Stiles–Crawford effect

The Stiles–Crawford effect (or Stiles–Crawford effect of the first kind ) is a term that has been applied to two perceptual illusions.

Effect 1
The first of these is one in which light that enters through the centre of the pupil appear brighter than light that enters near the edge.

This is due to a property of the cone photoreceptors of the human eye. It refers to the directional sensitivity of the cone photoreceptors; specifically to the phenomenon that light passing near the edge of the pupil is less efficient at evoking sensation than light passing through the center of the pupil. That is, the retina is not lambertian, and the effective acceptance angle of the cones is smaller than that subtended by the pupil.

A stimulus' peak effectiveness is at the center of the pupil (for the normal eye) and falls in effectiveness in a symmetrical pattern.

Rays of light passing through the centre of the pupil are less oblique to the cone after refraction and stimulate them more strongly than rays passing through peripheral areas of the pupil. A photoreceptor acts like a retinal optic fibre, it captures light that hits it at a narrow angle from its normal. The acceptance angle of a cone is narrow, approximately 5°, rods have larger acceptance angles.

The "Stiles-Crawford" effect reduces the detrimental effects of light scatter on the retina at photopic levels.

This effect does not occur in the dark-adapted eye so is not affected by the activity of the rods.

Background
Stiles and Crawford devised an experiment whereby a double pinhole grating was placed before the eyes, one pinhole would be at the centre of the eye, the other at the periphery. A variable filter is placed in front of the central pinhole, this reduced transmission, until the two beams of light appeared equally bright. The difference in brightness is recorded as the relative efficiency of the two pupillary points. This value differs depending whether or not the conditions were photopic or scotopic. Under scotopic conditions, the relative efficiency is much less prominent, different entry points in the pupil are almost equally effective.

Effect 2
Is related to the above and occurs when monochromatic light there is a change in hue depending on the angle at which the light entered the eye. At long and short wavelenths oblique angled light striking the periphery of the retina appears with a slight positive shift along the spectrum so appears more red than the same light entering the centre of the eye. At medium wavelengths there is a negative shift