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A. Alarcon, R. Sabesan, G. Yoon; Adaptive Optics Asymmetric Double Pass Method to Assess the Ocular Point Spread Function. Invest. Ophthalmol. Vis. Sci. 2010;51(13):3943.
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The conventional asymmetric double pass with a small entrance pupil (EnP) and a large exit pupil (ExP) is unable to capture higher spatial frequencies in the ocular point-spread-function (PSF) due to an extended laser beacon on the retina induced by diffraction. The goal of this study is to assess the ocular PSF by using adaptive optics (AO) in an asymmetric double-pass with a large EnP.
Aberrations of light going into the eye (the first pass) for a 6 mm EnP were corrected to produce a nearly diffraction-limited laser beacon at the retina. In the second pass, the reflected light from the retina was imaged through a 6 mm ExP onto a CCD camera to provide an estimate of the PSF. This method was compared to the conventional asymmetric double pass with unequal EnP (1 mm) and Exp (6 mm) and symmetric (EnP and ExP = 6 mm) double pass method. Three model eyes were simulated with varying magnitudes of aberration using phase plates. The PSFs were also obtained in 2 young normal eyes. The measured single pass PSF was used as a reference to assess imaging performance with each double pass technique in the model eye. The cross-correlation coefficients and area under the MTF (aMTF) were computed for comparison.
Two model eyes were dominated by Zernike coma of 2 µm and 0.5 µm respectively while the third model eye mimicked the aberrations of a real normal eye with 0.55 µm in total RMS, all over a 6 mm pupil. Cross-correlation coefficients of the conventional asymmetric and the AO asymmetric double PSFs with the single pass PSF were 0.87 ± 0.03 and 0.78 ± 0.01 on average for the model eyes. The aMTF up to 60 cyc/deg estimated from the single pass PSF, the symmetric and the AO asymmetric double pass were 9.9 ± 2.4, 14.2 ± 5.2 and 7.6 ± 2.9 respectively. Beyond the spatial bandwidth of the conventional asymmetric method, the AO asymmetric double pass also increases the aMTF by an average factor of 5.24 and 2.14 in model eyes and real eyes respectively.
AO asymmetric double pass provides a reliable estimate of the ocular PSF directly and has the potential to detect the optical defects that can not be detected by an ocular wavefront sensor. The ability to measure the PSF reliably facilitates an assessment of the ocular optical transfer function.
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