Coherent Response of a Human Eye: an Apodization Model of Stiles Crawford Effect of the first Kind

In Stiles Crawford effect of the first kind a beam entering a pupil from the edge shows a diminution in the effective brightness compared to an identical beam passing axially. The same reduction in brightness is also observed in a pupil becoming gradually less transparent from the centre toward its periphery, known as pupil apodization. In this study we have treated Stiles Crawford effect as pupil apodization and evaluated the modulation in the retinal image using a periodic object of infinite cycles with a saw-tooth wave transmission profile, both under incoherent and coherent illumination. Over a wide range of spatial frequencies, though the incoherent entering beam shows a gradual decrease in the modification, no change is observed for coherent illumination. Thus, the modeling of Stiles Crawford effect as pupil apodization is insensitive to coherent illumination and the retinal distribution of light is dependent on the coherence of the incident beam apart from pupil entry point of the beam in traditional Stiles Crawford effect. Hence, this study highlights the significance of employing coherence in the entering beam as an optical parameter in governing retinal response and understanding the spatial distribution of photon absorption in the photoreceptors of a human eye.