Wavefront Aberrometry: The Future of
We're entering a new era of vision care with adaptive optics.
By Louis J. Catania, O.D., F.A.A.O.
THINK FOR A MOMENT about technologies in your lifetime. Can you think of one that hasn't changed for 60 to 80 years? (Of course, this exercise depends on how old you are.) Well, you don't have to think further than the classic phoropter and its function in a basic refraction.
You've flipped lenses, spun dials and said, "Which is better, one or two?" more times than you can or ever will want to remember. To quote a famous comic, "Not that there's anything wrong with that." In fact, you've done a pretty good job of keeping people happy with their corrected vision, even if you often hear, "Yes, I can read the bottom line, but it's not clear."
The phoropter refraction has served
us well by providing a means of measuring vision, using the diopter as our primary
unit of assessing aberrations. But with the application of adaptive optics for measuring
and correcting vision, we
enter a new era of vision care the era of wavefront aberrometry and the use of the much more precise root mean square (RMS) value as unit of measure.
We could take up 1,000 pages describing the different methods and types of aberrometry and the strengths and weaknesses of each. But as clinicians, do we really need all that mathematical and technical information? Maybe. But I think three basic concepts give us all we need to know to feel comfortable and confident using wavefront aberrometry.
The first concept we need to understand is our goal with aberrometry: To measure points of power within a given pupil diameter. Some instruments measure 128 points, some as many as 15,000. Ultimately, these points construct an aberration model or a Zernike polynomial that's recorded in multiple forms: RMS units; two-dimensional color maps; three-dimensional models; or point spread function (PSF).
The next basic concept is the construction of this aberration model by creating and analyzing a wavefront of light from the eye. To create this wavefront, rays of infrared laser light (non-absorbable, collimated, non-bendable rays) projected through a given size pupil are bounced off the retina. These rays form a wavefront of light that travels along the visual axis and passes through each structure and medium as it exits the eye. This process results in the cumulative collection of the aberrations that each structure and medium produce.
The final concept to understand is fairly easy. Once a wavefront exits the eye with 100% of the eye's aberrations captured within it, it can be projected as an image of all the points measured. This is called a "centroid" dot pattern. Each point in this image reflects the distortions created by the aberrations at that corresponding point in the pupil.
Using piston (a non-aberrated wavefront) as a baseline, a computer uses the RMS formula to measure the distortions of each point. In about 0.2 seconds it has its measurements and, voila, the computer constructs the aberrations of that pupil mathematically (RMS values) and graphically (Zernike maps, models and PSF). How cool is that?!
If you're a true clinician, you're asking, "Can we build a corrective device from this exquisitely accurate measurement?" That's where the real fun begins.