The Sensitometric Properties of Emulsions at a Low Level of Chemical Sensitization*

Abstract

This review relates to the usual type of unsensitized emulsion (e.g., ammoniacal, neutral) in which the internal-image sensitivity is comparable with or superior to the surface-image sensitivity. Tor short exposure times (e.g., 10-4 sec.) and surface-image development such emulsions possess D vs. rel. Log E curves which rise smoothly to a density representing all or most of the grains. At long exposure times (e.g., 103 sec.) the curves are of stepped form, and heavy exposures, of the magnitude leading to re-reversal on normally sensitized emulsions, are required to render the less sensitive grains developable. Studies of double-exposure effects, and of the influence of halogen acceptors, indicate that the difficulty in forming surface image on many grains at long exposure times arises from preferential aggregation of silver at internal sites, and the inhibition resulting from the halogen evolved in forming this internal silver. Unsensitized emulsions frequently possess a surface-image reciprocity-failure curve having two maxima at low reference densities, but exhibiting simple low-intensity failure at higher densities. The same basic effect may be revealed as a small degree of low-intensity failure at low reference densities abruptly increasing to pronounced failure at moderate densities. Various experimental results indicate that the high-intensity maximum is associated with the most efficiently sulphur-sensitized grains (at the existing low level of sensitization), while the low-intensity maximum is provided by a different group of grains which have been sensitized primarily by reduction.