TECH CONNECTION
The Practical Side of Wavefront Science
If Zernike polynomials are getting you down, you'll appreciate the clarity our new columnist brings to the subject.
BY LOUIS J. CATANIA, O.D., F.A.A.O., Jacksonville, Fla.

We all know a little about wavefront analysis and higher-order aberrations (HOAs) -- at least in theory -- but new information and advances in technology and clinical applications have placed these concepts center stage in human vision care. Today, and more so in future years, how we use wavefront science to understand, measure and, ultimately, correct HOAs in clinical practice will make this one of the most important eyecare technologies.

In these columns, I'll discuss how we can use wavefront science and HOAs to diagnose and treat vision and eye disorders. If you're not up to speed on wavefront basics, I urge you to dust off your physical and physiological optics textbooks for a quick refresher course.

FROM THEORY TO PRACTICE

To apply wavefront theory and HOAs to everyday clinical practice, we need to think of the human visual system as a three-part system, including:

1. The eyeball, which collects and focuses light entering the eye

2. The retina and the optic nerve, which transmit light impulses from the eye to the brain

3. The visual cortex of the brain, which is the perceptual interpreter of light impulses and, in fact, our ultimate vision.

This approach helps us better understand what each element contributes; the strengths and weaknesses of each portion of the system; what we're measuring when we examine patients and listen to their symptoms, comments and responses; and how we can apply technologies like wavefront science most effectively.

Let's get real practical about how we examine patients. Will this "new" knowledge from wavefronts and HOAs help us learn more from our Snellen charts and phoropters?

LEARN TO LOVE THE PSF

To answer this question, let's briefly discuss the concept of point spread function (PSF). You probably learned about this physiological entity for your Boards and then promptly filed it away in the "never to be used again" center of your brain. Wrongo! It's back! But I promise this time PSF will be so much easier to understand that you'll love it, embrace it and never forget it again.

Instead of trying to measure and diagnose vision with all those insane Zernike polynomials (which I'll discuss in a future column), let's talk about what people actually see and describe during an exam. What do I mean? Simply, that after spending an interminable amount of time doing a refraction, when you're asking a patient to read the Snellen chart, you're really asking him to evaluate his PSF. What???

Point spread function is the perception of a point of light on a dark background (like viewing a bright star in the sky). Without lower- or higher-order aberrations in your visual system (or in the atmosphere, in our star analogy), the PSF produces a non-aberrated, distinct point source. Aberrations will distort that point in ways we can define, measure and describe.

In my next column, I'll define, measure and describe exactly what's happening. Meanwhile, think about what your patients describe to you as they try to read the Snellen chart.

Dr. Catania is associated with Nicolitz Eye Consultants in Jacksonville, Fla. He does clinical research on developing eyecare technologies; consults for ophthalmic companies and professional journals; holds academic ranks at numerous educational institutions; and writes and lectures worldwide. You can reach him at lcatania@bellsouth.net.