The Stiles—Crawford Effect of the Second Kind (SCII): A Review

The background which led Stiles to the discovery that the color of a monochromatic ray of light varied with its angle of incidence on the retina, and the developments in the subject since 1937 when this discovery was reported are summarized. Stiles's original measurements of the ‘hue shift’ did not successfully quantify SCII in every part of the spectrum, since it provided only two degrees of freedom while the color changes sometimes require three. Nevertheless, they were the paradigm for most subsequent work for the next quarter of a century. Full quantification of the effect was first obtained by trichromatic matching on Stiles's NPL trichromator almost 25 years after the initial discovery. The phenomena were then also fully described quantitatively with an elaboration of Stiles's original theory (‘self-screening’ theory), on the assumption that the ordinary laws of additivity are valid for color matches of three primaries striking the retina at one angle of obliquity to a test incident at a different angle. More recent experiments suggest that this initial assumption may not be generally true and that ‘self-screening’ theory may not generally provide a satisfactory description of the color changes throughout the visible spectrum, even for measurements of the effect obtained under conditions in which the additivity assumption seems to be valid. However, the available data strongly imply that absorptions of photons obliquely incident on foveal cones depend upon spectra clearly different from those upon which absorptions of normally incident photons depend.