p072fig02.17 Brightness constancy.<BR>||  Vision: brightness constancy, contrast normalization.  Compute RATIOS of reflected light.  Reflectance processing. p72c1h0.45 "... In other words, the perceived brightness of the gray disk is constant despite changes in the overall illumination. On the other hand, if only the gray disk were illuminated at increaing intensities, with the annulus illuminated at a constant intensity, then the gray disk would look progressively brighter.