RANK ORDERINGS OF PHOTORECEPTOR PHOTON CATCHES FROM NATURAL OBJECTS ARE NEARLY ILLUMINANT-INVARIANT

To examine the potential contribution of first-stage photoreceptor adaptation to color constancy, photon catches from 337 natural objects illuminated with phases of daylight and tungsten light were calculated for a model human fovea. The rank ordering of these photon catches within each cone photoreceptor class was examined. When first-stage adaptation is modeled using multiplicative or subtractive mechanisms, or a monotonic nonlinearity, no reordering of the rank ordering of photon catches is possible across illuminant changes. The observed rank orderings remained nearly invariant across illuminant changes for all three photoreceptor classes, although there was some local shifting in the rank orderings, thus ruling out the ability of von Kries adaptation alone to produce perfect color constancy. This means that for objects with natural reflectance spectra, the ordinal relationships between the photon catches within a class of photoreceptors exhibit only minor changes for these illuminant shifts. This result may be attributable to the fact that approx. 95% of the variance in these reflectance spectra is captured by the first principal component; objects that produce relatively few absorptions under one phase of daylight illumination will also produce relatively few absorptions under another phase. A geometric formalism for understanding these relationships is presented, and limitations on this analysis are discussed.