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Daniele Zavagno; Anomalous contours prevent brightness spreading in phantom illumination displays. Journal of Vision 2002;2(7):78. doi: 10.1167/2.7.78.
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© ARVO (1962-2015); The Authors (2016-present)
In the phantom illumination illusion a homogeneous dark background (B) shows two different brightness levels due to the presence of surfaces (S) shaded with a linear luminance gradient ranging from black to white. The spatial distribution of the shaded surfaces is so that all their light ends can be virtually connected with a contour. The portion of the background (B2) facing the light ends of S appears brighter than the rest of the background (B1).
Two experiments are discussed that show that when the distribution of S is so that B2 appears like an anomalous surface, then the brightness spreading that characterizes the illusion disappears. The first experiment was conducted on a CRT and the subject's task was to rate B2 with respect to B1 on a −20/+20 scale. The second experiment was conducted with paper displays using a paired comparison method with a forced choice task between which B2s appeared darker.
While the results strongly suggest that the brightness spreading is subsequent to figure-ground articulation, phenomenal observation of the experimental displays suggests also the presence of a low level component of the illusion which determines local effects for each S element. The formation of anomalous contours cannot overwhelm these local effects but it can prevent their spreading beyond S and its immediate surround (global effect).
The illusion is also discussed in relation to other brightness/lightness effects, such as color neon- spreading, assimilation and the glare effect.
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