the silver halide grains of said silver halide emulsion being predominantly homogeneous in crystal diameter, the sum of the projected areas of said silver halide grains being not more than about 50% of the surface area of said silver halide emulsion layer.

20. A photosensitive element as defined in claim 19 wherein said silver receptive layer is positioned between said transparent support and said silver halide emulsion layer.

21. A photosensitive element as defined in claim 19 wherein said silver halide emulsion layer is substantially free of overlapping silver halide grains.

22. A photosensitive element as defined in claim 19 wherein at least 90% of said silver halide grains have a diameter within ± 30% of the mean diameter.

23. A photosensitive element as defined in claim 19 wherein said silver halide grains have a mean diameter of about 0.70 to 1.5 micron, at least 90% of said silver halide grains having a diameter within ± 30% of said mean diameter.

24. A photosensitive element as defined in claim 19 wherein said silver halide grains have a mean diameter of about 0.9 micron.

25. A photosensitive element as defined in claim 19 wherein said silver halide emulsion layer contains about 90 to 125 mg./ft.2 of silver.

26. A photosensitive element as defined in claim 19 wherein said silver halide emulsion is a substitutedhalide silver halide emulsion prepared by replacing part of the chloride anions of a silver chloride emulsion with bromide anions or with bromide and iodide anions.

27. A photosensitive element as defined in claim 26 wherein said substituted-halide silver halide emulsion is a silver iodochlorobromide emulsion.

28. A photosensitive element as defined in claim 26 wherein said substituted-halide silver halide emulsion is a silver chlorobromide emulsion.

29. A photosensitive element as defined in claim 19 wherein said silver halide emulsion is a silver iodobromide emulsion.

30. A photosensitive element as defined in claim 19 wherein said silver halide gains have a mean projected area of about 0.6 square micron.

31. A photosensitive element as defined in claim 30 wherein at least 90% of said silver halide grains have a projected area between 0.5 and 1.5 times said mean projected area.

32. A photosensitive element as defined in claim 19 wherein said silver halide grains are generally spherical.

33. A photosensitive element as defined in claim 19 wherein said silver halide grains are generally cubic.

34. A photosensitive element for forming a color transparency by diffusion transfer processing to provide a developed negative silver image and a positive silver transfer image, said negative silver image and said positive silver transfer image being viewable as a positive image without separation, said photosensitive element comprising (a) a transparent support carrying a light-transmitting screen composed of minute optical elements, (b) a layer containing a photosensitive silver halide emulsion and (c) a silver receptive layer including a silver precipitating agent, the silver halide gains of said silver halide emulsion being predominantly homogeneous in crystal diameter, the sum of the projected areas of said silver halide grains being not more than about 50% of the surface area of said silver halide emulsion layer.

35. A photosensitive element as defined in claim 34 wherein said light-transmitting screen is an additive color screen composed of sets of interspersed minute color filter elements, the color filter elements of each set transmitting the same predetermined wavelength range of visible light.

36. A photosensitive element as defined in claim 35 wherein said additive color screen is composed of red, green and blue filter elements.

37. A photosensitive element as defined in claim 35 wherein said silver grains have a mean diameter of about one-fifth to one-tenth the width of said color filter elements, at least 90% of said silver halide grains having a diameter within ± 30% of said mean diameter.

38. A photosensitive element as defined in claim 34 wherein said silver halide grains have a mean diameter within the range of about 0.70 to 1.5 micron.

39. A photosensitive element for forming a transparency by diffusion transfer processing to provide a developed negative silver image and a positive silver transfer image, said negative silver image and said positive silver transfer image being viewable as a positive image without separation, said photosensitive element comprising (a) a transparent support, (b) a layer containing a photosensitive silver halide emulsion and (c) a silver receptive layer including a silver precipitating agent, said silver receptive layer being positioned between said support and said silver halide emulsion layer; the silver halide grains of said silver halide emulsion being predominantly homogeneous in crystal diameter, the sum of the projected areas of said silver halide grains being not more than about 50% of the surface area of said silver halide emulsion layer.

40. A photosensitive element as defined in claim 39 wherein said silver halide emulsion layer is substantially free of overlapping silver halide grains.

41. A photosensitive element for forming a positive transparency by diffusion transfer processing to provide a developed negative silver image and a positive silver transfer image, said negative silver image and said positive silver transfer image being viewable as a positive image without separation, said photosensitive element comprising (a) a transparent support carrying (b) a layer containing a photosensitive silver halide emulsion and (c) a silver receptive layer including a silver precipitating agent, said silver receptive layer being about 0.1 to 0.3 micron thick and positioned between said silver halide emulsion layer and said support; the silver halide grains of said silver halide emulsion being predominantly homogeneous in crystal diameter, the sum of the projected areas of said silver halide grains being not more than about 50% of the surface area of said silver halide emulsion layer.

42. A photosensitive element as defined in claim 41 wherein said silver halide emulsion layer is substantially free of overlapping silver halide grains and contains about 90 to 125 mg./ft.2 of silver.

43. A photosensitive element as defined in claim 42 including an additive color screen of alternating red, green and blue lines, said color screen containing approximately 1000 lines per color per inch.

44. A photosensitive element as defined in claim 43 wherein the mean diameter of the silver halide grains is within the range of about 0.70 to 1.0 micron, and at least 90% of the silver halide grains have a diameter within ± 30% of said mean diameter.

45. A photosensitive element as defined in claim 42 wherein the dispersion number of the grain size-frequency distribution curve for said silver halide emulsion is 0.40 or less.

46. A photosensitive element as defined in claim 45 wherein said photosensitive element includes an additive color screen of red, green and blue filter lines, said color screen containing about 550 to about 750 lines per color per inch.

47. A photosensitive element as defined in claim 43 wherein the mean diameter of said silver halide grains is within the range of 1.2 to 1.4 microns.

48. A photosensitive element for forming a positive transparency by diffusion transfer processing to provide a developed negative silver image and a positive transfer image, said negative silver image and said positive transfer image being viewable as a positive image without separation, said photosensitive element comprising (a) a transparent support carrying, (b) a layer containing a photosensitive silver halide emulsion and (c) an image-receiving layer for forming a positive transfer image; the silver halide grains of said silver halide emulsion being predominantly homogeneous in crystal diameter and habit, the sum of the projected areas of said silver halide grains being not more than about 50% of the surface area of said silver halide emulsion layer.

49. A photosensitive element as defined in claim 1 wherein said silver halide emulsion layer contains about 100 mg./ft.2 of silver and is substantially free of overlapping silver halide grains.

50. A photosensitive element as defined in claim 48 including a permeable polymer layer coated over said silver halide emulsion layer.

51. A photosensitive element as defined in claim 50 wherein said permeable polymer layer is a gelatin layer.

52. A photosensitive element as defined in claim 51 wherein said permeable polymer layer contains about 80 to 250 mg./ft.2 of gelatin.

