Cornsweet illusion
From The Art and Popular Culture Encyclopedia
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The Cornsweet illusion, also known as the Craik–O'Brien–Cornsweet illusion and the Craik–Cornsweet illusion, is an optical illusion that was described in detail by Tom Cornsweet in the late 1960s. Craik and O'Brien had made earlier observations in a similar vein.
In the image at right, the entire region to the right of the "edge" in the middle looks slightly lighter than the area to the left of the edge, but in fact the brightness of both areas is exactly the same, as can be seen by blacking out the region containing the edge. (The graded darker and lighter regions around the edge span only 14% of the total width of the image.)
This phenomenon is similar to the phenomenon of simultaneous contrast and Mach bands, but differs from it in two important respects.
- In Mach bands, the effect is seen only on areas that are close to the intensity gradient. In the Craik–O'Brien–Cornsweet illusion, a very small area (the central "edge") affects the perception of entire large areas, portions of which are distant from the edge.
- In the Cornsweet illusion, the region adjacent to the light part of the edge appears lighter, and the region adjacent to the dark part of the edge appears darker, just the opposite of the usual contrast effects.
