From Adaptive Optics for Vision Science
13.1.1 Refractive Error and Refractive Correction
The purpose of the eye’s optical system is to cast an image of the external
world onto the photoreceptor layer of the retina. If the system were perfect,
it would focus all rays of light from a distant point source into a single image
point on the retina. Real eyes, however, suffer from three types of optical
imperfections that degrade the quality of the retinal image: aberrations, diffraction,
and scattering. Since image formation in the eye is entirely refractive
in nature (i.e., dioptric), as opposed to reflective (i.e., catoptric), one might
suppose that the terms aberrations and refractive errors are synonymous.
However, in ophthalmic contexts, the term refractive error has been restricted
historically to spherical and astigmatic focusing errors. In the language of
Zernike wavefront analysis, these two types of refractive error are called
aberrations of the second Zernike order. In some cases, ophthalmic prescriptive
lenses may also contain prisms to overcome binocular difficulty in forming
an image of the point of regard on the foveal region of the retinas of the two
eyes. Such prisms are described as Zernike aberrations of the first order, but
they are generally disregarded for the purposes of assessing retinal image
quality because prismatic deviations shift the image laterally without blurring
it. The reason ophthalmic prescriptions have historically excluded the Zernike
aberrations of third and higher orders (e.g., coma, trefoil, spherical aberration,
etc.) is that these aberrations could not easily be measured or corrected
with spectacles or contact lenses. Consequently, refractive aberrations beyond
the second order were historically outside the domain of clinical practice.
However, this attitude has been reversed dramatically in recent years as
researchers and industry have sought new methods for correcting also the
higher order refractive imperfections of eyes to achieve unprecedented quality
in the retinal image and of spatial vision [1, 2].
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