Adaptive Optics for Vision Science

Chapter 18 - Design and Testing of a Liquid Crystal Adaptive Optics Phoropter

18.1   INTRODUCTION

Conventional phoropters are used by ophthalmologists and optometrists to
estimate and correct for the lower order aberrations of the eye, defocus and
astigmatism, in order to derive a prescription for their patients. An adaptive
optics phoropter measures and corrects the aberrations in the human eye
using adaptive optics techniques, which are capable of dealing with both the
standard lower order aberrations and higher order aberrations, including
coma and spherical aberration. This chapter describes the design and testing
of an adaptive optics (AO) phoropter based on a Shack–Hartmann wavefront
sensor
to measure the aberrations of the eye, and a liquid crystal spatial light
modulator
to compensate for them. The goal is to produce near diffraction-
limited image quality at the retina, which will enable the investigation of the
psychophysical limits of human vision. We will later show some preliminary
results from testing human subjects.

Corrective lenses can generally improve Snellen visual acuity to better than
20/20 in normal eyes by correcting the lower order aberrations, defocus, and
astigmatism, also known as sphere and cylinder [1]. Higher order aberrations
remain untreated, however, and continue to affect visual performance. One
of the goals of designing an AO phoropter is that it can correct higher order
aberrations to improve acuity beyond what can be achieved with conventional
spectacles or contact lenses. This improvement has been coined “supernormal
vision” [2]. The same design can be extended to produce in vivo images of
the human retina that are sharper, with higher resolution than conventional
fundus photography [2, 3].

Conventional deformable mirror (DM) devices, such as continuous faceplate
mirrors, are used in vision science and astronomical applications. In
addition to being expensive, these DMs typically have much larger apertures
than the eye. This leads to a large optical system in order to magnify the eye’s
dilated pupil to the larger size of the deformable mirror. This combination of
cost and size limits the suitability of an AO system using a conventional DM
for clinical trials and eventual commercialization.

Recently, new DM technologies have been developed based on both liquid
crystal (LC) devices and microelectromechanical system (MEMS) mirrors,
which are both compact and less expensive than the conventional DM devices.
The AO group at the Lawrence Livermore National Laboratory (LLNL) has
previously demonstrated very high order wavefront correction using LC and
other DM technology [4, 5]. This chapter demonstrates the use of new LC
technologies in the area of vision correction.

We start with a discussion of important design parameters related to the
wavefront sensor, light source, and spatial light modulator (SLM). Then, we
describe the testing of each subsystem followed by the testing of the combined
system. Results from human subjects testing are discussed at the end, along
with suggestions for future design improvement.

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