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Figure 4. Cornsweet effect makes identical regions look differently bright when separated by opposing luminance gradients that meet at an edge. Various stimuli, including a spinning black and white disk (a), can create this effect. The region adjacent to the lighter gradient appears brighter than the region next to the darker gradient (b), which is the opposite effect of the standard simultaneous brightness contrast. A graph of relative luminance (c) shows that the two areas on either side of the edge are physically identical, but a graph of the perception of brightness (d) indicates that the right side seems brighter than the left, which is the Cornsweet effect. Blocks showing the Cornsweet effect in a scene (e) enhance the illusion because the information presented increases the probability that the two blocks are differently reflective surfaces under different illuminants. The upper block now looks much darker than the lower one, even though they are identical.