The Relationship between Speed and Grain Size

Paper presented at a symposium on “Factors Limiting Photographic Performance” organized by the Royal Photographic Society on 11-13 Dec. 1968 in London.

Abstract

On the assumption that the average size of latent-image speck required for developability, and the average number of absorbed quanta required to form a developable speck, are independent of grain size, it follows that speed should increase indefinitely with grain size.

Speed/grain-size relationships for experimental series of emulsions have been derived and compared with theoretical trends. These comparisons show that the number of quanta per grain required for developability steadily increases with size within the normal size range. In one series this led to an optimum and then a decrease of speed despite increasing grain size.

On exposure of emulsions to X-rays of such energy as to render one grain developable for each quantum absorbed, speed increases up to the largest grain size. In this case the number of electrons released by an X-ray quantum is such that large inefficiencies in formation of a developable latent-image speck can be tolerated. Excellent agreement between theoretical and experimental speed/grain-size relationships for direct X-rays provides justification for the assumptions made in calculating speed/grain-size relationships for light exposures.

On the basis of experimental evidence several explanations for the decrease of quantum sensitivity at large grain-sizes are rejected. The only feasible type of explanation is one in which the latent-image silver is dispersed over a number of sites at the grain surface. Such an effect may arise in part because the grain size becomes large compared with the average diffusion distance of an electron from its point of release.

Notes

Paper presented at a symposium on “Factors Limiting Photographic Performance” organized by the Royal Photographic Society on 11-13 Dec. 1968 in London.