The optical resolution of the eye is fundamentally limited by the wave aberrations intrinsic to the cornea and crystalline lens and diffraction due to the finite size of the eye’s pupil. Conventional corrective methods such as spectacles, contact lenses, and refractive surgery provide a static amelioration of low-order sphere and cylinder. However, ocular image quality can be significantly improved by dilating the pupil to minimize diffraction and correcting the aberrations across the larger pupil, for example, using an adaptive optics (AO) system [1
AO has been successfully integrated into a variety of retina camera modalities, including conventional fundus cameras [1
], confocal scanning laser ophthalmoscopes (cSLO) [8
], and optical coherence tomography (OCT) [11
]. The technique enables routine, in vivo
observation of retinal structure at the cellular level, structure that could not otherwise be seen. AO has also been used to explore the limits of human visual acuity [18
] and to control the type and amount of aberrations to which the retina is exposed [19
]. For design considerations and a review of results using AO in vision science the reader is directed to Refs. 21
The effectiveness of AO fundamentally depends on its ability to measure, track, and correct the ocular aberrations. Performance of the last step is largely dictated by the AO system’s key component, its wavefront corrector. This device dynamically imparts an ideally conjugate aberration profile onto the passing wavefront, thus canceling the original aberrations. Numerous types of wavefront correctors have been employed in AO systems for the eye, but none have yielded diffraction-limited imaging for large pupils (≥6 mm). One problem is that the characteristics of the wavefront corrector necessary to achieve diffraction-limited imaging in the human eye are not well understood. Consequently, correctors have been employed somewhat arbitrarily with the expectation that image quality will improve, the extent of which is empirically determined.
Additionally, many of the wavefront correctors applied to the eye have been developed primarily for compensation of atmospheric turbulence. A common example being macroscopic discrete actuator deformable mirrors (DMs), such as those manufactured by Xinetics Inc. [23
]. Specifically, their actuator number, stroke, influence functions, and speed have been tailored to the spatial and temporal properties of the atmosphere [24
] rather than that of the eye [26
]. The high temporal fluctuations of atmospheric turbulence are roughly 2 orders greater than the microfluctuations in the eye; also their dynamic range is often too small for compensation of ocular aberrations. While wavefront correctors represent a small fraction of the total cost of ground-based telescopes in which they are employed, they represent a significant fraction of the total cost of most commercial retina cameras. Atmospheric wavefront correctors are also generally bulky, with large mirror surfaces (approximately several centimeters or more) that require long focal length relay optics to magnify the pupil of the eye. A smaller corrector comparable with the dilated pupil of the eye (4–8 mm) can substantially reduce the instrument size and is commercially attractive.
Alternative wavefront corrector technologies, which are more cost effective and smaller, have been explored. Burns et al.
] evaluated a customized phase plate to correct static higher-order aberrations for a cSLO. Significant improvement, however, can be realized if the correction is performed dynamically [2
]. Various types of dynamic wavefront correctors have hence been applied to the eye. Bimorph mirrors [24
] having 13–35 actuators have been investigated by several groups [3
]. Recently, Fernandez et al.
] evaluated a magnetic membrane mirror with 52 actuators. Microelectromechanical systems (MEMS) [30
], promises batch fabrication of low cost, compact wavefront correctors. Bulk micromachined membrane MEMS mirrors [31
] employing 37 electrodes have been successfully applied to the eye [32
]. Although both bimorph and membrane mirrors have a large dynamic range (~8 and 16 μm, respectively [32
]) for low-order aberrations, this falls rapidly with increasing spatial frequency. For an analysis of several commercial bimorph and bulk micromachined MEMS mirrors for the eye, see Dalimier and Dainty [35
Surface micromachined devices [30
] are another class of MEMS mirror whose mode of operation is comparable with discrete actuator DMs. Doble et al.
] employed a surface micromachined MEMS DM [37
] and successfully imaged human cone photoreceptors, demonstrating that wavefront correctors other than the macroscopic form are capable of this task.
Liquid-crystal spatial light modulators (LC-SLMs) are an alternative wavefront corrector technology. Transmissive, pixelated designs with 69 and 127 pixels were examined by Thibos and Bradley [38
] and Vargas et al.
], respectively. Prieto et al.
] and Fernández et al.
] used an optically addressed LC-SLM [41
]. Such devices have high spatial resolution (480 × 480 piston-only pixels) and low control voltages (~5 V) but are limited to phase-modulating polarized light with typical modulation confined to 2π. Phase wrapping [38
] must be used to extend their dynamic range.
While most of these correctors hold considerable promise for vision science, it remains unclear what the optimal parameters are to achieve a specified performance level in the eye, e.g., diffraction-limited imaging. Miller et al.
] provided a performance evaluation of piston-only segmented wavefront correctors using a limited population of 12 human eyes. Here we considerably extend this analysis to cover two separate large populations, each comprising 70 eyes. Two additional wavefront correctors, discrete actuator and piston/tip/tilt segmented devices are also examined. Required actuator stroke and number for diffraction-limited imaging is determined for various pupil sizes, second-order aberration states, and imaging wavelengths.