Simultaneous constancy, lightness, and brightness.

An achromatic surface in a complex scene has both an apparent reflectance attribute (lightness) and an overall intensitive attribute (brightness). We studied changes of these two attributes as a function of changes in illumination level and pattern complexity. Subjects observed simultaneously two arrays of simulated achromatic surfaces with identical reflectance distributions. The left-hand array (standard) was always illuminated at a moderate level. The right-hand array (test) had different illuminances from trial to trial. The subjects adjusted patches in the test array to match the corresponding patches in the standard array in either lightness or brightness. In complex patterns (32 grays) lightness constancy was nearly perfect; test reflectance settings were invariant over illuminance. In disk/annulus patterns (two grays), the lightness-match data confirmed previously published reports. At high illuminances, the standard patches could be matched with a smaller range of test-array reflectances than at low illuminances, i.e., lightness constancy was imperfect. Brightness matches varied substantially as a function of illuminance in all conditions.