Photographic Aspects of the Theory of Three-Color Reproduction*

The calorimetric and photographic principles of additive color reproduction are reviewed. The significance of the concept of photographic spectral sensitivity is examined, and the desirability of emulsions having contrast independent of wave-length is emphasized. Although the desirability of partially negative spectral sensitivities, such as described by Hardy and Wurzburg, is acknowledged, the photographic methods proposed by those authors for the realization of such sensitivities are shown to be rendered useless by fundamental difficulties. Two masking methods are described, one of which utilizes unusual types of emulsions to realize completely the desired partially negative spectral sensitivities, and the other of which utilizes conventional materials and techniques to realize a close approximation to the desired sensitivities. The errors of additive color reproductions resulting from several different compromises in which negative portions of the spectral sensitivity curves are ignored or avoided are computed and compared. The simple omission of all sensitivity in the wave-length regions where the theory calls for negative sensitivity, utilizing only the positive portions of the theoretical spectral sensitivity curve, results in the smallest errors. Finally, the effect of increase of contrast is examined, with the conclusion that purity can be increased, compensating for the loss of purity resulting from the failure to realize partially negative spectral sensitivities, but that considerable errors in dominant wave-length as well as in brightness result from the excessive contrasts.