In referring to silver halide grain structures and emulsions containing two or more halides the halides are named starting with the halide present in the lowest concentration and ending with the halide present in the highest concentration.
Emulsions containing grains formed of silver combined with chloride, bromide, iodide or any possible mixture of these halides are known to form latent images upon exposure to visible or near ultraviolet radiation and can be employed for photographic purposes. The photographically least used silver halide grain compositions are high iodide (&gt;90 mole percent iodide, based on silver) silver halides. The iodide ion released during development inhibits further development and, unless addressed in developer formulation, can effectively arrest grain development. Although the remaining silver halide compositions have application specific advantages, the most widely employed photographic emulsions are silver iodobromides, since they exhibit the highest attainable levels of photographic sensitivity and exhibit superior speed-granularity relationships. The incorporation of iodide ions into the face centered cubic silver bromide crystal lattice of the grains has been demonstrated to enhance the efficiency of latent image formation. Silver iodobromide emulsions are almost universally used for camera speed photography.
Kofron et al U.S. Pat. 4,439,520, Wilgus et al U.S. Pat. 4,434,226 and Solberg et al U.S. Pat. 4,433,048 were the first to point out the photographic advantages of tabular grain silver iodobromide emulsions. Among numerous advantages resulting from the tabular shape of the grains were improvements in photographic sensitivity and speed-granularity relationships. As clearly shown in FIG. 1 of Wilgus et al and FIG. 3 of Kofron et al, the iodobromide tabular grain emulsions contained a variety of grain shapes and exhibited considerable variance among the tabular grains. The tabular grain emulsions were not monodisperse, regardless of whether the entire grain population or only the tabular grain population is considered in assessing grain dispersity.
Tsaur et al U.S. Pat. No. 5,210,013 discloses the preparation of highly monodisperse tabular grain silver bromide and iodobromide emulsions. Tabular grain emulsions are disclosed exhibiting a coefficient of variation (hereinafter also referred to as COV) of less than 10 percent based on the total grain population. The tabular grain emulsions of Tsaur et al exhibit a higher level of monodispersity than previously realized in preparing tabular grain emulsions. In comparative Example 14 Tsaur et al remade the most monodisperse of the emulsions reported by Saitou et al U.S. Pat. No. 4,797,354 to demonstrate that the total grain dispersity (COV) of the emulsions of Saitou et al are in excess of 30 percent.
It is realized by those skilled in the art that the iodide content of silver iodobromoide grains can be uniformly distributed or varied. A variety of iodide placements in tabular grain emulsions are known. The varied forms of iodide placement are illustrated by Solberg et al, cited above; Sugimoto et al U.S. Pat. No. 4,665,012; Hayakawa et al U.S. Pat. No. 4,883,748; Saitou U.S. Pat. No. 4,945,037 and Aida et al U.S. Pat. No. 4,962,015.
Tsaur et al, cited above, in Examples 3 and 4 prepares silver iodobromide tabular grain emulsions by a procedure in which iodide concentrations are increased following the formation of host tabular silver bromide grains.