Adaptive Optics for Vision Science

Chapter 2 - Wavefront Measurement And Correction

Aberration Structure of the Human Eye

PABLO ARTAL, JUAN M. BUENO, ANTONIO GUIRAO,
and PEDRO M. PRIETO

Universidad de Murcia, Murcia, Spain

2.1   INTRODUCTION

The image-forming properties of any optical system, in particular the eye, can
be described completely by the wave aberration. It is defined as the difference
between the perfect (spherical) and the actual wavefronts for every point over
the eye’s pupil. A perfect eye (without aberrations) forms a perfect retinal
image of a point source (Airy disk). In reality, however, imperfections in the
refracting ocular surfaces generate aberrations that produce a larger and, in
general, asymmetric retinal image. The monochromatic aberrations of the
complete eye, considered as one single imaging system, can be measured
using a large variety of wavefront sensing techniques (see also Chapter 3).

Every ocular surface contributes differently to the overall quality of the
retinal image. The relative contribution to the eye’s aberrations of the main
ocular components (the crystalline lens and the cornea) can be obtained by
the combined use of ocular and corneal aberration data.

The monochromatic aberrations of the eye depend on a variety of factors
that will be reviewed in this chapter: in particular, accommodation, aging,
and retinal eccentricity. Beyond monochromatic aberrations, in normal whitelight
illumination, chromatic aberrations also play an important role that will
be discussed. The effect of polarization and scatter in the retinal image is also
considered in the latter sections of this chapter.

In adaptive optics (AO) applications, it may be useful to have a statistical
description of the aberrations of the system to be corrected. In the case of
the atmosphere in astronomical applications, this approach is widely used and
is very successful. Although, in the case of the eye, the situation is rather different,
we considered a similar statistical approach for the aberrations in a
population of normal eyes.

If the aberrations of the eye are known, it is possible to correct them using
a wavefront correcting device that compensates for the eye’s aberrations in
real time. This is a direct application of AO to the eye. In the ideal case, the
system of corrector + eye becomes permanently aberration free, producing
perfect retinal images. In different laboratories, AO in the eye has been demonstrated
using deformable mirrors or liquid crystal spatial light modulators
as corrector devices [1–5]. These systems are still laboratory prototypes that
include a wavefront sensor and a corrector, allowing vision science investigators
to perform visual psychophysics (see also Chapter 14) or to record
high-resolution retinal images (see also Chapters 1 and 10) through near to
aberration-free ocular optics. However, for practical applications, aberration
correction for the eye probably needs to be performed using simpler
approaches than those already demonstrated in research laboratories. The
most promising options are customized ablations in refractive surgery and
customized contact lenses or intraocular lenses. These cases are examples of
static corrections that are permanent and fixed, such as customized corneal
ablations (see also Chapter 12) or intraocular lenses (see also Chapter 11), or
are reversible and mobile, such as customized contact lenses (see also Chapter
11). These technologies will be reviewed in other chapters of this book, but
their correct implementation in the eye depends heavily on the understanding
of the nature of ocular aberrations.

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