...continued from Wavefront Technology in Eye Exams

A wavefront diagnosis of the eye
is performed by a device called an aberrometer. Currently, five different
kinds are in clinical use, including:

• Shack-Hartmann (or Hartmann-Shack) sensor
• Tscherning aberrometer
• Ray tracing aberrometer
• Scheiner aberrometer
• Double-pass aberrometer (slit skiascopy)

At present, the Shack-Hartmann sensor is the most popular with manufacturers and, therefore, the one you are most likely to encounter. With this device, you are required to sit and place your chin on a chin rest. Next, you are asked to peer into the device and focus your eyes on a point of light. After a few seconds of silence (no feedback will be required of you), the examination is completed.

Within minutes, a prescription prints out and you are free to go. This is a remarkably quick procedure compared with the conventional eye exam, which takes up to 30 minutes to complete.

A similar diagnostic experience can be expected of most types of aberrometers. The exception is the Scheiner aberrometer, which is subjective and requires patient response. This method can take about 12 minutes to complete.

While you are waiting in silence for the aberrometer to finish checking your eyes, you may wonder what is happening inside the device.

In general, an aberrometer uses a three-step process:

1. Because a wavefront passes through the opening in the front of the eye (pupil), the diameter of this opening is first measured to derive a reference wavefront shape representing a theoretically perfect eye that has the exact same pupil size as yours.
    
2. The round shape of a wavefront of light that has passed through your eye is then captured. Because no eye is actually perfect, all such wavefronts will contain at least some distortions. Some aberrometers capture the wavefront the instant it reaches the back of the eye or retina, while others capture the wavefront as it exits the pupil or front of the eye after being reflected off the retina. Some aberrometers capture the wavefront at both locations.
    
3. An aberration map of the eye is created by comparing the shape of the captured wavefront to that of the pre-programmed reference wavefront, measuring all points of difference between the two. A wavefront map of your eye, sometimes known as an "optical fingerprint," then is created.

When you start to interpret the results of your eye's wavefront map, keep in mind that the reference shape used for comparison is flat or two-dimensional. This flat, circular plane represents a theoretically perfect (emmetropic) eye that exactly matches the diameter of your eye's pupil. Your eye's actual three-dimensional wavefront map is created through comparison with this theoretically perfect, flat wavefront map.

The size of your pupil is important in creating a reference map, because this is where the wavefront of light first enters your eye and also where the wavefront exits the eye after it is reflected off the inner back of the eye or retina.

A wavefront of light that travels through a perfect eye is represented as a flat plane. But no eye is perfect. Therefore, three-dimensional distortions occur in the actual wavefront map based on the way light rays have been bent, impeded, or focused as they travel through the different refractive or focusing components of your eye.

Imperfections in the different focusing components of your eye, primarily the cornea and the lens, can alter the path of light rays by causing them to speed up, slow down, or incorrectly bend (refract). Other eye problems also can cause higher-order aberrations, including dry eye that alters the eye's surface moisture (tear film) that also helps refract light rays. Even cataracts clouding the eye's natural lens or scarring of the eye's surface (cornea) from eye surgery, disease, or trauma can cause higher-order aberrations and focusing problems.

Rather than reflecting back and emerging from the plane of the pupil as a two-dimensional, flat circle, the wavefront appears as a distorted, three-dimensional shape, bounded by the edge of the pupil. The failure of all the points of the wavefront's light rays to arrive at the pupil simultaneously and in an orderly fashion causes the shape of the wavefront to be a distorted version of the ideal flat circle. This distortion is the result of some light rays arriving in advance of what would be the ideal location, others lagging behind, and still others arriving off course.

These are the more common shapes of aberrations created when a wavefront of light passes through eyes with imperfect vision. (Images courtesy of Alcon Inc., LADARVision CustomCornea.)

Based on the many different patterns that can occur in a wavefront that acquires distortions as it passes through the eye, more than 60 different aberrated shapes have been identified and classified as vision errors. [Read more about higher-order aberrations.]

Using computer software, an aberrometer draws the shape of the wavefront after it has passed through an aberrated eye. Then the automated software measures the difference between the shape of the wavefront of the aberrated eye and the reference shape of a theoretically perfect eye.

An aberration map is a complete and accurate description of aberrations in an individual eye. It is sometimes referred to as an "optical fingerprint," because no two eyes produce the same prescription or aberration map. More importantly, it is a blueprint for custom designing vision treatment and correction, having applications for refractive surgery (LASIK and PRK), intraocular lenses, contact lenses, and spectacles.

Remember that you can never achieve a perfect wavefront, because no eye is perfect. Even if you are diagnosed with a certain number of higher-order aberrations, this is not necessarily cause for concern unless the degree of aberration causes significant vision symptoms such as difficulty seeing at night.