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Checker shadow illusion

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Squares A and B are the same color.
File:Optical.greysquares.arp.gif
As the cyclinder's shadow forms square B appears to lighten.

Same color illusion, also known as Adelson’s "Checker shadow illusion" or Checker shadow illusion or just Checker shadow, is a visual illusion. The squares A and B on the illusion are of the same color (or shade), although they seem not to be. You can check it out with an eyedropper in a graphic program. By moving a copied fragment of B with mouse you can even see live how the shade changes in front of your eyes (animation of this effect in WMV, AVI_XVID, SWF formats, as well as EXE file can be downloaded from this page).

"When interpreted as a 3-dimensional scene, our visual system immediately estimates a lighting vector and uses this to judge the property of the material." [quotation from this page]

It was first published by Ted Adelson in 1995. The original figure. Ted Adelson's explanation.

You can also see the illusion at [1] and [2] and [3]

Other illusions involving color

Wikimedia Commons has media related to Optical illusion.
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