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N. Doble, C. Kempf, M. Helmbrecht, A. Roorda; Closed Loop Adaptive Optics in the Human Eye Using a Segmented MEMS Deformable Mirror. Invest. Ophthalmol. Vis. Sci. 2008;49(13):4195. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
Most vision adaptive optics (AO) systems use deformable mirrors (DMs) with continuous front surfaces where moving one DM actuator causes deformation at adjacent ones. Using microelectromechanical systems (MEMS) technology, we have developed a high stroke DM with a segmented front surface. Each segment is individually positioned and has 3 degrees of freedom - piston, tip and tilt. The array of segments then forms the conjugate wavefront profile. Segmentation enables reliable operation in either a pure open loop or the normal closed loop configuration. We report the first use of such a DM for closed loop correction in the living human eye.
The AO system consists of a Shack Hartmann wavefront sensor (WFS) and the Iris AO MEMS DM. Each of the 37 DM segments has a one-to-one mapping to a WFS subaperture. A superluminescent diode (SLD) at 830nm provided the WFS beacon, giving 15 microWatts through a 1mm diameter beam delivered slightly off the corneal pole.We tested the AO performance on 3 normal eyes. Subjects were instructed to fixate on the WFS beacon, with the wavefront sensing being performed over a dilated 6.4mm diameter pupil. A series of WFS measurements were taken with and without AO correction. Zernike coefficients (ANSI standard, up to 5th order) were recorded before and after correction and subsequently used to reconstruct the wavefront and the point spread function.
It is common practice to report AO system performance based on WFS measurements and these are presented. Subjects were not refracted prior to correction. Errors given are ±1 standard deviation. For subject 1, the Strehl Ratio (SR) (from the calculated PSF) increased from 0.01±0.01 to 0.56±0.17 with a corresponding decrease in the peak-to-valley (PV) error of 8.10±0.31 to 0.74±0.20 microns. For subject 2, SR increased from 0.04±0.01 to 0.47±0.16, PV from 8.42±0.35 to 0.75±0.22 microns. Subject 3 showed a similar trend, SR increasing from 0.04±0.01 to 0.47±0.16, PV of 7.01±0.13 reduced to 0.63±0.23 microns. As with all WFS based measurements, careful interpretation is required as the measured SRs are highly dependant on the level of system calibration.
We have shown good correction performance for testing on human subjects based on WFS measurements. The next steps are to (i) increase the closed loop convergence speed, the slow speed is reflected in the large standard deviations (ii) measure correction performance in the far-field and correlate this with the measured WFS values. Such a measurement will provide an independent measure of system performance that is independent of any WFS calibration.
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