Photochemical investigations of the photographic plate

Abstract

Many theories of the latent image and of the means by which light impressions are produced in the photographic plate have been developed from time to time, but most of these theories are so indefinite that they cannot be tested experimentally, and are of no use in forecasting what will happen under fixed conditions. A quantitative theory was put forward by Hurter and Driffield, and their theory was further advanced by Channon. Some of the difficulties inherent in this theory have been pointed out by Bloch and Renwick and by Channon, Renwick, and Storr. There is no need to go into details of this theory of Hurter and Driffield, as the experiments described below show that two of their fundamental assumptions are not true. The gelatine dry plate consists of a skin or pellicle of dried gelatine, containing particles of silver bromide or a mixture of silver bromide and silver iodide. The average commercial plate contains about 0·2 gramme of gelatine and 0·1 gramme of silver bromide per square decimetre of surface. On such plate the coating of dried gelatine is about 25 microns (micron = μ = 0·001 mm.) thick, and if the plate is a process plate containing grains about 0·7 μ in diameter, there will be about three and a half layers of grains in the coating. If it is a fast plate with grains varying in size up to 2 μ or 3 μ, it will contain only one to two layers of particles. These layers are of course distributed at random through the film.