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Abstract-Toobtain the Stiles-Crawford colour change. the absorption of guided and unguided light by a cone outer segment as a function of the angle of incidence is estimated. The model is specified in two wals: the HE,,, model. based on a waveguide modal analysis. and the geometrical optics model, The measurements by Stiles (1937) are fitted with reasonable success. both for the intensity effect and for the colour change. using the phenomenon of selfscreening to explain the colour change.The Stiles-Crawford colour change has been explained in the literature as due to selfscreening of the visual pigment (Brindley. 1953: Walraven andBouman, 1960). Walraven and Bouman (1960) have obtained a specific expression for the absorption of light by the photopigment. attributing the Stiles-Crawford intensity effect to leakage from the outer segment and leakage from the inner segment. In this paper the Stiles-Crawford intensity effect is attributed to leakage from the inner segment. The expression for the absorption of light by the pigment in the outer segment. which will be used here. is specified in two ways. In the geometrical optics model the parameters. except the absorption coefficients. are not wavelength-dependent.In the HE, ,, model the parameters are obtained from the characteristics of the HE, .I dielectric waveguide mode. The Stiles-Crawford colour change is attributed to selfscreening.

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