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Prepared for Wikipedia by Adrian Pingstone in January 2004, based on the original archive copy at the Wayback Machine created by Edward H. Adelson. See also same color.

This is possibly one of the most uncanny optical illusions in existence. No matter how hard you look, you may not be able to convince yourself that the two squares are the identical shade of grey. To check, try one of these methods:

1. Right click the large picture, save it to your hard drive, put it into your image editor, "select" square A and then pull it down onto square B. There will be an exact match in colour! An example of this proof can be seen at de:Bild:Optical greysquares proof.png
2. The presentation of this illusion here is linked to an animated GIF that demonstrates the color match. That animation demonstrates that B is also an exact match to the two "dark" squares straddling the corners closest to and farthest from the viewer (A is one of the latter two). Furthermore, it makes it clear that none of the "light" colored squares matches B (though the one diagonally touching B to the left comes close).
3. As above but use the Color Sampler Tool or whatever means you have of sampling the RGB values of a point on the picture. Both squares produce the same RGB values. JPEG artifacts may cause slight variation.
4. There is also a Flash animation of this illusion here.
5. Another animation in WMV, AVI_DIVX, SWF (flash) formats, and EXE file are available to download from this page
6. Right click the picture, choose Print, then cut out square A and position it on square B.
7. Easiest of all, get a piece of paper and cut windows in it so that only squares A and B are showing through!
8. Perhaps even easier than the above, if you place your finger covering the two to three squares that separate A and B, it can clearly be seen that they are the same color. In other words, forcefully ignoring the borders between the squares.

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"©1995, Edward H. Adelson. These checkershadow images may be reproduced and distributed freely."

http://web.mit.edu/persci/people/adelson/checkershadow_illusion.html archive copy at the Wayback Machine

Explanations and other animated examples of "lightness perception and lightness illusions" can be found in a paper by Adelson.