Cone oil droplets are highly retractile, optically dense spheres located in front of the cone outer segment. Because of their refractile strength, absorbance, and size, these spherical oil droplets have a pronounced influence on the light reaching the photopigment. We used electromagnetic scattering theory to interrelate these factors, and we calculated the light energy distribution around cone oil droplets modeled as isolated spheres. The necessary electromagnetic parameters were determined from anatomical and optical measurements in the turtle retina. Cone and oil-droplet dimensions were determined with a light microscope. The index-of-refraction values for the red, yellow, orange, and clear oil droplets were measured to be 1.69, 1.55, 1.51, and 1.48, respectively, by the technique of immersion matching. The application of the electromagnetic scattering theory suggests that oil droplets significantly increase cone sensitivity. Cone action spectra predicted by this analysis show that the largest increases occur around the absorption maximum of each cone’s photopigment.
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