Wavefront Analysis Explained

by David A. Wallace MD

Wavefront analysis represents an entirely new way of thinking about correction of optical imperfections in the human eye. To understand how it is different from methods that came before, a little history is helpful.

Nearsightedness and farsightedness are referred to as spherical refractive errors and are corrected by spherical lenses. Lenses that correct nearsightedness are concave, and that correct farsightedness are convex. Spherical lenses are radialy symmetric. Astigmatism identifies a condition wherein the cornea is not spherically curved (everywhere equal) but like the side of a donut has different curvature in a steep axis (vertical in the case of the donut) and a flat axis (horizontal). Taken together, 'sphere' and 'cylinder' are considered " low order " aberrations of the human eye's visual system.

For approximately the past 200 years, refractive errors of the eye have been measured using combinations of spherical and astigmatic lenses. This measures the optimal combination of sphere and cylinder to achieve sharpest vision, sometimes referred to as the 'sphero-cylindrical error' .

We all know that there is much more to the perception of sharp, clear vision than the strength of correction in a prescription lens. Even wearing optimum prescriptive correction, some people notice glare , halo , ghosting , and the like. These and other imperfections of vision can be understood only if we look at the higher-order aberrations perceived by the whole eye as an imaging system.

The tools that precisely analyze aberrations in optical systems were developed by astronomers (desiring to view the heavens without distortion) and by optical engineers (who design equipment to manufacture lenses, cameras, projectors, microscopes, etc.). These instruments are called aberrometers or wavefront analyzers .

Wavefront analysis of the human eye measures sphere, cylinder (the "low-order" aberrations), and much more. Higher order aberrations include spherical aberration , coma, chromatic aberration, trefoil, and other types that are impossible to describe in lay terms. A mathematical representation of higher order aberrations was developed by an astronomer named Zernicke.

The process of correcting the aberrations of the human eye involves three steps:

• Measure the wavefront profile of the eye;
• Match the captured image to the eye being treated; and
• Treat with the computer-guided laser based upon the wavefront measurement.

Wavefront analysis of the human eye has only recently become possible. The commercial wavefront systems currently available are very costly and very complex. Laser treatment guided by wavefront analysis clearly has technical advantages, once all the kinks have been worked out of the systems. In my opinion, wavefront-based imaging and treatment is an important part of present and future vision corrective surgery.