Discrimination information in natural radiance spectra
Wilson S. Geisler

Vision is based upon detecting and identifying differences in the radiance spectra arriving at the eye or camera from different directions. Radiance spectra can differ in total radiance (intensity), in shape (color), or both. Ideal-observer theory was used to measure the relative amounts of intensity and chromatic information available to discriminate pairs of natural radiance spectra created by illuminating natural surfaces (Krinov, 1947) with a natural daylight source (D65). Two ideal observers were evaluated -- one operating at the input to the eye or camera, and one at the level of the photopigments in the human cone photoreceptors. The relative amount of intensity and chromatic information in each discrimination pair was quantified by the effective increase in contrast due to the chromatic information. The analyses showed that chromatic information increases the effective contrast by an average of 30% to 70% at the input to the eye, but only by 5% to 10% at the level of the photopigments. It appears that much of the chromatic information in natural radiance spectra is lost between the cornea and photopigments, for the purpose of discriminating between regions or detecting region boundaries, is usually small relative to intensity information. (This research was supported in part by National Institutes of Health grant EY02688 and AFOSR grant F49620-93-1-0307 )