The Nature of Photographic Sensitivity and the Mechanism of Formation of the Latent Image

Abstract

In a new physical-chemical theory of the photographic process in silver-halide emulsions, the model system is regarded as a supersaturated solid solution of metallic silver in the silver halide. The silver arises mainly as the result of chemical sensitization, while the existence of an energy barrier to the formation of nuclei of a new solid silver phase causes supersaturation. This barrier leads to the appearance of two different types of silver speck: primary centres (silver atoms and aggregates of a few silver atoms), and secondary centres (colloid silver particles).The energy barrier can be overcome and silver nuclei of a critical size formed in two different ways: (a) by the absorption of photon energy to give latent-image silver, and (b) by spontaneous fluctuations.Contradictions appearing in the older theories of the photop,raphic process are reviewed and it is shown that they are successfully dealt with by the new theory, the various effects considered arising from the interplay of factors influencing the degree of supersaturation and the lowering or surmounting of the barrier to nucleation.