The Wavefront Analyzer



What is a wavefront analyzer?
Wavefront analyzers are used to map aberrations in the eye. Several types of visual imperfections, referred to as lower- and higher-order aberrations, exist within the eye and can affect both visual acuity and the quality of vision.

In the past, only lower-order aberrations such as myopia, hyperopia, and astigmatism could be measured and treated. However, these do not account for all potential vision imperfections. Higher-order aberrations can also have a significant impact on one’s quality of vision and are often linked to visual glare and halos that may cause night-vision problems.

Higher-order aberrations cannot adequately be corrected with glasses, contact lenses, or conventional LASIK treatments. In fact, some researchers have found that such aberrations may actually be increased by laser refractive surgery, while other aberrations are naturally occurring.

Ophthalmologists are just beginning to understand how these higher-order aberrations affect vision. The wavefront analyzer software performs complicated measurements and projects a precise map for the surgeon to evaluate. The data is transferred to the laser, which generates a “treatment table,” or an outline of the patient’s refractive error and higher-order aberrations. (A perfect wavefront would be completely flat.) When light rays enter the eye and traverse the different refractive indices, the wavefront surface changes, taking on a shape unique to that eye. These variations are called wavefront errors. Treating a patient with the information taken from the wavefront analyzer can result in greater clarity of vision and less complaints of glare or night halos.

How does a wavefront analyzer work?
Alcon, Bausch & Lomb, and VISX all use wavefront analyzers based on Shack-Hartmann aberrometry and use a Shack-Hartmann sensor, which measures the slope of the wavefront across the pupil of the eye. The Shack-Hartmann aberrometry method maps both lower- and higher-order aberrations by projecting waves of light into a patient’s eye and mapping the waves that bounce back.

Uncorrected Aberrations
Uncorrected Aberrations
Corrected Aberrations
Corrected  Aberrations

One way the data is converted is by using Zernike polynomials, also called modes. Each mode describes a certain three-dimensional surface, and the Zernike polynomials correspond with ocular aberrations. For instance, second-order Zernike polynomials represent the conventional aberrations such as defocus and astigmatism. Zernike polynomials above the second order represent the higher-order aberrations that are suspected of causing night glare and halos. Zernike polynomials help to simplify the wavefront technology by combining all aberrations into one simple map. This is called Zernike decomposition.

Zernike Polynomials Shapes
Zernike Polynomials Shapes

Eye care professionals are also given information through conventional refraction in diopters as well as in Zernike form. Surgeons don’t necessarily need to understand all of the mathematics or particulars of how the wavefront analyzer arrives at its wavefront map. The map is very similar to a topographical map and can easily be read by doctors.

This map is then transferred to the laser, enabling the surgeon to address the patient’s unique visual requirements.