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W. Gao, B. Cense, C. Zhu, R. S. Jonnal, D. T. Miller; Impact of Fundus Structure on Wavefront Sensing of Ocular Aberrations. Invest. Ophthalmol. Vis. Sci. 2008;49(13):2836.
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© ARVO (1962-2015); The Authors (2016-present)
Objective wavefront sensors, in particular those based on Shack-Hartmann (S-H) technology, are routinely used for measuring ocular aberrations in the eye. Accuracy and precision of these instruments depend on the instrumentation, but also on properties of the eye. In particular, wavefront sensors rely on a focused beam of near infrared light that reflects from the thick fundus. Current sensors assume light is reflected from a single retinal plane presumed to be near the photoreceptor apertures. This assumption fails to account for errors potentially introduced by reflections from many depths in the thick fundus. To predict this error, we developed an optical model of the fundus reflection in conjunction with the physical principles that govern light propagation through the eye and wavefront sensor.
An axial reflectance profile of the fundus was constructed from averaged A-scan images acquired in several subjects at 2 degree retinal eccentricity using spectral domain OCT (c=840 nm, Δ=50 nm). The optical Stiles-Crawford effect was incorporated using directionality parameters of 0.06 and 0.12 for the reflections at the cilia junction and posterior outer segment tip, respectively. The contribution of the choroid was systematically varied over a range of expected values that account for possible differences in detection between SD-OCT and S-H wavefront sensors. Reflections from the thick fundus were propagated through an ideal eye (no ocular aberrations) and then sampled by an 11x11 lenslet array that spanned an 8 mm pupil at the eye. Centroids were obtained from the lenslet focal spots. Corresponding wavefront slopes were fit to the derivatives of Zernike polynomials up through sixth order.
The numerical simulation predicts that reflections from the thick fundus produce rotationally symmetric aberrations in the wavefront sensor measurements. The dominant component is defocus followed by spherical aberration. Their combined magnitude does not exceed ¼ diopter for the parameters considered here though the value is highly sensitive to the choroid contribution and the axial location at which the light exits the cone apertures.
Reflections from the thick fundus are predicted to induce ghost aberrations in S-H sensor measurements.
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