D7 Aberrometry: Measurement of the Ocular Wavefront

Topic

The Hartmann-Shack aberrometer—marketed under names such as COAS or iTrace—directs a very narrow beam of light into the eye; this beam passes through the cornea and the crystalline lens, reflects off the retina, and exits the eye, traversing the entire pupil rather than just the narrow entry point. Upon exiting the full pupil, the reflected beam is no longer a narrow beam but a complete ocular wavefront—representing the shape the light assumes at every point across the pupil after having passed through the cornea and crystalline lens twice.

If an eye's cornea and crystalline lens introduced no distortion beyond what is already corrected by the patient's standard optical prescription, the ocular wavefront exiting the eye would be perfectly flat, showing no irregularities from one point of the pupil to the next. The Hartmann-Shack aberrometer passes the ocular wavefront through an array of microlenses—a grid of tiny lenses—each of which focuses its corresponding portion of the wavefront onto a fixed reference point. When the ocular wavefront is not flat, each microlens focuses its portion onto a point offset from that fixed reference; the complete pattern of these offsets, measured microlens by microlens, reveals the exact shape of the wavefront distortion across the entire pupil. Another technique, the Tscherning aberrometer, operates differently: instead of analyzing the ocular wavefront as it exits after retinal reflection, it projects a known grid onto the retina through the eye's own cornea and crystalline lens, then analyzes how that projected grid is distorted on the retina to infer—in reverse—the distortion of the ocular wavefront itself. The ocular wavefront distortion revealed by the microlens array is too complex to be described by a single number; therefore, the Hartmann-Shack aberrometer decomposes it into Zernike coefficients—a series of numbers, each indicating the extent to which a specific, standardized type of distortion is present within the total ocular wavefront distortion. Some Zernike coefficients correspond to the same low-order aberrations corrected by a standard optical prescription—specifically, the defocus corrected by the sphere component and the astigmatism corrected by the cylinder component—whereas other coefficients correspond to high-order aberrations, which are more complex forms of distortion that cannot be corrected by any combination of sphere and cylinder.

Among the high-order aberrations revealed by Zernike coefficients, spherical aberration is one of the most common. It occurs because light rays passing through the periphery of a simple curved surface—such as that of a conventional spherical lens—converge at a different point than rays passing through the center of that same surface; consequently, there is no longer a single focal point for the entire pupil, but rather one for the center and another, slightly different one, for the periphery. This same spherical aberration detected in an eye by the Hartmann-Shack aberrometer can also be introduced by an intraocular lens with a simple curved surface if that lens is used to replace the natural crystalline lens following cataract surgery. While conventional refractive surgery—based on the standard model of ametropia—corrects only low-order aberrations (sphere and cylinder), wavefront-guided refractive surgery utilizes the full map of Zernike coefficients provided by a Hartmann-Shack aberrometer to reshape the cornea; this corrects not only the standard optical prescription but also the specific high-order aberrations unique to that particular eye. Similarly, a premium intraocular lens is designed with a surface that is not merely spherical but specifically calculated to cancel out the spherical aberration that a simple curved surface would otherwise introduce; consequently, the ocular wavefront exiting an eye implanted with such a lens more closely approximates the ideal, flat wavefront against which all Hartmann-Shack aberrometer measurements are compared.

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