53. A photosensitive element for forming a positive transparency by diffusion transfer processing to provide a developed negative silver image and a positive transfer image, said negative silver image and said positive transfer image being viewable as a positive image without separation, said photosensitive element comprising (a) a transparent support carring (b) a layer containing a photosensitive silver halide emulsion and (c) a silver receptive layer; the silver halide grains of said silver halide emulsion being predominantly homogeneous in crystal diameter, the sum of the projected areas of said silver halide grains being not more than about 50% of the surface area of said silver halide emulsion layer.

54. A photosensitive element as defined in claim 53 wherein said silver halide grains have a mean diameter within the range of about 0.7 to 1.5 microns.

55. A photosensitive element as defined in claim 54 wherein said silver halide grains have a mean diameter of about 0.9 micron, at least 90% of said silver halide grains having a diameter within ± 30% of said mean diameter.

56. A photosensitive element as defined in claim 55 wherein said silver halide emulsion layer contains about 100 mg./ft.2 of silver.

57. A photosensitive element as defined in claim 53 wherein said silver halide grains have a mean projected area of about 0.6 square micron.

58. A photosensitive element s defined in claim 57 wherein at least 90% of said silver halide grains have a projected area between 0.5 and 1.5 times said means projected area.

59. A diffusion transfer process comprising the steps of exposing a photosensitive element comprising (a) a transparent support carrying (b) a layer containing a photosensitive silver halide emulsion and (c) an image-receiving layer for forming a positive transfer image, said image-receiving layer being positioned between said support and said silver halide emulsion layer; the silver halide grains of said silver halide emulsion being predominantly homogeneous in crystal diameter and habit, the sum of the projected areas of said silver halide grains being not more than about 50% of the surface area of said silver halide emulsion layer, developing the exposed silver halide emulsion to a negative silver image having a maximum density of no more than about 0.3, said development being effected in the presence of a silver halide solvent to form a positive silver transfer image in said image-receiving layer, and maintaining the layers containing said negative and said positive images together as a permanent laminate, said images being viewed together as a positive image.

60. A diffusion transfer process comprising the steps of exposing a photosensitive element comprising (a) transparent support carrying (b) a layer containing a photosensitive silver halide emulsion and (c) a silver receptive layer; the silver halide grains of said silver halide emulsion being predominantly homogeneous in crystal diameter, the sum of the projected areas of said silver halide grains being not more than about 50% of the surface area of said silver halide emulsion layer, developing said exposed silver halide emulsion in the presence of a quaternary ammonium compound to form a negative silver image having a maximum density of no more than about 0.3, forming a positive silver transfer image in said silver receptive layer, and maintaining the layers containing said negative and positive silver images together as a permanent laminate, said images being viewed together as a positive transparency.

61. A diffusion transfer process as defined in claim 60 wherein said silver halide emulsion layer contains about 90 to 125 mg./ft.2 of silver.

62. A diffusion transfer process as defined in claim 60 wherein the silver halide grains of said silver halide emulsion have a mean diameter of about 0.70 to 1.5 microns, at least 90% of said silver halide grains having a diameter within ± 30% of said mean diameter.

63. A diffusion transfer process as defined in claim 62 wherein said mean diameter is about 0.90 micron and said silver halide emulsion layer contains about 100 mg./ft.2 of silver.

64. A diffusion transfer process as defined in claim 62 wherein said quaternary ammonium compound is N-benzyl-α-picolinium bromide.

65. A diffusion transfer process as defined in claim 64 wherein development is effected using tetramethyl reductic acid.

Description:
This invention is concerned with diffusion transfer 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. In 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. 2,614,926 issued Oct. 21, 1952, U.S. Pat. No. 2,707,150 issued Apr. 26, 1955, U.S. Pat. No. 2,726,154 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 11, 1961 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 extent but this is generally unobjectionable, particularly for projection purposes, due to the considerable difference in density between the positive and negative images.

and U.S. Pat. No. 2,992,103 states:

Althuogh not as preferred, the invention contemplates indefinite maintenance of the sandwich structure for viewing purposes following the formation of negative and positive images therein. The copending application of Edwin H. Land, Ser. No. 265,413, 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 the silver 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 Ser. No. 265,413 issued as the above-noted U.S. Pat. No. 2,726,154. 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. It 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 J. Weed. In 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 U.S. 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 U.S. 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 predominantly 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 effective 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 transparencies.

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 1,000× magnification of a transmission view through an unexposed diffusion transfer additive color film embodying the present invention;

Fig. 3 reproduces an optical photograph at 1,000× 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 10,000× 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 10,000× magnification of replicas of undeveloped silver iodobromide grains of a silver halide emulsion used in a commercial silver diffusion transfer process;

FIG. 6 reproduces an electron micrograph at 10,000× 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 10,000× 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 size-frequency 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 10,000× 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. 10 reproduces an electron micrograph at 10,000× magnification of a transmission view through the silver halide layer shown in FIG. 9 following exposure (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 U.S. 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 viewer's 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 of a 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 integral negative-positive 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 characteristics of the silver halide emulsion and the minimum dimension (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 transmission 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 90% of the silver halide grains having a diameter within ±30% of said mean diameter. Where the image format is larger, as in the case of 35 mm or 31/4 × 41/4 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 1:1.5 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 10% in projected area.

The delta (Δ) or difference between the maximum density of the positive silver transfer image and the maximum density of the negative silver image preferably 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 distributions 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 transparencies 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 of a 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 100 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 100 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 extent there is overlap of the silver or silver halide grains, the total projected area will be reduced, and the transmission density of the negative silver image will also be reduced. Since the silver halide grains in practice will not be perfect spheres, reference to 0.87 micron as the desired grain diameter when 100 mg. of silver is used is intended to be a guide to those skilled in the art in selecting and coating silver halide emulsions in the practice of this invention. The effect upon total projected area if one uses 100 mg. of silver in the form of silver halide grains of other diameters may be seen in FIG. 11 wherein there is plotted the percentage of area covered (total projected area) of 100 mg. of silver as a function of the grain diameter in microns, assuming the grains to be spheres. As one uses grains having a diameter larger than about 0.9 micron, the percentage of area covered is reduced, and conversely the total projected area increases as the grain diameter is reduced. (No overlapping of grains is assumed.) Of particular significance is the rapidity of the rate of increase in total projected area as the mean grain diameter is reduced, while the rate of change is much slower as the mean grain diameter is increased.

As indicated above, 100 mg. of silver per square foot is sufficient to provide a transmission density of approximately 3.0. It has further been found that positive silver transfer images having transmission densities of approximately 2.5 to 3.0 may be obtained using silver halide emulsion layers containing about 90 to 125 mg. of silver per square foot if the positive image silver is deposited in a high covering power form in a very thin, e.g., about 0.1 to 0.3, and preferably about 0.25, micron thick image-receiving layer.

Further understanding of the present invention will be facilitated by a discussion of this point of some of the accompanying drawings.

