See caption
The regions marked A and B are the same shade of grey
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The checker shadow illusion is an optical illusion published by Edward Adelson, professor of vision science at the Massachusetts Institute of Technology (MIT), in 1995.[1]

The image of a chequer-board has an object casting a partial shadow over the light and dark squares, in which the square labeled A appears to be a darker colour than the one marked B, although they are actually both the same shade of grey. The illusion demonstrates that the perceived lightness or darkness of an object is determined not only by its physical luminance but also by its context and surroundings, in this case with the visual system discounting the shadow.[1]

A region of the same shade connecting A and B
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The visual system is not very good at being a physical light meter, but that is not its purpose. The important task is to break the image information down into meaningful components, and thereby perceive the nature of the objects in view.
— Edward Adelson.[2]