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H Sakai, Y Hirata, S Usui; Effect of Pupil Size on the Quality of Retinal Images . Invest. Ophthalmol. Vis. Sci. 2002;43(13):2018.
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Purpose: The pupil diameter varies depending on the luminance of the external visual scene. In the current study, we present further evidence giving support to our hypothesis (Hirata et al., 2001) that the pupil diameter is controlled so as to optimize the quality of retinal images. Methods: The retinal information capacity (RIC) was utilized to quantify the quality of retinal images. The RIC has been defined as a logarithm of the number of images that the photoreceptor array around fovea can reconstruct or distinguish in a unit optic angle (Snyder,1977, Laughlin,1992). And it is a function of not only the receptive efficiency of photoreceptor but also the characteristics of eye optics. We modeled the eye optics with the pupil function which consists of the transmittance and the wave aberration (described as a 6th order Zernike polynomial expansion). The Stiles-Crawford effect due to the directional sensitivity of a photoreceptor was taken into account on the transmittance of the pupil. The wave aberration was measured in ten young males (22 to 28 years old) with various degrees of astigmatism, and then the RIC was calculated for each subject on the assumption that the external scene is 1/f noise. For comparison, the luminance - pupil diameter (L-P) relationship were measured in the same ten subjects. Results: At first, it was found that there exists an optimal pupil diameter that maximizes the RIC at each luminance level for each degree of astigmatism. The luminance - optimal pupil diameter characteristics show an inverse S-shaped relationship that is typically seen in human. Furthermore, as subject's eye optics is degraded, the relationship tends to shift toward lower luminance and become more irregular. These theoretical predictions are in good agreement with the L-P relationship measured experimentally. Conclusion: These results suggest that the pupil diameter at each luminance level may provide the optimum quality of retinal images in terms of the RIC, which support our hypothesis.
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