Adaptive Optics for Vision Science

Chapter 11 - Customized Vision Correction Devices

11.1   CONTACT LENSES

Contact lenses are a potential means of correcting the higher order aberrations
of the human eye. Their immediate advantage over spectacles, the most
commonly prescribed method of correcting lower order aberrations in the
eye, is that they are centered over the cornea, close to the visual axis, and this
alignment remains relatively constant in all positions of gaze. This optical
advantage was identified in the earliest days of contact lens prescribing [1],
and since then, contact lenses and the higher order aberrations of the eye have
been intrinsically linked. The presence of higher order aberrations in the eye
has been known for several decades [2–4] and, until recently, were primarily
identified as spherical aberration. However, the significant role of nonrotationally
symmetrical aberrations (such as coma) has been studied and identified more
recently as a major contributing factor to the less than ideal optical
performance of the human eye [5]. Current measurements using Shack–
Hartmann-based wavefront sensors have opened the door to identifying the
magnitude and form of higher order wave aberrations in large, preoperative,
physiological populations [6, 7]. Initial inspection of the average distribution
of higher order aberrations across a typical prepresbyopic population requiring
refractive correction shows a distinct deviation from zero for spherical
aberration while all other Zernike terms have an average close to zero, a value
expected of a biological optical system attempting to optimize itself across a
population (Fig. 11.1). This would suggest that an appropriate aspheric correcting
surface would reduce the wave aberration of the eye significantly if it
were incorporated into a contact lens. However, as Figure 11.2 shows, there
is a wide range of spherical aberration values across the patient population,
which is not related to another variable, such as degree of ametropia. To
realistically correct more than 38% of the population, spherical aberration
would have to be introduced as an additional parameter in the contact lens,
rather than an average level of correction added into every lens. Indeed,
attempts have been made to improve visual performance by correcting the
spherical aberration of the eye using soft contact lenses manufactured specifi-
cally for individual eyes [8, 9]. However, the results were mixed, with the
improvement in visual performance being less than expected.

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