Wavefront Aberrometry: The Future of
entering a new era of vision care with adaptive optics.
Louis J. Catania, O.D., F.A.A.O.
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.
Optometric Management, Issue: November 2005