An investigation has been made of the distribution of development centers among the grains of a simple experimental emulsion and of the influence of sulfur sensitization on this distribution. The distribution of image centers among the grains which have not been deliberately chemically sensitized is not random, but shows instead a strong preference for only one center per grain, regardless of the intensity of the exposing light. For low-intensity exposure, the same emulsion after sulfur sensitization still shows only one development center per grain. At higher intensities, the number of development centers increases. The transition in the distribution of centers in the sensitized emulsion coincides with the onset of high-intensity, reciprocity-law failure. At very high intensity, the observed distribution agrees with the Poisson formula for a random distribution. Latensification with a gold solution, which eliminates high-intensity, reciprocity-law failure, does not affect the form of the speck distribution. This difference resulting from sulfur sensitization is most easily explained in terms of an increase in stability at the sites where latent-image centers form, e.g., an increase in electron trap depth.
Although the emulsion does not solarize, an examination of the development centers reveals that prolonged, low-intensity exposure of the sulfur-sensitized emulsion causes regression of all but one center per grain.
H. E. SPENCER, L. E. BRADY, and J. F. HAMILTON, "Study of the Mechanism of Sulfur Sensitization by a Development-Center Technique," J. Opt. Soc. Am. 54, 492-497 (1964)