The pathways of light measured in fundus reflectometry

We measured the spectral reflectance of the fovea of ten normal subjects in four conditions, i.e. under dark-adapted and bleached conditions and at two retinal angles of incidence. The objective was to study optical pathways through the photoreceptor layer, resulting in a model that simultaneously explains spectral, directional and bleaching properties of the fovea. On theoretical grounds, we propose that small reflections from the stack of discs in the cone outer segments are the origin of the directional component of foveal reflection. Non-directional reflection occurs at the inner limiting membrane and at all layers posterior to the outer segments. With four reflectance spectra as input, the model allows determination of the density of the photostable absorbers, the lens, macular pigment, melanin and blood. Because of the simplified modeling of the layers posterior to the photoreceptor layer, the values for the density of melanin and blood are not necessarily comparable to physiological data. The density of the visual pigment calculated with this model is consistent with psychophysical data, with estimates for the ten subjects ranging from 0.41 to 0.80. The long wavelength sensitive cone fraction is calculated as 0.56.