The Effect of Light-Absorption on the Shape of the Photographic H and D Curve*

Abstract

A theoretical treatment is first given to show how the gamma-value of the photographic H. and D. curve varies with the light absorption in an emulsion. The treatment is based on the idea that an emulsion can be subdivided into a number of layers and that the total H. and D. curve of the entire emulsion can be obtained by summation of the elementary H. and D. curves for the separate layers. The absorption of the emulsion is expressed in terms of the ratio, logIT/IB, where IT and IB represent the intensities of photographically active light in the top and bottom layers, respectively, of the emulsion. Theoretical curves giving γ/Dmax as a function of the ratio, logIT/IB, are given for several widely different types of blue-sensitive emulsions. The curves are compared with experimental measurements carried out by varying the wave-length of the exposing light. The experimental results are found to be in fair agreement with the theoretical curves.The quantity, logIT/IB, can be varied also by varying the emulsion thickness, keeping the absorption coefficient of the emulsion constant. Under these conditions, this theory gives curves for the variation of γ/Dmax as a function of emulsion thickness. The curves are accordingly tested, using different layer thicknesses of emulsion, and the results are again found to be in agreement with the theoretical curves.Finally, it is shown, by assuming that the maximum density of an emulsion varies linearly with emulsion thickness, that curves predicting gamma as a function of emulsion thickness can be obtained. Curves of this type are derived and tested by experiment.