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Alan L. Gilchrist, Suncica Zdravkovic; Highest luminance defines illumination level as well as lightness. Journal of Vision 2002;2(7):553. doi: https://doi.org/10.1167/2.7.553.
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Converging evidence indicates that lightness is anchored by the highest luminance in a framework of illumination, not the average luminance. New findings indicate that the perceived level of illumination in the framework is also defined by the highest luminance and not the average luminance. We projected a spotlight across the upper half of a display consisting of five adjacent dark gray rectangles. Thus an illumination boundary with an obvious penumbra divided the display into a spotlit upper half and a roomlit lower half. Spotlight intensity was adjusted to equate the luminance of the lightest (middle gray) rectangle in the spotlight with a white surface in room light. Observers reported seeing a shadow on the lower half of five light gray rectangles, not a spotlight on the upper half of five dark gray rectangles. Thus, the part of the display that shared the same highest luminance with the room was seen as having the same illumination level as the room. For each rectangle, lightness matches made to the upper half and the lower half were very different. But matches made to each rectangle as a whole (by a separate group of observers) agreed with the matches for the spotlit parts, further indicating that the spotlit region was in fact seen as ambient illumination. Several additional experiments confirm that, in a 3D scene, separate planes appear to be equally illuminated when they are equal in highest luminance, not average luminance.
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