Stiles–Crawford effect

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. 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. 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. 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. 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. Both of the Stiles–Crawford effects of the first and second kind are highly wavelength-dependent, and they are most evident under photopic conditions. 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. 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. They constructed an apparatus where two independently controlled beams, both emitted by the same light source, entered the eye: a narrow beam through the center of the pupil, and a wider beam filling the whole pupil. The two beams alternated in time, and the subject was instructed to adjust the intensity of the wider beam until minimum flicker was observed, thus minimizing the difference in the visual stimulus between to the two beams. It was observed that the luminance of the pupil is not proportional to the pupil area. For instance, the luminance of a 30 mm2 pupil was found to be only twice that of a 10 mm2 pupil. In other words, to match the apparent brightness of light entering a 30 mm2 pupil, the luminance of light entering through a 10 mm2 pupil had to be increased by a factor of two, instead of the expected factor of three. 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. Using similar methods, the Stiles–Crawford effect has been verified by the scientific community. The Stiles–Crawford Effect is quantified as a function of distance (d) away from the center of the pupil using the following equation: η ( d ) = a m o u n t   o f   l i g h t   e n t e r i n g   t h r o u g h   t h e   c e n t e r   o f   t h e   p u p i l   t o   p r o d u c e   a   c e r t a i n   r e s p o n s e a m o u n t   o f   l i g h t   e n t e r i n g   a t   a   d i s t a n c e   d   a w a y   f r o m   t h e   c e n t e r   t o   p r o d u c e   t h e   s a m e   r e s p o n s e {displaystyle eta ,!(d)={frac {amount of light entering through the center of the pupil to produce a certain response}{amount of light entering at a distance d away from the center to produce the same response}}} , where η is the relative luminance efficiency, and d is defined as positive on the temporal side of the pupil and negative on the nasal side of the pupil.