Mach bands

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Mach bands are an optical illusion named after Ernst Mach. It refers to bands adjacent to a light to dark gradient that appear lighter or darker than justified by the underlying light (luminance, see figure).

Notice the dark band that appears immediately to the right and the light band that appears immediately to the left of the gradient.

The effect is one of increased, local, perceived brightness on either side of a luminance gradient, like that of a spatial high-boost filter. It is usually supposed that this effect is caused by lateral inhibition of the receptors in the eye. An alternative explanation is that the effect is explained by the fundamentally statistical strategy of visual perception, representing the common occurrence of highlights and lowlights in association with luminance gradients.

[edit] References

Lotto RB, Williams SM, Purves D (1999). "Mach bands as empirically derived associations". Proceedings of the National Academy of Sciences 96 (9): 5245-50. PMID 10220451.