Mach band

A Mach band is an optical illusion, named after Ernst Mach. It is the region encircling a bright light which in visual perception appears lighter or darker than its surroundings, in a zone where the luminance increases or decreases rapidly.

Related Topics:
Optical illusion - Ernst Mach - Luminance

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Although the physical brightness is constant over the distance from the ambient object, the observer perceives an "undershoot" and "overshoot" in brightness at what is physically a step edge. Thus, at a certain point, the observer perceives a slight decrease in brightness compared to the true physical value. After that point the observer undergoes a slight overshoot in brightness compared to the true physical value. The total effect is one of increased, local, perceived contrast at an edge in brightness.

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It is theorised that the Mach band may be responsible for moths' attraction to ambient objects.

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