Wavefront measurements for higher-order aberrations
Myopia and hyperopia are challenging the patience of millions of people worldwide. Most of these people have to use prescription eyeglasses or contact lenses on a daily basis, which require frequent lens care like cleansing, rinsing and even disinfection. Another big task for Rx eyewear users is to change periodically lenses, due to changes to their prescription. This problem is especially common among children and even teenagers, whose eyes are still under development. Young eyewear users are encouraged or forced to receive regular eye exams and have a check whether the prescription has changed. During such an exam, the prescription measurement and determination is important.
The overall advantage of wavefront measurement
Currently, most of the doctors change the lenses with different powers in front of the patient and get his response, in order to get the exact prescription. This method of prescription determination is quite simple and widely applied. In fact, the result is based on the patient’s subjective feeling, which can be inaccurate or even misguiding.
In contrast, new wavefront technology can automatically identify vision errors from the way light waves travel through the eye. This is absolutely an objective measurement. Wavefront measurement can also provide more information than conventional eyeglasses or contacts prescription. Actually, wavefront-guided LASIK can provide sharper vision than conventional LASIK. It is estimated that wavefront measurement will replace conventional prescriptions.
Higher-order aberrations can be detected only by wavefront technology
Lower-order vision errors such as farsightedness and nearsightedness can be diagnosed by conventional methods, while higher-order aberrations can only be defined by wavefront technology and cured by new kinds of contacts, intraocular lenses and refractive surgery. These aberrations include coma, trefoil and spherical aberration, which receive more concern because they are the sources of LASIK side effects such as halos and ghost images. The mentioned wavefront-guide LASIK can reduce such aberrations and improve visual performance. It is also worth mentioning possible causes of higher-order aberrations, e.g. dry eye, cataracts, eye surface scarring and trauma.
Operational details of wavefront measurement
The device used to conduct a wavefront measurement is called aberrometer, which just requires a patient to rest his or her chin on the chin rest for a few silent seconds. When the patient focuses the eyes on its light point, the aberrometer firstly measures pupil diameter, which is used to formulate a theoretically perfect reference wavefront shape. The second step is to draw an actual wavefront with potential distortions. The last job is to compare the two wave-fronts point-to-point, so as to work out the patient’s exact wavefront map.
The principle of wavefront measurement
After making a wavefront measurement, the doctor will get a wavefront map. It consists of the lines perpendicular to the tips of a bundle of light rays, which is perfectly flat under perfect vision and is irregular under an imperfect eye. Wavefront technology evaluates the focus distortions that occur on cornea, so that both lower- and higher-order vision errors can be diagnosed.
As mentioned before, the reference wavefront is theoretically perfect, which is also flat and circular plane. In contrast, the patient’s real wavefront map is three-dimensional and contains distortions, created by imperfections on the cornea and the lens. These irregular components refract and focus light rays abnormally.