In FIG. 1 there is schematically illustrated a film unit 30 for forming a diffusion transfer additive color transparency 30b. Film unit 30 comprises a transparent support 10 carrying an additive color screen 12 composed of alternating red, green and blue filter segments or elements (designated R, G and B, respectively), an image-receiving layer 14 and a silver halide emulsion layer 16. (It is assumed that the silver halide emulsion layer 12 contains approximately 100 mg. of silver per square foot as silver halide grains having a mean diameter of 0.9 micron, with at least 90% of the silver halide grains having a diameter within ± 30% of said mean diameter.) In Stage A, an exposure of the silver halide emulsion 16 through the additive color filter 12 is assumed to be only to red light, and of an intensity sufficient to fully expose the silver halide. Under these conditions, silver halide behind the red filter elements R will be exposed by red light transmitted therethrough, while the silver halide behind the green and blue filter elements G and B will not be exposed, as the red light will be absorbed before reaching the silver halide. The resulting exposed silver halide emulsion layer 16a (Stage B) contains the latent image of the red record. Development and diffusion transfer processing may be effected by application of a processing composition 18 onto the silver halide emulsion side of the exposed film unit 30a. As illustrated in Stage B, a processing composition 18 is applied from a reservoir or container 20 having a slot or nozzle adapted to meter on a predetermined quantity of the processing fluid as a function of the rate at which the exposed film is feed past the container 20 nozzle. The processing composition develops the latent image to a negative silver image 16b of low covering power (approximately 0.3 maximum transmission density) in registered relationship with the red filter elements R. The unexposed silver halide behind the green and blue filter elements G and B is not developed but is dissolved and transferred by diffusion to the superposed image-receiving layer where the silver complex is precipitated to form a positive silver image 14a in high covering power silver. The positive image silver is deposited in registered relationship with the green and blue filter elements G and B. Viewing of the final additive color transparency 30b by white light projected through the developed silver halide emulsion layer 16b, the positive silver image 14a and the additive color screen 12 provides a right reading positive red image reproducing the red record of exposure Stage A. (The low covering power negative silver image does not prevent the passage of light through the red filter elements R during subsequent viewing, while the high covering power positive silver prevents the passage of light through the green and blue filter elements G and B.

FIG. 2 reproduces an optical photomicrograph (1000x) of an unexposed diffusion transfer additive color film similar to that shown in FIG. 1 as film unit 30. The silver halide grains are visible as are the red, green and blue filter elements (lines). FIG. 3 reproduces an optical photomicrograph (1000x) of the film of FIG. 2 following exposure to red light and processing, as in FIG. 1. The heavy density or solid areas 40 in FIG. 3 are provided by the high covering power positive image silver areas (corresponding to the lack of exposure behind the blue and green lines). The low density areas 42 are provided by the low covering power negative silver areas (corresponding to the red expo ure), and the fact that silver particles cover no more than 50% of the surface of these fully exposed areas is visibly demonstrated.

FIG. 4 reproduces an electron micrograph (10,000x) of an area of a diffusion transfer additive color film like that shown in FIG. 2 and which has been given and intermediate exposure to red and blue light and a maximum density exposure to green light and diffusion transfer processed to an additive color transparency. (The film was prepared and processed as described in Example 2 below.) Visual inspection of the centrally positioned wide diagonal band (running from the upper corner of the electron micrograph) shows a combination of a large number of small silver particles (positive image silver) and a small number of relatively large silver particles (negative image silver). This band underlies the red and blue filter elements and is positioned between fully exposed areas, underlying green filter elements, containing a large number of relatively large silver particles and free of small silver particles; visual examination of these intermediate density red and blue exposed and developed areas shows a combination of high covering power positive image silver and low covering power negative image silver, thus producing an intermediate transmission density.

As pointed out above, the silver halide emulsions contemplated for use in this invention are predominantly homogeneous in grain size. The silver halide emulsions used in this invention may comprise any of the photosensitive silver halides or mixed halides, and are preferably in a gelatin binder. Particularly useful silver halides are silver iodobromide, silver iodochlorobromide and silver chlorobromide. Iodide if present in the silver halide preferably comprises about 1 to 10% iodide by weight of silver. Particularly preferred silver halide emulsions are chemically sensitized, substituted-halide mixed silver halides containing 1 to 50% chloride and 0 to 10% iodide, the remaining halide being bromide. A particularly useful predominantly homogeneous grain silver halide emulsion is one prepared by forming a silver chloride emulsion and replacing or "substituting" part of the chloride anions with bromide and/or iodide anions. Such silver halide emulsions are referred to herein for convenience as substituted halide silver halide emulsions. The preparation of predominantly homogeneous substituted-halide silver halide emulsions having a mean grain diameter within the range of about 0.7 to 1.5 microns, and preferably within the range of about 0.7 to 1.2 microns, with at least 90% of the silver halide grains having a diameter within ± 30% of said mean diameter, is the subject of the copending application of Vivial K. Walworth, Ser. No. 383,176, filed concurrently herewith, and said application is hereby incorporated by reference. The following example is reproduced from said Ser. No. 383,176 for purposes of illustration.

EXAMPLE 1

A solution of gelatin and potassium chloride (Solution A) was prepared by dissolving 205 g. of phthalic anhydride derivatized inert bone gelatin and 205 g. of potassium chloride in 5,750 ml. of distilled water. A solution of potassium chloride (Solution B) was prepared by dissolving 1,026 g. of potassium chloride in 5,336 ml. of distilled water. A silver nitrate solution (Solution C) was prepared by dissolving 2,000 g. of silver nitrate in 5,336 ml. of water. Solution A was heated to 80° C. and Solutions B and C were heated to 70° C. Solutions B and C were then added to Solution A simultaneously (by double jet addition) over a period of 8 minutes. The resulting mixture was digested 5 minutes at 80° C. After this digestion period, a solution of 1,337 g. of potassium bromide and 60 g. of potassium iodide dissolved in 5,336 ml. of water and heated to 70° C. was added over a period of 8 minutes keeping the temperature at 80° C. The mixture was then digested for 35 minutes at 80° C. After the digestion period, the mixture was cooled to 20° C. and the pH adjusted to about 2.7 with 10% sulfuric acid. The silver halide-gelatin flocculate was washed several times with chilled, distilled water until the conductivity of supernatant liquid reached 50-100 μmhos. After the last decantation of excess wash water, 950 g. of dry active bone gelatin was added and allowed to swell for 20 minutes. The temperature was then raised to 38° C. and held there for 20 minutes while the gelatin dissolved. After adjusting the pH to about 5.7, the temperature was raised to 54° C. and 24 ml. of a solution of an ammonium gold thiocyanate complex was added. (This chemical sensitizer solution was prepared by mixing a solution of 1.0 g. of ammonium thiocyanate n 99 ml. of water with 12 ml. of a solution containing 0.97 g. of gold chloride in 99 ml. of water.) The emulsion was then afterripened at 54° C. for 120 minutes. The emulsion was cooled to 38° C., optical sensitizer added and the emulsion digested for about 45 minutes before being chilled and set. The resultant silver iodochlorobromide emulsion contained approximately 85 mole percent bromide, 12 mole percent chloride and 3 mole percent iodide, as determined by X-ray fluorescence analysis. The silver iodochlorobromide grains had a mean diameter of about 0.86 micron, and 90% of the grains had a diameter within the range of about 0.63 to 1.08 micron, or within ± 26% of the mean diameter.

