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J. Shen, L. Thibos; Off-Axis Monochromatic Aberrations in a Wide-Angle Model Eye. Invest. Ophthalmol. Vis. Sci. 2008;49(13):3322.
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To evaluate the use of a clinical wavefront aberrometer (COAS, Wavefront Sciences, Inc.) for off-axis measurements of ocular wavefront aberrations.
A single-surface, emmetropic model-eye was constructed from a plano-convex lens cemented to a flat milled onto an acrylic sphere (25.4 mm dia). The outer surface of the sphere was sandblasted and tinted to provide a diffusely reflecting surface that models the human fundus. A Coddington pupil (7mm diameter, 3.5mm posterior to the lens apex) was the limiting aperture for light reflected from the model fundus. Trial lenses corrected refractive errors present in the peripheral field. Wavefront phase were reconstructed from slope measurements over a circular domain circumscribed around the elliptical entrance pupil.
Up to 30° of eccentricity, the wavefront sensor recorded slope measurements over the entire entrance pupil. Beyond 35º, the irradiance declined rapidly along the minor axis of the elliptical pupil, possibly due to the range-limiting-aperture in COAS that blocks highly aberrated rays. The change of RMS wavefront error over a circular mask from 0° to 30° of eccentricity was small for 2nd- to 6th- Zernike orders. However, RMS of all of Zernike orders increased dramatically, as did measurement variance, beyond 30º of field angle, again indicating unreliable measurements at large field angles. The model eye became increasingly myopic and astigmatic off-axis and the higher order aberrations of 3rd-order coma and 5th-order coma showed also increased off-axis. All of these changes were highly correlated.
Measurements of off-axis wavefront aberrations of a model eye from 0° to 30° of eccentricity are possible with the COAS clinical wavefront aberrometer and auxiliary lenses to correct astigmatism. Care is required to ensure that the entire entrance pupil is sampled by the wavefront sensor and that wavefront reconstruction is performed over the entire elliptical pupil.
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