Checkershadow

Checkershadow is a term most often used to refer to the checker shadow illusion, a well-known optical demonstration of how context and shading influence perceived lightness. The illusion was developed by psychologist Edward H. Adelson in the 1990s and is typically presented as a checkerboard with a circular region that creates a shadow over part of the pattern. In the standard setup, two squares of the checkerboard—one in the lit area and one in the shaded area—have the same luminance, yet they appear to be different brightness.

The core phenomenon is that surrounding context alters appearance. The visual system interprets the shaded region as being under a shadow, leading to a perceptual adjustment in which the square inside the shadow is judged differently from the same-colored square outside the shadow. As a result, two identical luminance values can look discrepant due to the inferred lighting conditions and surrounding contrast. The illusion is frequently illustrated with labeled examples to emphasize that surface reflectance and illumination are being interpreted rather than simply measured.

In vision science, checkershadow is used to study lightness constancy, context effects, and the limits of human color and brightness perception. It has educational value for explaining how the brain integrates local light information with global scene cues, and it is cited in discussions of computer vision, color science, and perceptual psychology. While the term checkershadow primarily refers to this specific illusion, variations and digital simulations are common in textbooks and online demonstrations to illustrate similar principles of perception.

See also: optical illusion, lightness constancy, visual perception, checkerboard.