Foveal cone spacing was measured in vivo using an objective technique: ocular speckle interferometry. Cone packing density was computed from cone spacing data. Foveal cone photopigment density difference was measured in the same subjects using retinal densitometry with a scanning laser ophthalmoscope. Both the cone packing density and cone photopigment density difference decreased sharply with increasing retinal eccentricity. From the comparison of both sets of measurements, the computed amounts of photopigment per cone increased slightly with increasing retinal eccentricity. Consistent with previous results, decreases in cone outer segment length are over-compensated by an increase in the outer segment area, at least in retinal eccentricities up to 1 deg.
Marcos, S., Tornow, R. P., Elsner, A. E., & Navarro, R. (1997). Foveal cone spacing and cone photopigment density difference: Objective measurements in the same subjects. Vision Research, 37(14), 1909–1915. https://doi.org/10.1016/S0042-6989(96)00295-7