The spectral power distribution of the light that reflects from a surface to the eye depends both on the reflectance function of the surface and on the spectral power distribution of the illuminant The human visual system actively adjusts to reduce the dependence of surface color appearance on the illumination. We use a matching paradigm to measure the change in the cone coordinates of a color signal necessary to maintain constant color appearance across an illuminant change. If the visual system made no adjustment, the measured cone coordinate change would be zero. If the visual system were perfectly color constant, the measured change would compensate exactly for the physical change in cone coordinates due to the change in illuminant
It is not possible to measure the visual system’s adjustment to all combinations of illuminant changes and surfaces. Therefore we develop and test a model of this adjustment. In our laboratory experiments two variables govern the adjustment: the cone coordinates of the reflected color signal and the change in the spectral power distribution of the illuminant We model the visual system’s adjustment using a bilinear function. We determine the parameters of the bilinear function from a small number of measurements. We show that the bilinear model predicts the visual system’s adjustment to many other surface and illuminant combinations.