ABSORPTION OF WHITE-LIGHT IN PHOTORECEPTORS

The fraction F of incident light absorbed by a photoreceptor of length l has traditionally been given by F = 1 - e(-kl), where k is the abso rption coefficient of the photoreceptor. Unfortunately, this widely-us ed expression is incorrect for absorption of the type of light most co mmon in natural scenes-broad spectrum ''white'' light-and significantl y over-estimates absorption, This is because the measured values of k are only valid at the absorbance peak wavelength of rhodopsin, whereas at other wavelengths (which the eye may also see) k is lower. We have accounted for the wavelength dependence of k and calculated the absor ption of white light from four different natural radiant sources: the quantal irradiances of natural daylight and a patch of very blue sky, and the quantal reflections of soil and green foliage irradiated by na tural daylight, Based on these results, a simple averaged correction f or white light stimulation is derived, F = kl/(2.3 + kl), which is val id for a wide range of k and l, and therefore applicable to both verte brate and invertebrate photoreceptors. (C) 1998 Elsevier Science Ltd.