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Vyas Akondi, Xiaojing Huang, Alfredo Dubra; Evaluation of the double-pass wavefront in model and human eyes. Invest. Ophthalmol. Vis. Sci. 2022;63(7):1481.
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© ARVO (1962-2015); The Authors (2016-present)
Model eyes are used in adaptive optics (AO) ophthalmoscopy to focus imaging channels, calibrate fields of view, distortion correction, and instrument troubleshooting. We evaluate model retinas and AO control strategies to mitigate the double-pass wavefront and allow stable and converging closed-loop operation when using AO scanning ophthalmoscopes.
We evaluated model eyes with 19 and 100 mm focal length achromatic doublets, and a variety of flat substrates: protected silver mirror, polished glass, polished opal glass, white diffusers ground with different grits, and paper. The double pass was quantified by poking an off-center actuator in the deformable mirror of the AO ophthalmoscope and comparing the relative heights of the wavefront peaks (see Fig. 1). These measurements were performed with a focused beam of 850 nm light that was at first stationary and then scanned across an ~1.1 deg angle using a 13.8 kHz resonant scanner. The same setup was used to estimate the model eyes field curvature by vertically steering the stationary beam.
Wavefronts from model eyes with paper or rough substrates as retinas (similar to a normal human eye), did not show center-symmetry that can be seen with a mirror retina, irrespective of beam scanning (see Fig. 1). The use of finely polished glass substrates as retinas results in a substantial attenuation of first-pass wavefront. The measured field curvature, with the 100 mm lens is ~7.8 smaller relative to the depth of focus (7.75 mm beam diameter) than for the 19 mm case, consistent with ray tracing. The AO control loop can be successfully operated provided that the first-pass wavefront is less than 100% of the second pass. The first pass wavefront induces AO control oscillations that can be mitigated by lowering the closed loop gain.
Decreasing the AO control gain improves convergence even with highly reflective materials, while the use of model eyes with lower field curvature improves the AO control stability. If the field curvature cannot be changed, then, the AO control stability can be improved by changing the wavefront sensor exposure to match the inverse of the scanning ophthalmoscope frame rate, or if the AO and imaging are synchronized, the exposure.
This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.
Wavefronts after poking a single deformable mirror actuator in an AO scanning ophthalmoscope with various model eyes and a human eye. The horizontal resonant scanner is either off or scanning at 1.1° angle.
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