The highest speed photographic emulsions are recognized to be silver bromoiodide emulsions. Because of their larger size, the presence of iodide ions in the silver bromide crystal structure of the grains is recognized to produce lattice irregularities that enhance latent image formation (observed as increased imaging sensitivity) on exposure to electromagnetic radiation.
Silver halide photography has benefited in this decade from the development of tabular grain silver bromoiodide emulsions. As employed herein the term "tabular grain emulsion" designates any emulsion in which at least 50 percent of the total grain projected area is accounted for by tabular grains. Whereas tabular grains have long been recognized to exist to some degree in conventional emulsions, only recently has the photographically advantageous role of the tabular grain shape been appreciated.
Tabular grain silver bromoiodide emulsions exhibiting particularly advantageous photographic properties include (i) high aspect ratio tabular grain silver halide emulsions and (ii) thin, intermediate aspect ratio tabular grain silver halide emulsions. High aspect ratio tabular grain emulsions are those in which the tabular grains exhibit an average aspect ratio of greater than 8:1. Thin, intermediate aspect ratio tabular grain emulsions are those in which the tabular grain emulsions of a thickness of less than 0.2 .mu.m have an average aspect ratio in the range of from 5:1 to 8:1.
The common feature of high aspect ratio and thin, intermediate aspect ratio tabular grain emulsions, hereinafter collectively referred to as "recent tabular grain emulsions", is that tabular grain thickness is reduced in relation to the equivalent circular diameter of the tabular grains. Most of the recent tabular grain emulsions can be differentiated from those known in the art for many years by the following relationship: EQU ECD/t.sup.2 &gt;25 (1)
where
ECD is the average equivalent circular diameter in .mu.m of the tabular grains and PA0 t is the average thickness in .mu.m of the tabular grains. The term "equivalent circular diameter" is employed in its art recognized sense to indicate the diameter of a circle having an area equal to that of the projected area of a grain, in this instance a tabular grain. All tabular grain averages referred to are to be understood to be number averages, except as otherwise indicated. PA0 AR is the average tabular grain aspect ratio and PA0 ECD and t are as previously defined, PA0 R-1: U.S. Pat. No. 4,414,304, Dickerson; PA0 R-2: U.S. Pat. No. 4,414,310, Daubendiek et al; PA0 R-3: U.S. Pat. No. 4,425,425, Abbott et al; PA0 R-4: U.S. Pat. No. 4,425,426, Abbott et al; PA0 R-5: U.S. Pat. No. 4,434,226, Wilgus et al; PA0 R-6: U.S. Pat. No. 4,439,520, Kofron et al; PA0 R-7: U.S. Pat. No. 4,478,929, Jones et al; PA0 R-8: U.S. Pat. No. 4,672,027, Daubendiek et al; PA0 R-9: U.S. Pat. No. 4,693,964, Daubendiek et al; PA0 R-10: U.S. Pat. No. 4,713,320, Maskasky; and PA0 R-11: Research Disclosure, Vol. 299, Mar. 10, 1989, Item 29945. PA0 R-12: U.S. Pat. No. 4,433,048, Solberg Piggin et al. PA0 R-13: U.S. Pat. No. 4,806,461, Ikeda et al to the extent pertinent is considered essentially cumulative with Solberg Piggin et al. PA0 R-14: Japanese Kokai SHO 63[1988]-106746, Shibata et al PA0 ECD is the mean effective circular diameter in .mu.m of the tabular grains and PA0 t is the mean thickness in .mu.m of the tabular grains
Since the average aspect ratio of a tabular grain emulsion satisfies relationship (2): EQU AR=ECD/t (2)
where
it is apparent that relationship (1) can be alternatively written as relationship (3): EQU AR/t&gt;25 (3)
Relationship (3) makes plain the importance of both average aspect ratios and average thicknesses of tabular grains in arriving at preferred tabular grain emulsions having the most desirable photographic properties.
The following illustrate recent tabular grain silver bromoiodide emulsions satisfying relationships (1) and (3):
Research Disclosure is published by Kenneth Mason Publications, Ltd., Dudley Annex, 21a North Street, Emsworth, Hampshire P010 7DQ, England.
The recent tabular grain emulsions have been observed to provide a large variety of photographic advantages, including, but not limited to, improved speed-granularity relationships, increased image sharpness, a capability for more rapid processing, increased covering power, reduced covering power loss at higher levels of forehardening, higher gamma for a given level of grain size dispersity, less image variance as a function of processing time and/or temperature variances, higher separations of blue and minus blue speeds, the capability of optimizing light transmission or reflectance as a function of grain thickness, and reduced susceptibility to background radiation damage in very high speed emulsions.
It has been recognized that still further improvements in emulsion sensitivity without any increase in granularity can be realized by forming recent tabular grain silver bromoiodide emulsions with iodide non-uniformly distributed within the grains. This is illustrated by the following patent:
Solberg Piggin et al, which contains teachings compatible with and in most instances forming a integral part of the teachings of R-1 to R-11 inclusive, discloses forming tabular grain emulsions with a lower proportion of iodide in a central region of the tabular grain structure than in a laterally offset region. When iodide concentrations are progressively increased as the grains are grown, the central region preferably forms a minor part of the tabular grain. On the other hand, with abrupt differences in iodide concentrations between the central and laterally displaced regions, the central region preferably forms the major portion of the tabular grain.
Investigations of tabular grain silver bromoiodide emulsions prepared according to the teachings of Solberg Piggin et al prepared by abruptly increasing iodide to form laterally displaced regions of the tabular grains has revealed that at least a portion of the iodide redistributes itself over the major faces of the tabular grains. Thus, higher iodide silver bromoiodide surface laminae have been identified on the tabular grains of these emulsions.
While the recent tabular grain emulsions have advanced the state of &he art in almost every grain related parameter of significance in silver halide photography, one area of concern has been the susceptibility of tabular grain emulsions to vary in their photographic response as a function of the application of localized pressure on the grains. As might be intuitively predicted from the high proportion of less compact grain geometries in the recent tabular grain emulsions, pressure (e.g., kinking, bending, or localized stress) desensitization, a long standing concern in silver halide photography, is a continuing concern in photographic elements containing recent tabular grain silver bromoiodide emulsions.
It is suggested by
that the pressure sensitivity of emulsions with average aspect ratios of greater than 2:1 can be reduced by forming silver halide laminae of differing halide content on the major faces of the grains. A tabular grain silver bromoiodide emulsion with higher iodide levels in the tabular grain laminae prepared under the closest temperature and pAg conditions to those of the present invention is EM-5. As demonstrated by the Examples below, EM-5, shown in FIG. 1 as point R-14, is clearly outside the range of preparation conditions yielding emulsions of improved constancy of sensitivity as a function of pressure applied. In most instances Shibata et al formed tabular grain laminae at much higher excesses of halide ion (higher pAg levels).