The same color illusion

This illusion illustrates that purely human observations in science may be ambiguous or inaccurate, even for such a seemingly direct perception as relative color. Similar illusions exist on the sky, such as the size of the Moon near the horizon, or the apparent shapes of astronomical objects. The advent of automated, reproducible, measuring devices such as charge-coupled devices (CCDs) have made science in general and astronomy in particular less prone to, but not free of, human-biased illusions.

Why does the illusion work?

The visual system needs to determine the color of objects in the world. In this case the problem is to determine the gray shade of the checks on the floor. Just measuring the light coming from a surface (the luminance) is not enough: a cast shadow will dim a surface, so that a white surface in shadow may be reflecting less light than a black surface in full light. The visual system uses several tricks to determine where the shadows are and how to compensate for them, in order to determine the shade of gray "paint" that belongs to the surface.

The first trick is based on local contrast. In shadow or not, a check that is lighter than its neighboring checks is probably lighter than average, and vice versa. In the figure, the light check in shadow is surrounded by darker checks. Thus, even though the check is physically dark, it is lighter when compared to its neighbors. The dark checks outside the shadow, conversely, are surrounded by lighter checks, so they look darker by comparison.

A second trick is based on the fact that shadows often have soft edges, while paint boundaries (like the checks) often have sharp edges. The visual system tends to ignore gradual changes in light level, so that it can determine the color of the surfaces without being misled by shadows. In this figure, the shadow looks like a shadow, both because it is fuzzy and because the shadow casting object is visible.

Colors such as gray, green and yellow are extremely difficult for the human eye-brain system to judge. Generally the cue as to what color is perceived is taken from the surrounding context. When the context is removed, the perception can change drastically.