It has been noted above that the preferred embodiments of this invention utilize a predominantly homogeneous grain size silver halide emulsion having a mean grain diameter of about 0.9 micron coated at a silver coverage of approximately 100 mg. per square foot. FIG. 9 reproduces an electron micrograph (10,000x) of a transmission view through an unprocessed silver halide emulsion layer containing 101.7 mg. of silver per square foot of the 0.86 micron mean diameter silver halide emulsion prepared in Example 1, coated at a 1.5:1 gelatin/silver weight ratio. Readily evident from a visual examination are the predominantly homogeneous grain size of the silver halide grains, the substantial lack of overlapping silver halide grains, and the fact that the sum of the projected areas of the silver halide grains is not more than about 50% of the surface area of the silver halide emulsion layer. FIG. 10 reproduces a similar electron micrograph (10,000x) of another portion of the same silver halide emulsion layer as shown in FIG. 9 following maximum density exposure and development. It will be seen that the grains have not grown appreciably and that the total projected area is maintained low in accordance with this invention.

Grain size distribution curves, or grain sizefrequency distribution curves as they are sometimes called, are frequently used to describe and define silver halide emulsions. Mees and James, The Theory of the Photographic Process, 3rd Edition, The Macmillan Company, New York, N.Y., 1966, pages 36-44, set forth a description of techniques of measuring the size of silver halide grains and of determining the frequency of grains of given sizes in a particular silver halide emulsion. Electron microscope size-frequency analysis of silver halide emulsions gives measurements particularly useful with grains too small to resolve well by light microscopy.

FIG. 8a reproduces the grain size-frequency distribution curve of particle sizes (1,000 grains) determined using a Zeiss TGZ-3 particle size analyzer to obtain counts from electron micrographs of the silver halide emulsion prepared in Example 1. The horizontal axis for the curve in FIG. 8a represents relative log diameter in microns of the silver halide grains, while the vertical axis represents the relative number of grains, with the dotted curve representing cumulative percentile. For the silver halide emulsion prepared in Example 1, the mean particle diameter was 0.86 micron. While the percent deviation from the mean diameter of 90% of the silver halide grains has been stated in Example 1, visual comparison of the grain size-frequency distribution curve reproduced in FIG. 8a graphically demonstrate far more clearly the narrow distribution, i.e., the homogeneous grain size, of the substituted-halide emulsion prepared in Example 1.

It is also possible to characterize the grain size distribution of a silver halide emulsion by use of the dispersion number of the grain size-frequency distribution curve, i.e., the number obtained as follows: the grain size diameter of the 16th percentile is subtracted from the grain size diameter at the 84th percentile, and the resulting number is divided by the median diameter. The smaller the dispersion number, the narrower will be the band width of the grain size-frequency distribution curve. The dispersion number for the silver halide emulsion prepared in Example 1 (see FIG. 8a) was 0.35. The silver halide emulsions useful in practicing the present invention using additive color screens having 1,000 or more color triplets per inch may be characterized as having a dispersion number of 0.4 or less, and preferably 0.35 or less, in addition to having a mean grain diameter within the range of about 0.7 to 1.0 micron.

Further graphic visual evidence of the homogeneous grain size distribution of the silver iodochlorobromide emulsion prepared in Example 1 may be obtained by examination of the electron micrograph (10,000x) reproduced in FIG. 7, of these grains replicated in carbon-platinum. The silver halide grains of this emulsion are far more homogeneous in grain size than silver halide emulsions used in commercially available silver diffusion transfer processes. This fact is readily apparent from a visual comparison of FIG. 7 with FIGS. 5 and 6 which reproduce electron micrographs (10,000x) of similar carbon-platinum replicas of silver iodobromide emulsions used, respectively, in Polaroid Land Type 42 and Type 47 films.

As indicated above and illustrated in FIG. 1, the diffusion transfer additive color transparency film of this invention comprises a transparent support carrying an additive color screen, a silver receptive layer and a silver halide emulsion, and these layers are retained together, in registered relationship, as a permanent laminate after processing.

The additive color screen per se may be formed by techniques well known in the art, e.g., by sequentially printing the requisite filter patterns by photomechanical methods. An additive color screen comprises an array of sets of colored areas or filter elements, usually from two to four different colors, each of said sets of colored areas being capable of transmitting visible light within a predetermined wavelength range. In the most common situations, the additive color screen is trichromatic and each set of color filter elements transmits light within one of the so-called primary wavelengths ranges, i.e., red, green and blue. The additive color screen may be composed of minute dyed particles, such as starch grains or hardened gelatin particles, intermixed and interspersed in a regular or random arrangement to provide a mosaic. A regular mosaic of this type may be made by the alternating embossing and doctoring technique described in U.S. Pat. No. 3,019,124 issued Jan. 30, 1962 to Howard G. Rogers. Another method of forming a suitable color screen comprises multi-line extrusion of the type disclosed in U.S. Pat. No. 3,032,008 issued May 1, 1962 to Edwin H. Land, David S. Grey and Otto E. Wolff, the colored lines being deposited side-by-side in a single coating operation.

A particularly useful and preferred additive color screen comprises red, green and blue stripes or lines in a regularly repeated pattern. The "width" of each of the color filter elements may be varied according to the use to which the final additional color transparency will be put. In general, the greater the expected enlargement of the additive color transparency, e.g., when projected on a viewing screen, the smaller the filter elements should be to ensure that the viewer will not "see", i.e., be able to resolve, the color screen independently of the additive color image. The width of the filter elements thus limits the degree of magnification acceptable in viewing the final image. In general, it has been found that screens composed of approximately 550 triplet sets of red, green and blue lines per inch (i.e., 550 lines per color per inch, each triplet set of lines having a combined width of about 45 microns) is useful if a 35 mm (24 × 36 mm) or a 31/4 × 41/4 inches transparency is desired, approximately 750 triplet sets per inch if the film is of the 16 mm type, and approximately 1,000 triplet sets per inch if the film is of the Super 8 type. Obviously, the finer screen also may be used with larger image films if so desired. In a typical additive color screen particularly useful in providing additive color movie film having a standard Super 8 image area, each red, green and blue line is about 8 microns wide, and each triplet set of red, green and blue lines is about 24-25 microns wide.

