Limits of lightness identification for real objects under natural viewing conditions.

We examined whether observers veridically perceive the reflectances of real objects under natural viewing conditions. A new forced-choice paradigm was used to measure observers' abilities to identify (not match) the reflectance of randomly crumpled gray papers across two levels of illumination, and also to simultaneously measure brightness discrimination thresholds for the same objects. Accuracy of lightness identification differed qualitatively among observers. By explicitly manipulating observer strategies, we show that when observers use brightness dissimilarity, their performance is similar to lightness identification. A brightness adaptation model simulates how instead of extracting lightness, observers can rely on perceived relative brightness to achieve the measured degrees of lightness identification.