The ocular fundus shows a significantly higher reflectance for longer wavelengths together with higher light diffusion. We investigated the spatial characteristics of the diffuse light at two different wavelengths and the extent to which this may affect red-green relative spectral sensitivity.
We measured the reflectance of the ocular fundus of seven subjects for different field sizes ranging from 0.18 to 7.28 degrees and for two different wavelengths, 560nm and 650nm. The experimental setup consisted of a double-pass instrument projecting uniform disks on the fundus, while the retinal images were recorded with a cooled electron multiplied CCD camera. Absolute reflectance was measured after the system was calibrated using an artificial eye with a spectrally flat reflector. Additionally, the relative spectral sensitivity of the subjects between the above-mentioned wavelengths was measured using heterochromatic flicker photometry in two different field sizes (0.4 and 6.5 degrees). Flickering frequency was 25 Hz while the retinal illuminance was 3.2 logTrolands for the test field and a superimposed white adapting field. For the analysis, subjects were classified in two groups with dark and light iris pigmentation.
Retinal reflectance depended on field size, wavelength and the iris color of the subject. Figure shows reflected light as a function of field size for red and green light and for the two groups of subjects. The ratio between reflected red and green light exhibited a mean value of 4.0% (SD=0.8%), and 3.0% (SD= 0.2%), for light and dark eye colored subjects respectively. The respective relative red-green sensitivity was 6.4% (SD=1.9%) and 3.7% (SD=0.9%). All subjects demonstrated an increase of green-red relative sensitivity (ranging from 2.9 to 8.4%) with increasing filed size.
Increased fundus reflectance and sensitivity at longer wavelengths are associated to the spatial extent of diffuse light. Although the effect can be observed under appropriate conditions, its small magnitude suggests that diffuse light from deeper fundus layers is unlikely to have practical relevance to vision.