A particularly preferred process for the production of the color screen comprises the process set forth in U.S. Pat. No. 3,284,208 which includes successively coating the smooth surface of a lenticular film with a plurality of photoresponsive layers and sequentially subjecting these coatings to radiation focused by the lenticules to provide selective exposure of the coating. Subsequent to each exposure, unexposed portions of the coating are removed and the resultant resist is dyed to provide a set of chromatic filter elements, after which the next succeeding photoresponsive layer is applied. Each such exposure is effected by radiation incident on the lenticular film at an angle calculated to provide the desired plurality of color filter element sets in substantial side-by-side or screen relationship, with each set of a color effective to filter predetermined wavelengths of light. Where the additive color screen is trichromatic, e.g., the conventional red, green and blue, the exposed portion of each photoresponsive area will generally comprise about one-third of that layer. Although all three exposures may be accomplished by radiation incident on the lenticules of the lenticular film at three separate angles so calculated that each exposure exposes about one-third of the area behind each lenticule, it will be seen that the final color filter element formation may be effected by exposing the last photoresponsive coating to diffuse radiation, relying upon the previously formed color filter elements to prevent undesired exposure.

Subsequent to formation of the first and second series of filter elements, the lenticular configuration is reconstituted as a continuous, smooth surface. If the lenticules comprise a separate stratum temporarily affixed to the surface of the support on which the color screen is formed, that separate stratum may be stripped from the support. Where the lenticules are integral with the film base or support and have been provided by pressure and/or solvent deformation of the base, a continuous smooth surface may be reconstituted by application of a suitable solvent to release the deformation pressures produced during the manufacturing of the lenticular film base; if desired, for example, for optical transmission purposes, the reconstituted surface may be polished, for example, by surface contact with an appropriate rotating polishing cylinder or drum, to provide the desired optical characteristics to the film base surfaces.

The external surface of the color screen is preferably overcoated with an alkali resistant protective polymeric composition, such as cellulose acetate butyrate, polyvinyl butyral, polyvinylidene chloride, and the like, to protect the screen filter dyes from attack by the diffusion transfer processing composition used to process the film. The other layers of the film may then be coated over this protective layer.

Suitable apparatus for effecting the described exposure of the lenticular film is disclosed, for example, in U.S. Pat. No. 3,318,220.

The transparent support or film base employed may comprise any of the known types of transparent photographically useful rigid or flexible supports, for example, glass, polymeric films of both the synthetic type and those derived from naturally occurring products, etc. Expecially suitable film bases comprise polyesters such as the polymeric films derived from ethylene glycol and terephthalic acid and commercially available under such tradenames as Mylar and Estar; and polymeric cellulose derivatives such as cellulose triacetate or cellulose acetate butyrate.

The preferred image-receiving layer, as noted above, is very thin and includes one or more silver precipitating agents to provide a vigorous silver precipitating system adapted to provide high covering power silver transfer images. This layer is preferably positioned between the silver halide emulsion layer and the additive color screen, but it is within the scope of this invention to position the silver halide emulsion layer between the image-receiving layer and the additive color screen. The image-receiving layer should exhibit little or no swelling during processing, thereby aiding in forming a compact silver deposit providing a high covering power silver transfer image and minimizing lateral diffusion of the transferring soluble silver complex, thereby increasing image resolution and maximizing color separation and saturation. A number of suitable binder or carrier materials for the silver precipitating agents are known in the art. A particularly useful image-receiving layer comprises a stratum of deacetylated chitin containing a silver precipitating agent; image-receiving layers of this type are described in detail in U.S. Pat. No. 3,087,815 issued Apr. 30, 1963 to William H. Ryan and Elizabeth L. Yankowski, and the disclosure of said patent is incorporated herein by reference.

It is also advantageous to limit the swelling of the silver halide emulsion layer to further assits in minimizing lateral diffusion of soluble complex and thus increase image resolution and color resolution and saturation. Suitable hardening and/or cross-linking agents are well-known in the photographic art. The film unit also may advantageously include an anti-static agent or coating. Wetting agents may be used in accordance with conventional practices.

Suitable silver precipitating agents are well-known in the art and are described, for example, in the several patents mentioned above as describing silver transfer processes. Particularly useful silver precipitating agents include the heavy metal sulfides and selenides and the colloidal metals described in said patents and in, for example, U.S. Pat. No. 2,698,237 issued Dec. 28, 1954 to Edwin H. Land. It is preferred to use sulfides whose solubility products in an aqueous medium at approximately 20° C. are between 10-23 and 10-30, and especially the sulfides or selenides of zinc, copper, cadmium and lead. The silver precipitating agents are used in low concentrations, e.g., in the order of about 1-25 × 10-6 moles per square foot. Where the silver precipitating agent is one or more of the heavy metal sulfides or selenides, it is desirable to prevent the diffusion and wandering of any excess sulfide or selenide ions, which may be present, by also including, in the silver precipitating layer or in a separate layer adjacent thereto, at least one metallic salt which is substantially more soluble in the processing agent than the heavy metal sulfide or selenide used as the silver precipitating agent and which is irreducible in the processing agent. This more soluble salt has, as its cation, a metal whose ion forms sulfides or selenides which are difficulty soluble in the processing agent and which give up their sulfide or selenide ions to silver by displacement. Accordingly, in the presence of sulfide or selenide ions the metal ions of the more soluble salts have the effect of immediately precipitating the sulfide or selenide ions from solution. These ion-capturing salts may be soluble salts of cadmium, cerium (ous), cobalt (ous), iron, lead, nickel, manganese, thorium, and tin. Satisfactory soluble and stable salts of the above metals may be found among the acetates, nitrates, borates, chlorides, sulfates, hydroxides, formates, citrates, or dithionates thereof. The acetates and nitrates of zinc, cadmium, nickel, and lead are preferred. In general, it is also preferable to use the white or lightly colored salts.

The above mentioned ion-capturing salts may also serve a function of improving the stability of the positive image provided they possess, in addition to the aforementioned characteristics, the properties specified in U.S. Pat. No. 2,584,030 issued Jan. 29, 1952 to Edwin H. Land. For example, if the ion-capturing salt is a salt of a metal which slowly forms insoluble or slightly soluble metallic hydroxides with the hydroxyl ions in the alkaline processing liquid, it may contribute to reducing the alkalinity of the film unit, thereby aiding in the prevention of undesirable developer stains.

Silver halide solvents useful in forming the desired soluble complex with unexposed silver are well known and, for example, may be selected from the alkali metal thiosulfates, particularly sodium or potassium thiosulfates, or the silver halide solvent may be a cyclic imide, such as uracil, in combination with a nitrogenous base as taught in U.S. Pat. No. 2,857,274 issued Oct. 21, 1958 to Edwin H. Land. While the silver halide solvent is preferably initially present in the processing composition, it is within this invention to initially position the silver halide solvent in a layer of the film unit, preferably in the form of a precursor which releases or generates the silver halide solvent upon contact with an alkaline processing fluid.

