Development of Adaptive Optics in Vision Science and Ophthalmology

DAVID R. WILLIAMS and JASON PORTER
University of Rochester, Rochester, New York

This chapter briefly reviews the history of ocular aberration measurement and
correction that paved the way to the development of adaptive wavefront correction
of the eye. While the focus of this book is on the engineering of adaptive
optics systems for the eye, this chapter describes recent applications of
adaptive optics and the scientific discoveries that adaptive optics has made
possible, encouraging the future development of this technology.

1.1   BRIEF HISTORY OF ABERRATION CORRECTION IN THE HUMAN EYE

1.1.1   Vision Correction

The first use of a transparent stone as a crude magnifying glass is not known,
though it has been suggested that this could have been as early as 5000 bc [1].
It is also unclear who first fixed simple lenses to the head. Though corrective
spectacles rank among the most important medical inventions in history, their
origins are obscure [2]. Most sources attribute spectacles to an unknown
Italian near the end of the thirteenth century. In any case, the invention of
spectacles seems to have been based on empirical observation of the effects
of glass held before the eye rather than theoretical insight, as their invention
preceded the first clear description of how the image was formed on the retina
[3] by at least 300 years. Kepler, an astronomer perhaps best known for his
laws of planetary motion, pointed out that the retinal image is inverted and
also clearly described the benefits of concave lenses for myopic correction and
convex lenses for hyperopic correction. This was a time of rapid advances in
the field of optics. The telescope was invented around this same time, though
again there is controversy about the inventor [4]. Most scholars attribute the
invention to Hans Lipperhey, a Dutch spectacle maker who produced a telescope
in 1608. Galileo would soon use one of the first telescopes to observe
the moons of Jupiter and sunspots.

The link between astronomy and the eye apparent in Kepler’s scientific
contributions and Lipperhey’s telescope is a recurring theme in the history of
vision science, culminating in the recent translation of adaptive optics from
astronomy to vision science. Galileo had a competitor, Christoph Scheiner,
who was also an astronomer with interests in physiological optics. Scheiner
demonstrated empirically that the retinal image was inverted by cutting a hole
in the back of an excised animal eye and viewing the retinal image directly
[5]. Scheiner also constructed what was arguably the first wavefront sensor
for the eye. Scheiner’s wavefront sensor evaluated the fate of light passing
through only two locations in the eye’s entrance pupil. Modern ophthalmic
wavefront sensors extend this concept by measuring the direction that light
takes as it passes through hundreds of different locations in the eye’s pupil.
Scheiner made two holes in an opaque disk. When held close to the eye, the
perceived image was doubled if the eye was defocused and single only if the
eye was in focus, providing subjective information about the eye’s most important
aberration.

It would be nearly 200 years before a clear understanding developed of
astigmatism, the eye’s second most important monochromatic aberration.
Thomas Young recognized the existence of astigmatism in his own eye and
determined that his astigmatism was predominantly lenticular in origin by
noting that it persisted even when he immersed his eye in water, largely neutralizing
the cornea [6]. In 1827, Sir George Biddell Airy, yet another astronomer,
fabricated the first spherocylindrical lenses to correct astigmatism. This
ultimately lead to the current ophthalmic practice of prescribing aberration
corrections with 3 degrees of freedom corresponding to a defocus correction,
the cylindrical power, and the cylinder axis. Helmholtz argued that the normal
eye contained more monochromatic aberrations than just defocus and astigmatism,
based in part on his own subjective observations of a bright point
source viewed in the dark [7]. These monochromatic, higher order aberrations
were often referred to as “irregular astigmatism” to distinguish them
from the regular astigmatism that could be corrected with a cylindrical lens.
Roughly one and a half centuries after Helmholtz’s description of the higher
order aberrations in human eyes, we are now equipped with the adaptive
optics (AO) technology that can systematically measure and correct them.