theory about how changes in the illuminant affect the color appearance of objects must specify how the visual
system's adjustments to illuminant changes vary with the surface collection in a scene. I report an experiment
designed to investigate this issue. The stimuli were CRT simulations of flat matte surfaces rendered under
diffuse illumination. For all combinations of 7 daylight illuminants and 12 collections of surface reflectances
the subject's achromatic locus was measured on an isoluminant plane in color space. For any surface collection
the changes in the achromatic locus could be well approximated by a linear transformation of the illuminant
changes. These linear transformations showed relatively small variation with the surface collection. To first
approximation, these results suggest that the effect of changes in the illuminant on color appearance can be
described linearly and that it can be separated from the surface collection. There was an effect of the surface
collection on the achromatic locus. The data rejected the idea that a surface collection's mean reflectance
function might capture this effect, ruling out models of color appearance that are based on this kind of averaging
assumption.