The processing compositin may contain a thickening agent, such as an alkali metal carboxymethyl cellulose or hydroxyethyl cellulose, in a quantity and viscosity grade adapted to facilitate application of the processing composition. The processing composition may be left on the processed film or removed, in accordance with known techniques, as is most appropriate for the particular film use. The requisite alkalinity, e.g., a pH of 12-14, is preferably imparted to the processing composition by the use of one or more alkali metal hydroxides, such as sodium, potassium and/or lithium hydroxide. A wetting agent may be advantageously included in the processing composition to facilitate application thereof, particularly where the processing composition is applied in a very thin layer of low viscosity fluid.

Suitable silver halide developing agents may be selected from amongst those known in the art, and may be initially positioned in a layer of the photosensitive element and/or in the processing composition. Organic silver halide developing agents are generally used, e.g., organic compounds of the benzene or naphthalene series containing hydroxyl and/or amino groups in the para or ortho positions with respect to each other, such as hydroquinone, tert-butyl hydroquinone, toluhydroquinone, p-aminophenol, 2,6-dimethyl-4-amino-phenol, 2,4,6-triaminophenol, etc. If the additive color transparency is one which is not washed after processing to remove unused silver halide developing agent, development reaction products, etc., the silver halide developing agent(s) should not give rise to colored reaction products which might stain the image or which, either unreacted or reacted, might adversely affect the stability and sensitometric properties of the final image. Particularly useful silver halide developing agents having good stability in alkaline solution are substituted reductic acids, particularly tetramethyl reductic acid, as disclosed in U.S. Pat. No. 3,615,440 issued Oct. 26, 1971 to Stanley M. Bloom and Richard D. Cramer, and α, β-enediols as disclosed in U.S. Pat. No. 3,730,716 issued to Edwin H. Land, Stanley M. Bloom and Leonard C. Farney on May 1, 1973.

It is also within the scope of this invention to utilize antifoggants and/or image toning agents in concentrations well known in the art, and incorporated in the photosensitive element and/or in the processing composition in accordance with well known practices.

The provision of a processing composition permeable layer, (sometimes referred to as an "overcoat" or "top coat"), free of silver halide or silver precipitating agent, as the outermost layer has been found to provide a number of useful benefits. Such a layer may be used to carry one or more reagents, such as anti-halation dyes and/or image stabilizing agents, useful in the process. It is believed that this overcoat may exert a useful modulating effect in the rate and/or concentration at which components of the processing composition contact the silver halide, particularly where this overcoat is coated directly over the silver halide emulsion, and promote a more uniform processing composition wave front and permeation. Suitable processing composition permeable polymers may be readily selected to provide the particular properties and the degree of modulation desired. As examples of preferred materials, mention may be made of gelatin and cellulose acetate hydrogen phthalate. The former may be deposited over the emulsion layer from a water solution whereas the layer may be deposited from a suitable solvent such as an organic solvent, e.g., an acetone/ethanol mixture. Other suitable polymers include polyvinyl alcohol and polyvinyl pyrrolidone. The polymeric layer may be cross-linked or hardened to control the rate of permeation and degree of swelling. As examples of hardening agents which have been found useful, particularly with gelatin layers useful in practicing this invention, mention may be made of chrome alum and alginates, such as propylene glycol alginate. The presence of an overcoat layer also is advantageous in minimizing "salting out" of components of the processing fluid on the surface of the developed film where the processing fluid is not removed. A particularly useful overcoat layer is a coating of about 80 to 250 mg./ft.2 of gelatin.

The particular dye or dyes used to provide the individual color screen filter elements may be selected in accordance with principles well known in the art and per se form no part of the present invention. It is also known in the art that the individual filter elements need not be equal in area, and some variation in the relative areas occupied by the individual colors may be desirable to obtain a color balance as the result of the color transmission properties of individual dyes. Examples of suitable dyes for use in forming additive color screens are set forth in the previously cited patents related to additive color photography and in other patents, e.g., U.S. Pat. No. 3,730,725 issued May 1, 1973 to E. M. Idelson, the copending application of E. M. Idelson, Ser. No. 319,905 filed Dec. 29, 1972, and the copending application of L. Locatell, Jr., Ser. No. 319,223 filed Dec. 29, 1972.

The following examples of the preparation of an additive color transparency in accordance with this invention are set forth for illustrative purposes only.

EXAMPLE 2

A transparent polyethylene terephthalate film base bearing an additive color screen composed of approximately 1,000 triplet sets of red, green and blue dyed dichromated gelatin filter lines was prepared by the procedure described in the above-mentioned U.S. Pat. No. 3,284,208. A thin layer of cellulose acetate butyrate was coated over the additive color screen, followed by an image-receiving layer comprising approximately 4.4 mgs./ft.2 of deacetylated chitin and approximately 0.25 mgs./ft.2 of cupric sulfide. A photosensitive silver halide layer was then applied over the image-receiving layer, using a predominantly homogeneous substituted-halide mixed halide chemically sensitized silver halide emulsion (means diameter 0.70 micron) prepared substantially as described in Example 1 except that the silver chloride precipitation time was 4 minutes instead of 8 minutes, the silver halide being panchromatically sensitized and having essentially the same grain size characteristics. The silver halide layer contained approximately 120 mgs./ft.2 of gelatin and approximately 102 mgs./ft2 of silver and 0.2% propylene glycol alginate by weight of the gelatin. The silver halide emulsion layer was then overcoated with an anti-halation layer. This photosensitive element was photoexposed to a multicolor step wedge and a layer approximately 0.0014 inch thick of processing composition was applied between the antihalation layer and a polyethylene terephthalate spreader sheet. The processing composition comprised:

Sodium hydroxide 4.43 g. Lithium hydroxide 1.48 g. Sodium carboxymethyl cellulose (medium viscosity) 3.13 g. 2,6-dimethyl-4-amino-phenol 0.44 g. Tetramethyl reductic acid 4.71 g. Sodium sulfite 5.17 g. Sodium thiosulfate 9.10 g. 2,4,6-triaminophenol 0.22 g. 6-nitrobenzimidazole 0.69 g. Wetting agent (reaction product of nonyl phenol and glycidol) 2.25 g. Water to make 100 cc.

After about one minute the polyester spreader sheet was removed and the additive color positive transparency in the developed additive color film was projected without separating the silver halide emulsion and image-receiving layers. The neutral column of the additive color transparency exhibited the following transmission densities:

Red Green Blue ______________________________________ Dmax. 2.55 2.65 2.71 Dmin. 0.39 0.35 0.39 ______________________________________

EXAMPLE 3

An additive color diffusion transfer film was prepared substantially as described in Example 2 but without the anti-halation layer, using a silver iodochlorobromide emulsion having a mean grain diameter of 0.94 micron. This emulsion was prepared in a manner similar to that described in Example 1 except that the mole percent bromide (based on silver) added was 85%. The dispersion number for a grain size-frequency distribution curve of this emulsion (shown in FIG. 8b) was 0.33. The silver coverage was 101 mg./ft.2, with a silver-to-gelatin ratio of about 1:1.2. The processing composition used comprised:

Sodium hydroxide 4.44 g. Lithium hydroxide 1.48 g. Sodium carboxymethyl cellulose (medium viscosity) 2.84 g. 2,6-dimethyl-4-amino-phenol 0.45 g. Tetramethyl reductic acid 4.96 g. Sodium sulfite 5.18 g. Sodium thiosulfate 9.11 g. 2,4,6-triaminophenol 0.22 g. 6-nitrobenzimidazole 0.46 g. 2-mercaptobenzothiazole 0.16 g. Wetting agent (reaction product of nonyl phenol and glycidol) 3.75 g. Water to make 100 cc.

