Relative luminance is fundamental to lightness perception, but can be used to predict specific lightness values only when coupled with an anchoring rule. Empirical results indicate that, in simple displays, lightness is anchored by the highest luminance (white) rather than by the average luminance (middle gray). This implies that increasing the luminance range of a stimulus causes grayness induction: the lower luminance values become darker gray while the highest luminance remains unchanged, as many so-called brightness induction experiments have shown. Yet sometimes increasing the luminance range of a stimulus causes luminosity induction: the highest luminance becomes increasingly self-luminous while the lowest luminances remain unchanged. Whether grayness induction or luminosity induction results from an increase in stimulus contrast depends on relative area. A few simple, yet hitherto unrecognised, rules that describe how anchoring by highest luminance combines with anchoring by largest area appear to be consistent with the many published reports on area and lightness/brightness. These findings add to the accumulating evidence that many phenomena previously attributed to contrast are much better understood in terms of anchoring.
Robust brightness enhancement across a luminance range of the glare illusion