Adaptive Optics for Vision Science

Chapter 4 - Wavefront Correctors for Vision Science

Wavefront Correctors for Vision Science

NATHAN DOBLE
Iris AO Inc., Berkeley, California

DONALD T. MILLER
Indiana University, Bloomington, Indiana

4.1  INTRODUCTION
Aberrations of the ocular media and diffraction generated by the finite size
of the eye’s pupil fundamentally limit our ability to resolve fine retinal structure
when looking into the eye. Conversely, with the light path reversed, diffraction
and aberrations limit visual acuity to well below the spatial bandwidth
imposed by the neural visual system, such as that dictated by the sampling of
the photoreceptor mosaic. Conventional corrective methods, such as spectacles,
contact lenses, and refractive surgery, provide a static amelioration of
prism, sphere, and cylinder, which correspond to the lower order Zernike
aberrations of tilt, defocus, and astigmatism. Image quality in the eye, however,
can be significantly increased by dilating the pupil to minimize diffraction
and subsequently correcting the ocular aberrations across the large pupil
using, for example, an adaptive optics (AO) system.

In recent years, AO has been successfully applied to correct both the lower
and higher order ocular aberrations in a variety of retinal camera architectures.
These include conventional fundus cameras [1–3], confocal scanning
laser ophthalmoscopes (cSLOs) [4], and optical coherence tomography
(OCT) [5–7]. The increase in contrast and resolution permits observation of
retinal structure at the single-cell level, which could not otherwise be seen in
the living eye. AO has also been used to improve vision by controlling the
type and amount of aberrations to which the retina is exposed. Specifically,
AO provides a means to directly assess the visual impact of individual types
of aberration [8, 9] and allows patients to experience beforehand the predicted
visual benefit of invasive surgical procedures, such as refractive surgery
[10, 11]. In general, the ability of AO to improve resolution in the eye makes
it a key enabling technology for probing the microscopic living retina and
enhancing vision.

The extent to which AO can effectively improve resolution, however, fundamentally
depends on its ability to accurately measure, track, and correct
the ocular aberrations. This chapter focuses on the last step, correction. While
all of the steps are critical, the performance limiter of current AO systems
for vision science appears to be the wavefront corrector. This limitation,
coupled with the expense of wavefront correctors ($7000 to >$100,000),
motivates the need for discussion of this device. This chapter attempts to
bring together wavefront corrector information that is important for the
design of AO systems for vision science. Much of this is not readily found in
the adaptive optics literature, which is heavily centered on atmospheric applications.
Section 4.2 introduces the principal components of an AO system.
Section 4.3 presents the primary types of wavefront correctors, and Section
4.4 surveys versions that have been applied or are in the process of being
applied to vision science AO systems. Section 4.5 contains theoretical performance
predictions for the most common types of wavefront correctors. The
section summarizes results already in the literature with new predictions for
corrector types not yet evaluated, all within the mathematical framework of
Fourier optics. Predictions are based on modeling the correctors’ principal
operation in conjunction with measured wave aberrations collected on two
large populations. This theoretical analysis extends that already reported for
segmented piston-only devices [12].

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