The characteristic curves of the red, green and blue densities of the neutral column are reproduced in FIG. 12.

EXAMPLE 4

800 g. of inert bone gelatin was swollen in 8,800 ml. of distilled water for 20 minutes. The temperature was raised to 40° C. and the gelatin was dissolved with agitation. The pH of the gelatin solution was adjusted to a pH of 10.0 with 50% sodium hydroxide. While maintaining the temperature at 40° C., 88 g. of phthalic anhydride dissolved in 616 ml. of acetone was gravity fed into the gelatin solution over a 30 minute period, maintaining the pH at 10.0 with 50% sodium hydroxide. The solution was slowly agitated to 40° C. for another 30 minutes, after which the pH was adjusted to 6.0 with sulfuric acid. A gelatin solution (Solution A) was prepared which comprised 6,000 ml. of distilled water, 2,560 g. of the above-prepared phthalic anhydride derivatized gelatin and 205 g. of potassium chloride. A potassium chloride solution (Solution B) was prepared by dissolving 1,026 g. of potassium chloride in 5,336 ml. of distilled water. A silver nitrate solution (Solution C) was prepared by dissolving 2,000 g. of silver nitrate in 5,336 g. of distilled water. Solution A was heated to 80° C. Solutions B and C were heated to 60° C. and added to Solution A by double jet addition at a rate of 1,750 ml. per minute over 31/2 minutes, maintaining Solution A at 80° C. The resulting mixture was digested 5 minutes at 80° C. After this digestion period, a solution of 1,337 g. of potassium bromide and 40 g. of potassium iodide dissolved in 5,336 ml. of water and heated to 60° C. was added over a period of 31/2 minutes, keeping the temperature at 80° C. The mixture was then digested for 35 minutes at 80° C. After the digestion period, the mixture was cooled to 20° C. and the pH adjusted to about 2.7 with 10% sulfuric acid. The silver halide-gelatin flocculate was washed several times with chilled, distilled water until the conductivity of the supernatant liquid reached 50-100 μmhos. After the last decantation of excess wash 950 950 g. of dry active bone gelatin was added and allowed to swell for 20 minutes. The temperature was then raised to 38°C. and held there for 20 minutes while the gelatin dissolved. After adjusting the pH to about 5.7, the temperature was raised to 54° C. and 24 ml. of a solution of an ammonium gold thiocyanate complex was added as in Example 1. The emulsion was then afterripened at 54° C for 150 minutes. The emulsion was cooled, chilled and set. The resultant silver iodochlorobromide emulsion contained approximately 85 mole percent bromide, 13 mole percent chloride and 2 mole percent iodide. The silver iodochlorobromide grains had a mean diameter of about 0.92 micron, and a grain size-frequency distribution curve of the emulsion had a dispersion number of 0.36. An additive color photosensitive element was prepared substantially as described in Example 2, the silver halide emulsion layer containing the above-prepared 0.92 micron mean diameter silver iodochlorobromide emulsion panchromatically sensitized prior to coating and coated at a coverage of approximately 92.1 mg./ft.2 of silver, 200 mg./ft.2 of gelatin and 4.8 mg./ft.2 of propylene glycol alginate. (No anti-halation layer was present.) Following exposure, the film was processed as in Example 1 using a processing composition comprising: Sodium hydroxide 3.94 g. Sodium carboxymethyl cellulose (medium viscosity) 3.77 g. Sodium thiosulfate 10.07 g. Sodium sulfite 4.08 g. 6-nitrobenzimidazole 0.26 g. toluhydroquinone 3.14 g. 2,4,6-triaminophenol 0.23 g. Wetting agent (reaction product of nonyl phenol and glycidol) 2.28 g. Water to make 100 cc.

The red, green and blue characteristic curves of the neutral column of the resulting additive color transparency are reproduced in FIG. 13. The neutral column exhibited the following transmission densities:

Red Green Blue ______________________________________ Dmax. 3.01 3.26 3.34 Dmin. 0.30 0.36 0.32 ______________________________________

The following example describes the preparation of a predominantly homogeneous grain size silver halide emulsion useful with additive color screens having 750 or fewer color triplets per inch.

EXAMPLE 5

800 g. of inert bone gelatin was swollen in 8,800 ml. of distilled water for 20 minutes. The temperature was raised to 40° C. and the gelatin was dissolved with agitation. The pH of the gelatin solution was adjusted to a pH of 10.0 with 50% sodium hydroxide. While maintaining the temperature at 40° C., 88 g. of phthalic anhydride dissolved in 616 ml. of acetone was gravity fed into the gelatin solution over a 30 minute period, maintaining the pH at 10.0 with 50% sodium hydroxide. The solution was slowly agitated at 40° C. for another 30 minutes, after which the pH was adjusted to 6.0 with sulfuric acid. A gelatin solution (Solution A) was prepared which comprised 6,000 ml. of distilled water, 2,260 g. of the above-prepared phthalic anhydride derivatized gelatin, 101 g. of potassium bromide and 60 g. of potassium iodide. A potassium bromide solution (Solution B) was prepared by dissolving 1,470 g. of potassium bromide in 13,600 ml. of distilled water. A silver nitrate solution (Solution C) was prepared by dissolving 2,000 g. of silver nitrate in 13,600 g. of distilled water. Solution A was heated to 80° C. Solutions B and C were heated to 60° C. and added to Solution A by double jet addition at a rate of 233 ml. per minute for 60 minutes, maintaining Solution A at 80° C. The resulting mixture was cooled to 20° C., and the pH adjusted to 2.7 with 10% sulfuric acid. The silver halidegelatin flocculate was washed with chilled distilled water until the supernatant had a conductivity of 50-100 μmhos. 893 g. of dry active bone gelatin was added and allowed to swell for 20 minutes. The temperature was raised to 38° C. and held there for 20 minutes while the gelatin dissolved. The pH was adjusted to 5.70 with 10% sodium hydroxide and gold sensitizer added as in Example 1. The temperature was raised to 51° C. and the emulsion was afterripened for 180 minutes. The emulsion was cooled to 38° C., a panchromatic optical sensitizer added and the emulsion was digested for 45 minutes before being chilled and set. The resulting silver iodobromide emulsion contained approximately 97 mole percent bromide. The silver iodobromide emulsions had a mean diameter of about 0.93 micron and the grain size-frequency curve reproduced in FIG. 8c had a dispersion number of 0.47. 80% of the silver halide grains had a diameter within + 33% and - 30% of the mean diameter.

