Corneal topography can measure the shape of the surface of an irregular cornea, but it cannot measure the actual refractive topography of the entire lens–cornea optical system. For such measurements, instruments traditionally used in astronomy to measure optical distortions, or wavefront aberrations, induced by the inhomogeneous and turbulent earth atmosphere, have been applied to the examination of the human eye. These instruments are called wavefront aberrometers or simply aberrometers.
Wavefront aberrometers are essentially ray-deflection autorefractors (the prototype of which was the Scheiner disk) that measure the deflection of light rays passing through many pupil locations, rather than just a few locations sufficient to determine only spherocylindrical errors of the eye, as described previously in the section on autorefractors.
For ophthalmic applications, the most popular method employed for wavefront aberrometry is Hartmann-Shack aberrometry, which was first demonstrated by Josef Bille and colleagues in 1994.
Before getting into the details of the underlying principles, let us first review the nature of a wavefront. It is an artificial construct connecting points in a light ray bundle of equal travel time from a common source. This is perhaps best understood in the example of water waves, as illustrated in Figure 8-7. The perfect wavefront shown on the top nicely illustrates that, in general, wavefronts are perpendicular to the ray direction. The concept of wavefront aberrations is simulated on the bottom, where we can see an irregular wavefront that becomes distorted by a floating twig.
In Hartmann-Shack aberrometry, ocular wavefront distortions are measured using a Hartmann-Shack wavefront sensor (Fig 8-8), which consists of a micro-lenslet array and a charge-coupled device (CCD) camera. A micro-lenslet array is a lattice of tiny lenses that can be thought of as a multifaceted lens, like an insect eye (Fig 8-9). An object imaged through such a lens results in multiple images of the same object arranged in an array. For example, in aberrometry, the light reflected out of the eye is partitioned into multiple beams by the micro-lenslet array, forming multiple images of the same retinal spot on the CCD camera. An ideal eye produces a perfect emerging wavefront and thus a regular array of spot images, with each image falling on the grid produced by the optical center of each facet of the multi-lenslet array (Fig 8-10A). An eye with aberrations, however, produces an irregular wavefront and thus an irregular array of spot images, displaced from where they should otherwise fall on the grid (Fig 8-10B). From the displacement of each spot, the shape of the wavefront can be reconstructed and represented in the form of a 2-dimensional wavefront map.
Figure 8-7 Pictorial explanation of wavefront aberrations by means of water waves. A, A perfect, regular wavefront is shown. B, An irregular wavefront is shown that became distorted by a floating twig.
(Part A by Sang Pak, modified by Kristina Irsch, PhD. Part B by Alain Vagner.)
Figure 8-8 Schematic of a Hartmann-Shack wavefront sensor. In Hartmann-Shack aberrometry, wavefront distortions are measured using a Hartmann-Shack sensor (HSS) that consists of a micro-lenslet array and a charge-coupled device (CCD) camera.
(Redrawn by Mark Miller from a schematic image courtesy of Abbott Medical Optics.)
Figure 8-9 Pictorial explanation of a micro-lenslet array (right) by means of an insect eye (left).
(Courtesy of Thomas Shahan, modified by Kristina Irsch, PhD; inset reproduced with permission from Axetris AG.)
Figure 8-10 An object imaged through a multi-lenslet array. In aberrometry, the light reflected out of the eye is divided into multiple beams by the micro-lenslet array, forming multiple images of the same retinal spot on the charge-coupled device (CCD) camera or video sensor. A, An ideal eye produces a perfect undistorted emerging wavefront and thus a regular array of spot images, with each image falling on the grid produced by the center of each facet of the multi-lenslet array. B, An aberrated eye produces a distorted wavefront and thus an irregular array of spot images, displaced from where they should otherwise fall on the grid.
(Illustration reproduced from Thibos LN. Principles of Hartmann-Shack aberrometry. J Refract Surg. 2000;16(5):S563–S65.)
Liang J, Grimm B, Goelz S, Bille JF. Objective measurement of wave aberrations of the human eye with the use of a Hartmann–Shack wave-front sensor. J Opt Soc Am A Opt Image Sci Vis. 1994;11(7):1949–1957.
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.