The Stiles–Crawford hue shift following photopigment depletion

Abstract

THE colour of a light is determined by such factors as its dominant wavelength, luminance, colorimetric purity, locus of retinal stimulation, surround chromaticity and preceding adapting field. Stiles and Crawford¹ were first to note that the position at which a small pencil of light enters the pupil also affects the light's colour. In general, light that enters through the centre of the pupil is seen as brighter than the same light impinging on the same retinal area but passing through the periphery of the pupil. This is known as the Stiles–Crawford effect of the first kind (S–C I). It has been demonstrated that this effect is due to the directional sensitivity of the receptors^2–4 and is much more pronounced in cones than in rods^5–8. Stiles⁹ also showed that two monochromatic lights of the same wavelength, one passing through the centre of the pupil (on-axis), the other entering through the periphery (off-axis), will differ in hue even after they are equated for brightness. (There is also a small change in saturation that becomes almost negligible at the long-wavelength end of the spectrum.) This is known as the Stiles–Crawford effect of the second kind (S–C II). For example, to match in hue an on-axis light of 610 nm an observer might require an off-axis light of 605 nm. The direction and magnitude of this hue shift varies with the wavelength of the on-axis light (Fig. 1). We have measured S–C II in conditions of varying levels of bleached photopigment. An explanation of the effect based on the absorbance of the photopigment can account for much but not all of the data. For the long-wavelength end of the spectrum, when the photopigment is significantly depleted, some other factor must have a role in mediating this effect.