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Note, that although the gray rectangles are all of equal luminance, the ones seen in the context with the dark stripes appear brighter than the ones seen in the context with the bright stripes. Note that this effect is opposite to what would be expected from a simple physiological explanation on the basis of simultaneous contrast (in that case the rectangles sharing the long borders with the dark stripes should appear brighter).
Television screens depend on this visual illusion. Pixels that are not illuminated are seen as black, when they are really the same dim gray seen when the television is turned off. An even more extreme example is an overhead projector screen, which is bright white in reality but whose less-illuminated regions appear black.
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