As noted above, the present invention is of particular value in providing additive color movie film since it is not necessary to remove the developed silver halide emulsion layer or the applied layer of processing composition. In this embodiment, the film is preferably exposed, developed and projected without being removed from the cassette within which it is provided. The cassette contains a supply reel, a take up reel, a reservoir of processing fluid and a suitable aperture for exposure and projection. After exposure of the complete reel of film, the exposed film is advanced past a fluid application station (cf. container 20 in FIG. 1, Stage B) where a processing fluid is applied as the film is rewound, i.e., returned from the take up reel to the supply reel, with the applied processing fluid confined between film convolutions. After a suitable time lapse following completion of the film rewinding to permit completion of development and transfer image formation, the film is again advanced from the supply reel to the take-up reel past a projection station permitting viewing of the finished additive color movie film. The processing fluid is not removed from the developed film, and the wet developed film is dried during the projection stage. In a typical embodiment of this type, the processing fluid is applied in a layer approximately 0.0005 inch thick, and the elapsed time between the application of processing fluid to the end of the exposed film and the projection of that portion of the film is about 10 inches. It will be understood that the development and transfer image formation should be completed during this time period, notwitstanding the fact that much longer time may elapse between application of the processing fluid and projection of the other end of the strip of movie film. Details of such in-cassette processing are described in a number of patents including U.S. Pat. Nos. 3,608,455; 3,615,127; 3,616,740; 3,643,579 and 3,687,051 to which reference may be made for more detailed descriptions. If desired, of course, such film may be developed and viewed in a continuous operation without an intervening wind up operation by providing a suitable dark storage area for the developing film between the processing fluid application station and the viewing or projection station.

It has been found that diffusion transfer processing in the presence of a quaternary ammonium compound, e.g., N-benzyl-α-picolinium bromide, is advantageous in keeping the maximum density of the developed negative image low. Indeed, there is evidence that the developed silver grains possess smaller projected areas when developed in the presence of a quaternary ammonium salt, particularly where the principal silver halide developing agent is an α,β-enediol such as tetramethyl reductic acid. The following example illustrates this embodiment as well as the utilization of this invention in the formation of additive color movies.

EXAMPLE 6

A transparent polyethylene terephthalate film base bearing an additive color screen composed of approximately 1,000 triplet sets of red, green and blue dyed dichromated gelatin filter lines was prepared by the procedure described in the above-mentioned U.S. Pat. No. 3,284,208. A 1.5 micron layer of "Saran" polyvinylidene chloride polymer (Saran is a trademark of Dow Chemical Co.) was coated over the additive color screen, followed by a 0.5 micron layer of "Formvar" polyvinyl formal polymer (Formvar is a trademark of Shawinigan Products Co.) and an image-receiving layer comprising approximately 4.4 mgs./ft.2 of deacetylated chitin and approximately 0.25 mgs./ft.2 of cupric sulfide. A photosensitive silver halide layer was then applied over the image-receiving layer, using a panchromatic sensitized predominantly homogeneous substituted-halide mixed halide chemically sensitized silver halide emulsion prepared in a manner similar to that described in Example 1, and having a means diameter of 0.86 micron, a grain size-frequency distribution curve (reproduced in FIG. 8d) dispersion number of 0.34, with 90% of the grains having a diameter within the range 0.62 and 1.09 microns or + 27% and - 28% of the means diameter. The silver halide layer contained approximately 120 mg./ft.2 of gelatin and approximately 98.5 mgs./ft.2 of silver and about 0.2% propylene glycol alginate by weight of the gelatin. The silver halide emulsion layer was then overcoated with an anti-halation layer containing 250 mg./ft.2 of gelatin. This photosensitive element was slit to Super 8 movie size, perforated and placed in a cassette similar to those described in the above-mentioned patents. Following photoexposure, the following processing composition was applied over the anti-halation layer at a coverage of approximately 1.15 g./ft.2 and the film wound on itself on the take-up reel: Sodium hydroxide 6.87 g. Hydroxyethyl cellulose (Natrosol 250, high viscosity) 0.69 g. Tetramethyl reductic acid 10.83 g. Sodium sulfite 2.01 g. Sodium thiosulfate 12.03 g. Potassium bromide 0.84 g. N-benzyl-α-picolinium bromide (50% solution) 1.55 g. Water to make 100 cc.

About 10 seconds after the processing composition was applied to the end of the exposed film, the film was rewound onto the supply reel and then projected without washing or removing the applied processing composition. A high quality additive color movie film was obtained. The red, green and blue characteristic curves of the neutral column in the thus processed additive color movie are reproduced in FIG. 14 and have the following transmission densities:

Red Green Blue ______________________________________ Dmax. 2.01 2.35 2.47 Dmin. 0.25 0.25 0.25 ______________________________________

The sodium thiosulfate used in the above examples was the pentahydrate.

In a particularly useful embodiment, the additive color transparency film includes an anti-halation layer as the outermost layer distal from the transparent support. The provision of such anti-halation layers in diffusion transfer additive color film is the subject of the copending application of Edwin H. Land, Ser. No. (383,261), filed concurrently herewith. The anti-halation dyes are selected for their ability to be rendered colorless by the processing composition, e.g., by contact with sodium sulfite. The transmission density of the anti-halation layer used in the above examples was about 0.5 to 0.6 to red, green and blue light. In Examples 2 and 3 above, the anti-halation dyes were disposed in a layer of cellulose acetate hydrogen phthalate, while gelatin was used as the binder in Example 6. The anti-halation layer has been found to provide extended color separation, particularly in the toe portion of the positive image characteristic curve. The anti-halation layer may also, and when used in the above examples did, include a noble metal image stabilizing agent, i.e., a substantially waterinsoluble gold compound, of the type described in U.S. Pat. No. 3,704,126 issued Nov. 28, 1972 to Edwin H. Land, Stanley M. Bloom and Leonard C. Farney.

While the additive color film has been illustrated as including an additive color screen, it will be understood that the inventive concepts herein disclosed may be utilized in combination with lenticular screens to obtain additive color images.

The prior art fails to disclose the desirability or advantages of using a predominantly homogeneous grain size silver halide emulsion as herein disclosed and claimed. Indeed, the prior art, as illustrated by the previously cited U.S. Pat. No. 3,536,488, has considered it to be desirable to use silver halide emulsions having grain diameters ranging from about 1 to 3.5 microns, in contrast to the 0.7 to 1.5 and preferably 0.7 to 1.0 micron mean diameter homogeneous grain size silver halide emulsions herein contemplated.

Since certain changes may be made in the above product and processes without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description or shown in the accoompanying drawing shall be interpreted as illustrative and not in a limiting sense.