Photographic sensitivity and the reciprocity law at low temperatures

Abstract

The experiments described below were undertaken in order to examine a theory by Gurney and Mott (1938) on the formation of the latent image. The theory adopts the old conception, that the latent image consists of a speck of silver, which is produced from the silver halide by a two-stage process. The first stage consists in the lifting of an electron from a bromine ion into the conductivity band in which the electron moves until it is trapped by a region of low potential, the “sensitivity speck”. The second stage consists in the movement of the positive silver ions up to the charged sensitivity speck. The ionic movement should depend on temperature as e - E/kT and one should therefore expect the sensitivity of photographic materials almost to disappear at sufficiently low temperatures. Experiments at liquid air temperature have been made by several investigators (see, for instance, Sheppard, Wightman, and Quirk 1934). The sensitivity found is of the order of 5‒10 % of room temperature sensitivity and appears surprisingly high in view of the above considerations. It seemed desirable to lower the temperature still further by using liquid hydrogen and to test in particular for the effect of change in intensity and time, while keeping the exposure, i. e. The product of intensity with time, constant. A great amount of work has been done on the reciprocity failure of sensitive materials, by which is meant the fact that for a constant exposure the density obtained after processing depends on the intensity level at which the exposure was given. (Webb 1933 a , b ; Jones and Webb 1934). It appears that for every material there is an optimum rate of reception of energy. The position of the optimum depends on temperature (Webb 1935), and for this reason alone the investigation of the reciprocity failure at as low a temperature possible seemed desirable. It was hoped that some information on the mechanism of the latent image formation could be obtained in this way.