Heuristic analysis of von Kries color constancy

If the eye had three photopigments with narrow, nonoverlapping spectral sensitivities, integrated-reflectance (von Kries) adaptation could achieve perfect color constancy. However, human spectral sensitivities are broad and overlapping, which impairs constancy.1 A broad receptor sensitivity enables two lights to be metameric to that receptor system (i.e., two lights differing over the bandwidth of the receptor can produce the same response). Lights that are metameric to a trichromatic observer must be metameric to at least one receptor system. Hence the breadth of receptor sensitivities permits metamerism. When this takes the form of object-color metamerism, it will limit constancy by any mechanism. If the receptor sensitivities were broad but nonoverlapping, restricting the illuminant to three narrow bands2 in the respective spectral-sensitivity domains would eliminate object-color metamerism and would also permit von Kries color constancy. However, an overlap in human spectral sensitivities incurs off-diagonal elements in the (invertible) transformation between object reflectance and receptor stimulus. Hence, adjustment of a three-narrow band illuminant cannot be compensated by a von Kries (= diagonal) transformation. Since human red and green spectral sensitivities overlap greatly, the red-green opponent system shows mediocre color constancy even for three-band illuminants.1