The variation of photographic sensitivity (as measured by the index 10/ E m) with different light sources of equal visual intensity, but having different distributions of energy, was experimentally obtained. Ordinary, orthochromatic, and three new fast panchromatic plates were investigated. Distinction was made between two types of sensitivity comparisons. Thus it was found that, first, the ratio of the sensitivity of any one of the panchromatic plates to that of the ordinary plate was greater with "incandescent" lighting than when " sunlight" was used, and second, in all cases the panchromatic plates were less sensitive to " incandescent" lighting than to "sunlight." Approximate factors are included (for the particular emulsions studied) by which visual exposure meter readings should be multiplied when certain types of illuminants are encountered. CONTENTS Page I. Introduction 843 II. Procedure and results 844 III. Discussion of results 846 I. INTRODUCTION The sensitivity of photographic emulsions to various light sources having different spectral energy distributions has recently become of considerable interest. This is due, in part, to the discovery of new dyes which make possible increased sensitivity to the red end of the spectrum. Artificial illuminants, particularly incandescent lamps, have a relatively large amount of energy in the longer wave lengths. With such illumination, therefore, it is natural to expect the increase in red sensitivity to make the new emulsions more efficient than former ones, i. e., less exposure will be required to obtain the same photographic effect. Comparisons of sensitivity may be made in one of two ways. First, one may compare the sensitivity of two different emulsions using the same source. Second, the sensitivity of an emulsion to one source may be compared with its sensitivity to another source of equal "intensity" but having a different relative spectral energy distribution. 1 The question arises as to whether equal "intensity" should be taken to mean equal energy or equal visual intensity. That the two would not be necessarily the same is obvious. In certain problems, such as, for example, determining the type of illuminant which is * The terms "relative spectral energy distribution" and "energy distribution" have the same meaning in this paper.
