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W. Gao, R. S. Jonnal, B. Cense, O. Kocaoglu, Q. Wang, D. T. Miller; Photoreceptor Directionality Measured With Shack-Hartmann Wavefront Sensing. Invest. Ophthalmol. Vis. Sci. 2009;50(13):2741.
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© ARVO (1962-2015); The Authors (2016-present)
The directional sensitivity of the retina, known as the Stiles-Crawford effect (SCE), originates from the waveguide property of photoreceptors. This effect has been extensively studied in normal and pathologic eyes using highly customized optical instrumentation. Here we investigate the utility of a Shack-Hartmann wavefront sensor (SHWS) - an instrument that is commercially available and employed in clinics - as a new approach for measuring the optical SCE.
An existing laboratory SHWS was modified for use in this study. The sensor consisted of a 788 nm beacon that entered the eye with a 2 mm diameter and that delivered 3 µW. A 17 by 17 lenslet array sampled the reflected light across a 6.8 mm pupil at the eye. The SHWS was modified to enable control of the beacon entry position (XY) in the pupil of the eye as well as to include a pupil camera, which provided confirmation of the beacon location. Four subjects were recruited, all free of ocular disease and with normal corrected vision. Pupil dilation was realized with 1% Tropicamide.Near-IR wavelengths penetrate deeper into retinal tissue and generate more multiply scattered light than visible wavelengths, both effects that mask the directional contribution of the photoreceptors. To offset these, we took advantage of the SH lenslet array to spatially separate the waveguided light (core of SH spots) from the multiply scattered light (side lopes of SH spots). The bright core of each SH spot was extracted from the acquired frames and used to compose a spatial distribution map of the retinal reflectance in the pupil plane of the eye. Using a non-linear least square algorithm, the reflectance spatial distribution was fit to a five-parameter model composed of a constant bias and a Gaussian function with directional parameter, ρ. Measurements of ρ were acquired at the fovea and retinal eccentricities of 1, 2, and 3 degrees.
The average ρ across the four subjects was 0.08, 0.12, 0.17 and 0.21 mm-2, starting at the fovea. ρ increases monotonically with retinal eccentricity and is consistent with published values after taking into account the longer wavelength used here.
The SHWS has been traditionally employed for measuring wavefront phase (aberrations) in the eye. Here we demonstrate that intensity information can also be extracted from the same measurements and the spatial distribution of which is consistent with that produced by the optical SCE.
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