WAVEFRONT ABERROMETRY – MEASURING THE REFRACTIVE POWER OF THE EYE
Wavefront Aberrometry technology is developed for the LASIK procedure, but it is also routinely used in examinations to diagnose vision problems. In the outpatient clinic, we use a phoropter – an ophthalmic testing device with several lenses of different power. Trying out different lenses before the patient’s eye, the ophthalmologist asks to confirm which lens is the most suitable, i.e. what image is the clearest. This information is very subjective, based on what the patient thinks he sees instead of what he really does see. As opposed to that, wavefront measurement is precise and objective because the visual errors are automatically detected as the light rays travel through the eye.
What does wavefront mean?
Imagine light projects as a beam of light rays. If we drew a line at an angle of 90% to the beam of light, we would get a wavefront map. In the eye with the perfect vision the wavefront is flat, whereas in the eye with optical aberrations, it is irregular. Distortion types of the wavefront created as the light goes through the eye give important information about the refractive error and the ways to correct it.
What is wavefront technology (aberrometry)?
Aberrometry is a technique measuring how the wavefront lights go through the cornea and crystalline lens (refractive components of the eye). The distortion during the travel of light is called aberration and presents a specific refractive error. Wavefront technology or aberrometry diagnoses higher- and lower-order aberrations. It represents the way the eye focuses or refracts the light.
Nowadays higher-order aberrations can be precisely defined by wavefront technology and corrected with some special types of glasses, contact lenses, or refractive surgery. Hence, they have become an important part of the examination. These aberrations became important when ophthalmologists noticed they are a source of discomfort for the patients who had undergone LASIK procedures. They can cause halos, so-called “ghost images”, or other visual disturbances, especially in conditions of poor lighting such as nighttime driving.
We use an aberrometer for wavefront measurement. Measuring lasts a couple of seconds. As a result, a wavefront map is created. It works as an “optical fingerprint” because there are no two eyes with an identical optical map.
A flat circular surface is a theoretically perfect eye (an emmetropic eye – an eye without a refractive error), the diameter of which matches the diameter of the pupil. In fact, the eye is a three-dimensional map created in comparison with a perfectly flat map.
Three-dimensional distortions occur on a real wavefront map. They are based on an individual passage of light rays and change as the light goes through the eye. Imperfections in the shape of the cornea and the lens make the waves speed up, slow down, or refract.
Higher-order aberrations can also arise due to dry eyes because the tear film distorts the light rays as they go through the tear film onto the cornea. Even cataract (a clouding of the normally clear lens) or a scar in the cornea causes higher-order aberrations and focusing problems.
There are more than 60 different types of aberrations identified and classified as refractive errors. Wavefront measurements identified as refractive errors can be calculated in different ways. Zernike polynomials are a method of choice. A Dutch physicist Fritz Zernike defined them in 1934. Every Zernike’s polynomial describes a specific shape on the light path through the eye. The sum of these polynomials is equal to all aberrations or it presents the total refractive error of the eye. The test result is presented as a topographic map with a detailed shape of the wavefront with aberrations. The ophthalmologist uses the map to decide how to correct the diopter – with LASIK or PRK refractive surgery, intraocular lenses, contact lenses, or glasses.
Bear in mind one can never have the perfect wavefront because no eye is perfect. Even if you have a certain number of higher-order aberrations, there is no need to worry unless they cause a serious problem such as night driving problems.