Color Constancy

The Problem of Color Constancy

Vision obtains information about objects through the light reflected from them. If the reflected light were in one-to-one correspondence with physical object properties, then extracting stable object percepts would be straightforward. But the reflected light confounds properties of the illumination with those of the object. In the case of color, the relevant object property is its surface reflectance function S(λ): the fraction of incident illuminant power that is returned to the eye at each wavelength λ. The relevant property of the illuminant is its spectral power distribution, I(λ): the amount of power at each wavelength that arrives at the object. The spectrum reflected to the eye is thus C(λ) = I(λ) S(λ). C(λ) is called the color signal. Ambiguity arises because of the symmetric role played by I(λ) and S(λ) in the formation of the color signal. For example, a banana seen in skylight might reflect the same spectrum to the eye as does grass under direct sunlight because the effect of the illuminant change on the color signal can be counteracted by the change in surface reflectance. The perceptual challenge of color constancy is to make object color appearance stable against changes in I(λ) while making it sensitive to changes in object reflectance S(λ).

Empirical Observations

To what extent does the visual system actually stabilize object color in the face of illuminant changes? This has been studied with scaling and naming paradigms where observers describe the color appearance of objects seen under different illuminants, as well as with matching paradigms where observers adjust a test object seen under one illuminant to match the appearance of reference object seen under a different illuminant. A few generalizations may be drawn from the large empirical literature. First, color appearance does vary somewhat when the illuminant is changed: color constancy is not perfect. Second, the variation in object appearance is small compared with what would be predicted for a visual system with no constancy.

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