Integration of the Stiles–Crawford effect of the first kind

Abstract

In 1932/1933, Walter Stanley Stiles and Brian Hewson Crawford, sought to measure the area of the entrance pupil of their subject's eyes. They assumed all portions of the entrance pupil of the eye contributed equally to visual excitation. Their pupillometer did not function as hypothesized. Using photopic stimuli, they found light-rays entering the pupil center had greater capability to stimulate vision than rays penetrating peripheral portions of the pupil. They deduced the reason for this failure, and discovered the Stiles–Crawford effect of the first kind (SCE-1), or ‘the directional sensitivity of the retina’. This important finding resulted in numerous fine studies, but integration of excitation across the entrance pupil of the eye, per se, has been rarely considered. At a later date, Enoch, then Drum, addressed this problem. We address integration and specification of visual stimuli, consider confounds, such as the effects of aberration-induced blur, and the means of mitigating confounds encountered. We consider use of adaptive optics for these purposes.

Keywords:

• visual stimulus
• photometry
• retinal illuminance
• troland (and photopic and scotopic trolands)
• Stiles–Crawford effect of the first kind (SCE-1)
• integration of the SCE-1
• methods of comparison of visual stimuli (bipartite fields, flicker photometry)
• aberrations of the eye (e.g. spherical aberration, chromatic aberration(s))

Notes

Note

1. (1) In most experiments measuring the SCE-1 performed in recent years, very small-size projections of the aperture stop of the test apparatus have been imaged in the plane of the observer's entrance pupil (i.e. they are usually less than 1 mm in diameter). (2) This discussion is limited to monocular testing. (3) Measured SCE-1 functions obtained vary with wavelength (please see Figure 8). (4) We do not consider here recent discussions of reflected, re-emitted, and projected light gathered from the retinal photoreceptor waveguides which can be assessed by reverse path irradiation/illumination). This relationship is commonly termed ‘the optical SCE-1’ (e.g. 25–27).