White's illusion

White's illusion is an optical illusion illustrating the fact that the same target luminance can elicit different perceptions of brightness in different contexts.

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).

To see that the gray bars are the same color, you can stare at the image until your eyes begin to cross, then control your eyes until the gray bars line up, making one big gray bar, proving that the gray bars are the same color.

An even more extreme example is an overhead projector screen, which is bright white in reality but whose less-illuminated regions appear black. Note that this is an effect of simultaneous contrast, not White's illusion, per se.