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.

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.

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

External links

• Demonstration of the Mach band
• R. Beau Lotto, S. Mark Williams, and Dale Purves, Mach bands as empirically derived associations Proc. Natl. Acad. Sci. USA. 1999 April 27; 96 (9): 5245–5250