Mach Bands, Fig. 1
From: Mach Bands
The achromatic contrast effect is noticeable at the edges where the two similar gray rectangles meet each other. (a) The darker of the two regions (i.e., gray squares) seems to have another even darker band near the border, while an extra light band appears on the lighter region. (b) The gray rectangles therefore do not appear to be of uniform color. This illusion is one of the clearest demonstrations of the difference between physical stimulation and the perception. The two graphs represent actual luminance profile and (below) perceived lightness profile on the position of an edge. The underlying mechanism serves an important function of enhancing edge detection