On the spectral relation between the first and second Stiles–Crawford effect

Abstract

The first Stiles–Crawford effect describes the psychophysical visibility dependence on pupil point for a narrow beam of light incident on the eye. Its distribution function commonly attributed to photoreceptor waveguiding is characterized by a point of maximum visibility typically near the pupil centre and a wavelength-dependent directionality parameter. The second Stiles–Crawford effect describes a minor hue shift of obliquely incident light commonly attributed to pigment screening that accompanies observations of the first Stiles–Crawford effect. Here the first Stiles–Crawford effect and the wavelength dependence of its directionality parameter are examined in terms of photoreceptor waveguiding. Its distribution function is analyzed to derive characteristic features of the second Stiles–Crawford effect directly from the spectrum of the directionality parameter. The results obtained are independent of the exact physical mechanisms underlying both effects but do support a major role played by waveguiding. Finally, the influence of bleaching is discussed in relation to the model and to pigment screening.

Keywords:

• Stiles–Crawford effect
• hue shift
• bleaching
• photoreceptor waveguiding
• retinal modelling
• vision modelling

Acknowledgements

The author wishes to thank Professor J.M. Enoch for very stimulating discussions and inspiration on the Stiles–Crawford effects, Professor V. Lakshminarayanan and the organising committee for the invitation to the ‘75 years of the Stiles–Crawford effect’ symposium at Frontiers in Optics (Rochester 2008) where parts of this work were presented 12, and an anonymous reviewer for valuable comments to the manuscript. Financial support from Science Foundation Ireland (07/SK/B1239a) and UCD seed funding 2008 is also gratefully acknowledged.