Three faces of the cube were painted white and the other three were painted different shades of gray. When the observer sees three white faces, the appearance of illuminations is being measured. When the three visible faces have different reflectances, then the appearance of objects and illuminations is being measured. The results of matching experiments show that humans make the same match for radiance changes caused by illumination as for those caused by reflectance. The results are consistent with the hypothesis that maxima in the field of view change slowly with radiance and that all areas darker than the maxima change rapidly with radiance.
Humans can successfully identify changes in whites due to illumination.
They also can identify changes due to reflectance of darker objects. However, they cannot discriminate between dark appearances caused by either illumination or reflectance.