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Checkerboard shading illusion The two diamonds a and b have the same luminance, or light intensity, yet look dramatically dierent in shade of grey (the diamonds in any vertical column all have the same luminance, so by following the connector between the diamonds at the bottom of the gure you can see that a and b are the same). The diamonds have the same luminance because a gradual luminance gradient has been added to the checkerboard, going from dark on the left to bright on the right. The result is that the addition of ‘dark-part-of-gradient’ plus ‘light-diamond’ on the left equals the addition of ‘bright-part-of-gradient’ plus ‘dark-diamond’ on the right. However the gradual luminance gradient is barely perceptible and our visual system treats it as illumination, specically shading. We tend to discard shading when estimating the color of surfaces, so we see the two diamonds as if the gradual luminance gradient were not there, i.e. as very different shades of grey.

The illusion was designed by Professor Frederick Kingdom of the McGill Vision Research Unit, Department of Ophthalmology, McGill University. It is based on similar illusions designed by Professors Ted Adelson of the Massachusetts Institute of Technology and Alexander Logvinenko of Glasgow Caledonian University. Professor Kingdom has a longstanding interest in brightness illusions, as part of his on-going research on color vision funded by the Canadian Institute of Health Research.

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