Stiles-Crawford effect

The Stiles-Crawford effect is 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.

The original description was by Stiles, W. H. and B. H. Crawford (1933), "The luminous efficiency of rays entering the eye pupil at different points," ''Proc. R. Soc. Lond'' B 112:428-450.