What is "chromostereopsis"

n. A visual illusion whereby an impression of depth is conveyed in two-dimensional colour images, usually a combination of red with either green or blue.

Chromostereopsis is a visual illusion whereby the impression of depth is conveyed in two-dimensional color images, usually of red-blue or red-green colors, but can also be perceived with red-grey or blue-grey images. Such illusions have been reported for over a century and have generally been attributed to some form of chromatic aberration.

Chromatic aberration results from the differential refraction of light depending on its wavelength, causing some light rays to converge before others in the eye (longitudinal chromatic aberration or LCA) and/or to be located on non-corresponding locations of the two eyes during binocular viewing (transverse chromatic aberration or TCA).

Chromostereopsis is usually observed using a target with red and blue bars and an achromatic background. Positive chromostereopsis is exhibited when the red bars are perceived in front of the blue and negative chromostereopsis is exhibited when the red bars are perceived behind the blue. Several models have been proposed to explain this effect which is often attributed to longitudinal and/or transverse chromatic aberrations. However, recent work attributes most of the stereoptic effect to transverse chromatic aberrations in combination with cortical factors.

It has been proposed that chromostereopsis could have evolutionary implications in the development of eyespots in certain butterfly species. Additionally, some stained-glass artists were probably aware of this effect, using it to generate protruding or receding, sometimes referred to as "warm" and "cold", color images.