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Alexander D Logvinenko; Is luminance contrast necessary to perceive lightness?. Journal of Vision 2003;3(9):417. doi: https://doi.org/10.1167/3.9.417.
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The same luminance border can be perceived as a lightness edge homogeneously illuminated, or an illumination edge (a shadow) over a surface of homogeneous lightness (e.g., Logvinenko & Menshikova, 1994 Perception 23, 1007–1023). In this report I show that such a phenomenon can be observed even when contrast of the edge equals zero.
Observers looked at a grid of vertical strips made of white paper which was presented against a large black background the illumination of which was set so as to make it equiluminant (100 cd/m2) with the strips. The strips looked white but dimly lit. The background looked dark grey but highly illuminated. This is a typical “lightness constancy” result. The Munsell scale was used to measure the lightness of both.
So, two adjacent equiluminant areas (with no luminance contrast between them) were perceived as areas of different lightness and different apparent illumination. In other words, zero luminance contrast invokes a lightness edge and an apparent illumination edge, which cancel each other. It follows that luminance contrast is not necessary for perceiving lightness.
Note that a pattern of apparent illumination (as one of the “intrinsic images”) was rather different from the luminance pattern. Hence, segmentation into strips and background was not based on the luminance contrast between them (as it was zero). The strips and the background emerged as different areas because of the abundant illumination cues that signalled that there were two differently illuminated areas.
When observers looked through the grid, fixating the background, the wallpaper illusion was observed. The strips were seen at the same distance as the background, appearing attached to the background. Belonging now to the background they seemed to have the same apparent illumination as the background. As a result of such “misjudgement of illumination” the strips appeared to be of the same lightness as the background (dark grey).
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