The human photopigments are photolabile and undergo bleaching at moderate and high light intensities. The amount of pigment bleached is proportional to the quantum catch of the pigment. Bleaching the photopigment reduces the effective pigment concentration of the photoreceptor and, consequently, affects its spectral sensitivity (see also Photopigment optical density and Photoreceptor outersegment length). As more pigment is bleached, the effective pigment density decreases and the spectral sensitivity becomes narrower (though the λmax does not change). In the rods, the effects of bleaching on spectral sensitivity are relatively insignificant because only very small changes in rhodopsin density accompany the very large adaptive changes in rod sensitivity: Less than 5% of rhodopsin is bleached at the level at which the photoreceptor completely saturates (ca 3.0 log scotopic trolands), which ends the dynamic range of the rods. In the cones, however, the effects of bleaching on pigment concentration can have large effects on spectral sensitivity and must be taken into account when evaluating chromatic adaptation and color-matching data.
Formulae are available for calculating the amount of photopigment present for both long- (equilibrium) and short-duration bleaching conditions. They rely on knowing the half-constant of bleaching (i.e. the intensity for which the fractions of bleached and unbleached pigment are 0.5). Estimates of the half-constant of bleaching have been determined from fundal reflectometry for the rods (Rushton, 1972) and for the combined M- and L-cones (Rushton & Henry, 1968). Separate estimates of the half-bleaching constants of the M-cones (Rushton, 1963) and of the L-cones (Rushton, 1965) have also been obtained, but these are questionable.
A reliable estimate of the half-bleaching constant of the S-cones has not been obtained. This is problematic because there is evidence that the S-cones may be vulnerable to photic damage, even at relatively moderate levels of blue light. Exposing the S-cones to blue lights that bleaches between 50 - 75% of their photopigment on an intermittent schedule may cause a long-lasting or permanent loss of sensitivity (e.g., Harwerth & Sperling, 1971; 1975; Sperling, 1986).
Harwerth, R.S. & Sperling, H.B. (1971). Prolonged color blindness induced by intense spectral light in rhesus monkeys. Science 174 520-523.
Harwerth, R.S. & Sperling, H.G. (1975). Effects of intense visible radiation on the increment-threshold spectral sensitivity of the rhesus monkey eye. Vision Research 15, 1193-1204.
Rushton, W. A. H. (1963). The density of chlorolabe in the foveal cones of a protanope. Journal of Physiology, 168, 360-373.
Rushton, W.A.H. (1965). Cone pigment kinetics in the deuteranope. Journal of Physiology, London, 176, 38-45.
Rushton, W.A.H. (1972). Pigments and signals in colour vision. Journal of Physiology, London, 220, 1-31P.
Rushton, W.A.H. & Henry, G.H. (1968). Bleaching and regeneration of cone pigments in man. Vision Research 8, 617-631.
Sperling, H.G. (1986). Intense spectral light induced cone specific lesions of the retina and the effects of anesthesia. In: Hazards of Light (eds. Cronly-Dillon, J.R. et al.), pp. 153-167. Oxford: Pergamon.