Photoreceptor spectral sensitivities: common shape in the long-wavelength region

Previous measurements of mammalian photoreceptor spectral sensitivity have been analysed, with particular attention to the long-wavelength region. The measurements selected for study come from rod and cone systems, and from human, monkey, bovine and squirrel sources. For the spectra from photoreceptor electrophysiology and from psychophysical sensitivity, the frequency scaling applied by Mansfield (1985, The visual system, pp. 89-106. New York: Alan Liss) provides a common shape over a range of at least 7 log10 units of sensitivity, from low frequencies (long wavelengths) to frequencies beyond the peak. The same curve is applicable to the absorbance spectrum of bovine rhodopsin, although the absorbance can only be measured down to about 2 log10 units below the peak. At the longest wavelengths the results exhibit a common limiting slope of 70 loge units (or 30.4 log10 units) per unit of normalized frequency. A simple equation is presented as a generic description for the alpha-band of mammalian photoreceptor spectral sensitivity curves, and it seem likely that the equation may be equally applicable to retinal1-based pigments in other species. Despite the lack of a theoretical basis, the equation has the correct asymptotic behaviour at long wavelengths, and it provides an accurate description of the peak. It also accounts accurately for the experimentally observed "yellowing" of long-wavelength lights that occurs beyond 700 nm.