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Adaptive optics with a programmable phase modulator: applications in the human eye

Pedro M. Prieto, Enrique J. Fernández, Silvestre Manzanera, and Pablo Artal

Open Access Open Access

Abstract

Adaptive optics for the human eye has two main applications: to obtain high-resolution images of the retina and to produce aberration-free retinal images to improve vision. Additionally, it can be used to modify the aberrations of the eye to perform experiments to study the visual function. We have developed an adaptive optics prototype by using a liquid crystal spatial light modulator (Hamamatsu Programmable Phase Modulator X8267). The performance of this device both as aberration generator and corrector has been evaluated. The system operated either with red (633nm) or infrared (780nm) illumination and used a real-time Hartmann-Shack wave-front sensor (25 Hz). The aberration generation capabilities of the modulator were checked by inducing different amounts of single Zernike terms. For a wide range of values, the aberration production process was found to be linear, with negligible cross-coupling between Zernike terms. Subsequently, the modulator was demonstrated to be able to correct the aberrations of an artificial eye in a single step. And finally, it was successfully operated in close-loop mode for aberration correction in living human eyes. Despite its slow temporal response, when compared to currently available deformable mirrors, this device presents advantages in terms of effective stroke and mode independence. Accordingly, the programmable phase modulator allows production and compensation of a wide range of aberrations, surpassing in this respect the performance of low-cost mirrors and standing comparison against more expensive devices.

©2004 Optical Society of America

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Supplementary Material (1)

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Figures (13)
Fig. 1.
Fig. 1. Double-pass adaptive optics apparatus used to test the PPM.

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Fig. 2.
Fig. 2. Left panel: Intensity distribution on the PPM plane in intensity modulation mode when a flat image is displayed in the modulator for an illumination wavelength of 633nm. The bright square is the whole 20×20 mm active area of the PPM. Right panel: False color representation of the local estimates of the gain across the PPM surface for 633nm.

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Fig. 3.
Fig. 3. Fitting to a sinusoidal function of the intensity in “intensity modulation mode” as a function of the gray level for red (left panel) and infrared light (right panel).

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Fig. 4.
Fig. 4. Measured coma terms (red dots) generated by displacing a pure spherical aberration map across the PPM plane horizontally (left) and vertically (right) together with the respective linear fittings (blue lines). The bold lines show the displacements for 0 coma generation, which are used to precisely align the displayed phase maps with the AO system. The amount of spherical aberration was arbitrarily set to 0.63 µm.

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Fig. 5.
Fig. 5. Temporal response of the PPM when switched between two flat images. Red line: intensity transmitted in “intensity modulation mode”; cyan lines: asymptotic intensity transmitted for each image; green line: 90% of the phase shift; blue lines: times of image switch and 90% of phase change.

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Fig. 6.
Fig. 6. Zernike terms measured with the HS sensor when inducing with the PPM different amounts of defocus (C(2,0)), astigmatism (C(2,2)), coma (C(3,1)), spherical aberration (C(4,0)) and pentafoil (C(5,5)). All the coefficients are expressed in microns over a 5.5 mm pupil in the eye pupil plane.

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Fig. 7.
Fig. 7. Experimental aerial images for pure Zernike coma (C(3,1)), spherical aberration (C(4,0)) and pentafoil (C(5,5)). The upper panels correspond to 1 µm of the corresponding mode and the lower panels to 3 µm. The pupil size was 5.5 mm in all cases. A gamma correction (γ=2) has been introduced in every image to modify the contrast in order to make the faint fine structure apparent.

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Fig. 8.
Fig. 8. Wavefront aberration map wrapped in the range [-0.97π, 0.97π] (left), associated PSF (center), and experimental aerial image (right) for the artificial eye with the adaptive optics OFF.

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Fig. 9.
Fig. 9. Wavefront aberration map wrapped in the range [-0.97π, 0.97π] (left), associated PSF (center), and experimental aerial image (right) for the artificial eye with the adaptive optics ON after one single iteration (open-loop mode).

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Fig. 10.
Fig. 10. Temporal evolution of the RMS of subject JB during close-loop corrections of the ocular aberrations with the PPM for different values of the close-loop attenuation.

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Fig. 11.
Fig. 11. Temporal evolution of the RMS of subject JF, PA, PP, and SM during close-loop corrections of the ocular aberrations with the PPM for a close-loop attenuation of 0.3.

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Fig. 12.
Fig. 12. Aberration maps and PSFs at the beginning of close-loop AO correction and after 3 sec. of operation. The associated wavefront RMS and the Strehl ratio for each PSF can be found in table 1. The images correspond to the series displayed in Fig. 10 (attenuation=0.3) for subject JB and Fig. 11 for the rest of the subjects.

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Fig. 13.
Fig. 13. Multimedia file (2.81 Mb) showing the evolution of the wavefront map and the associated PSF during a close-loop AO correction on subject JB using the PPM. The video rate is set to 4 Hz to approximately match the PPM operation rate. Each individual PSF is normalized to its maximum.

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Tables (1)

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Table 1. RMS and Strehl ratio associated to the AO correction for each subject. The initial values are those found for the uncorrected eye. The mean corrected values correspond to the asymptotic average obtained by discarding the first 2 seconds of each series. The best values are the higher Strehl and the associated RMS in the series.

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Equations (1)

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I = I 1 + I 2 cos [ α ( g − g 0 ) ] ,
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