Cone ratios cannot discriminate between reflectance and illuminant borders

Invariance of cone ratios has been proposed to be a possible basis for perceptual color constancy. Cone excitation ratios were computed across reflectance edges viewed under different illuminants as well as across illuminant edges. The results of the authors' computations suggest that such a mechanism is incomplete. Cone ratios maintain color constancy across reflectance edges under a variety of illuminants, yet they are unable to discern whether a border is a reflectance edge or an illuminant edge.