[1] Measurements indicate that the peak photoreceptor sensitivity does not occur for light entering the eye directly through the center of the pupil, but at an offset of approximately 0.2–0.5 mm towards the nasal side.
[1] There are several factors that contribute to the Stiles–Crawford effect, though it is generally accepted that it is primarily a result of the guiding properties of light of the cone photoreceptors.
[1] In the 1920s, Walter Stanley Stiles, a young physicist at the National Physical Laboratory in Teddington, England, examined the effects of street lighting and headlight features on automobile traffic accidents, which were becoming increasingly prevalent at the time.
Stiles, along with his fellow National Physical Laboratory researcher Brian Hewson Crawford, set out to measure the effect of light intensity on pupil size.
[1] Stiles and Crawford subsequently measured this effect more precisely by observing the visual stimulus of narrow beams of light selectively passed through various positions in the pupil using pinholes.
[1] Experimental data are fit accurately using the following empirical relationship: where p(λ) is a wavelength dependent parameter which represents the magnitude of the Stiles–Crawford effect,[2] with larger values of p corresponding to a stronger falloff in the relative luminance efficiency as a function of distance from the center of the pupil.
[8] Light entering the fovea center, which is composed only of cones and Müller cells, at an angle of 0 degrees causes a very bright spot after passing through this area.
[8] A different approach, introduced by Vohnsen, considers the photopic Stiles-Crawford effect of the first kind to be a consequence of leakage rather than waveguiding by the dense and optically irregular photoreceptors.