Stiles–Crawford effect
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The Stiles–Crawford effect (subdivided into the Stiles–Crawford effect of the first and second kind) is a property of the human eye that refers to the directional sensitivity of the cone photoreceptors.[1]
The Stiles–Crawford effect of the first kind is the phenomenon where light entering the eye near the edge of the pupil produces a lower photoreceptor response compared to light of equal intensity entering near the center of the pupil. The photoreceptor response is significantly lower than expected by the reduction in the photoreceptor acceptance angle of light entering near the edge of the pupil.[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.[2]
The Stiles–Crawford effect of the second kind is the phenomenon where the observed color of monochromatic light entering the eye near the edge of the pupil is different compared to that for the same wavelength light entering near the center of the pupil, regardless of the overall intensities of the two lights.[1]
Both of the Stiles–Crawford effects of the first and second kind are highly wavelength-dependent, and they are most evident under photopic conditions.[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. The reduced sensitivity to light passing near the edge of the pupil enhances human vision by reducing the sensitivity of the visual stimulus to light that exhibits significant optical aberrations and diffraction.[1]