Recording, Amplification and Discrimination in Silver Halide Imaging

Abstract

The photographic process is de-scribed bp a unified theory that links light detection to chemical amplification. In this theory, named the Photoelectrochemical (PEC) model, exposure to light or chemicals is recorded in the silver halide grain as a change in its energy level. This concept was derived from the well known experimental fact that photographic sensitivity and the redox environment of the grains are closely correlated. The model considers environmental influences on grain defects, termed oxidized states. The amplified detector quantum sensitivity is derived to be inversely proportional to the number of oxidized states in the grains. Oxidized states are therefore concluded to be the main detector loss channel and sensitization is taken to be the process of decreasing their number. Amplification is controlled, by the grain's energy level through the induction period of development. Fog (or noise) is described and quantified by the newly introduced concept of amplified image to background discrimination, where the chemical developer differentiates in an analogue manner. As a consequence, the presence of a silver speck is not a decisive, digital requirement for the amplification process to be initiated. The highest photographic quantum sensitivity of two photons/grain is predicted, limited by photonic noise. Quantitative agreement between the PEC-model’s predictions and published experimental data is presented in the case of polyvalent metal doping and sulphur plus gold-hydrogen hypersensitized silver bromide emulsions. The model is used to examine current shortcomings of latent image theories from a different viewpoint.