Comparison of double-pass estimates of the retinal-image quality obtained with green and near-infrared light.

We compared retinal point-spread functions obtained by the double-pass method with two different wave-lengths, green (543 nm) and near-infrared (780 nm), in both cases under the best conditions of focus. The best refractive state at each wavelength was determined with two procedures: subjective refraction and analysis of the recorded double-pass images as a function of focus. Since the refraction results agree quite well, we assume that in both cases, green and near-infrared light, most of the light of the central core in the double-pass images comes from a layer close to that of the photoreceptors. The central spread of the double-pass images was also quite similar for the two wavelengths: a width of approximately 2-3 arcmin at half-intensity relative to the peak. However, larger differences were found in the tails of the images, with the infrared images presenting a larger scattering halo, probably as a result of a more important contribution of retinal and choroidal scattering for that wavelength. By using the central core in the double-pass images and ignoring the tails, we can use the near-infrared data to predict the modulation transfer function measured with the use of green light. These results raise the possibility of using near-infrared illumination the double-pass method to estimate the optical performance of the human eye.