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Compensative Retinal Response to Departures from Unit Contrast and Unit Spot-Size Ratio


  • Department of Physics, Institute of Technical Education & Research, Siksha ‘O’ Anusandhan University, Khandagiri Square, Bhubaneswar – 751030 Orissa, India


The objective of this work is to show how the retina responds to the loss of luminance efficiency due to increasing departure from either unit contrast or unit spot-size ratio in a compensative fashion. The methods adopted are two-wave interference and the single mode wave guiding of a photoreceptor cone. A correlation is established between loss of luminance efficiency due to oblique incidence and contrast in the interference pattern. Similarly, a relation between the fraction of the power that is not able to be coupled to a cone due to the peripheral entry of light and the spot-size ratio departure is developed. The findings are that due to maximum departure from unit contrast, the retinal response takes the traditional Stiles- Crawford route, but with no departure the Stiles-Crawford Effect of the first kind (SCE I) becomes totally irrelevant for the retina and for intermediate situations (between unit and zero contrast) the retinal response is controlled by a modified SCE I weakened proportionate to the contrast of the interference pattern on the retina. Likewise, when the incident spotsize and the waveguide mode spot-size match, the spotsize ratio is unity, departure is zero, the coupling of power is 100% and visibility loss is zero. But with gradual enhancement in the spot-size ratio departure, the visibility loss also increases. Again, a visibility loss of 90 % corresponds to a departure of either 6-fold or 1/6-fold (equivalent to a pupil entry point of 4 mm). This suggests that a pupil entry point of 3.5 mm may point to a loss of 70-80 %, a result reached with departure from contrast also. This way of correlating the retinal response to either contrast or spot-size ratio has the advantage of employing them as potential biomarkers for early detection of diseases affecting the photoreceptors.


Biomarker Contrast, Retinal Response, Spot-size Ratio, Stiles-Crawford Effect, Visibility Loss

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