Refraction is the physical phenomenon whereby light changes direction in crossing from one medium to another. For that reason, because of shape and power, a lens can alter the direction of the light rays coming from the environment to focus them on the retina. “Refraction” is the name given to the measurement of the optic status of the eye, resulting from the relationship between the refractive power of the cornea and crystalline lens in relation to the distance that separate these lenses from the retina.

The ideal optics is found in the so-called emmetropic eye where the focus is on the retina. The ametropic eye, on the other hand, is out of focus. Incorrect focusing is used in Wavefront Technology to refer to the optical lower order distortion or aberration produced by myopia and hypermetropia.

Myopia is a frequent defect. As a result of its condition, the eye acquires excess power and light is focused in front on the retina without reaching it. Myopes have good near vision but distant objects appear blurred and sometimes they cannot discern them. Myopia can be hereditary. It starts in childhood and progresses into adulthood. Usually in myopia, there is excess distance between the eye’s lenses and the retina and they eye is very long. In some cases, myopia may be caused by excess power of the cornea or the crystalline lens.

We are all hyperopic when we are born, but the eye reaches its normal size at around 3 years of age. Hyperopes can see distant objects very well, by they have to make an effort to see objects that are near. In hypermetropia, the eye is smaller than average and the distance between the eye’s lenses and the retina is very short. Usually, the cornea too is flatter and has a lower power. Because of its condition, the eye lacks power and light focuses behind the retina. Hypermetropia may be inherited and it is a frequent cause of strabismus in children who go untreated.

It is a relatively common defect where light never focuses like a spot, giving rise to image distortions. The most important cause of astigmatism is the shape of the cornea, although the crystalline lens may be the origin. In astigmatism, the cornea has an oval shape, giving rise to an uneven refraction of light rays.

As we age, the eye’s focusing system weakens, preventing us from focusing on objects that are near. People realize that presbyopia has began when they are unable to read the menu in a restaurant or when they can no longer read the small print in the phone book. The method for correcting this defect is to prescribe spectacles for near vision. New Excimer laser techniques are being developed in order to allow some near vision in some cases, thus avoiding the daylong dependence on spectacles.

Se refiere a defectos naturales a la óptica del ojo, en general de menor importancia que las ametropías (miopía, hipermetropía y astigmatismo), pero que limitan la calidad óptica del sistema, especialmente en condiciones de baja iluminación.

La medida y el análisis de las aberraciones ópticas de Orden Superior (Aberrometría) específicas de cada ojo, son un nuevo campo en las ciencias de la visión, que está siendo desarrollado con la Tecnología del Frente de Onda.

One of the most important instruments in ophthalmology is the “Keratometer”, which read the curvature radius of the cornea. The eye’s refraction may be derived from those measurements.

We know that the cornea has 48.33 diopters of converging power, meaning that it is responsible for 3/4 of the eye’s refractive power. Hence the importance of knowing its configuration, of analyzing individual shape variations, and of recognizing any changes induced as a result of trauma or surgery.

When refractive surgery became popular inn the late 1970’s and early 1980’s it became necessary to improve the clinical methods available to measure the anterior surface of the cornea. With the keratometer as a basis, photokeratoscopes were developed in order to record the image of the rings of Placido’s Disc reflected on the cornea and to have a qualitative impression of the inherent shape distortions of individual corneas. (Photo 1)

Considering that the ability to quantify perceived distortion is of clinical importance, videokeratoscopes were developed in order to capture the keratoscopic image by means of a computer system and then reconstruct the anterior corneal surface, drawing plots that can be easily understood and used by the ophthalmologist. This technology is known as Corneal Topography.

With this method, high powers of the corneal surface are represented in warm colors such as yellow, orange and red (Photo 2) where as the low-power areas are represented in cooler colors such as green and blue (Photo 3), making it easy to visualize the effects of refractive surgery and understand its outcome (Photos 4 and 5).

This technology continues to evolve with the needs of the medical practice and other types of topography machines have been designed based on a sweeping horizontal or rotational movement of a light ray projected through a slit. The images obtained can then be digitally reconstructed into 3D maps that analyze elevation surfaces. These are known as Elevation Topographers. This new approach is used to analyze corneal aberrations as well (Photo 7) on the basis of Zernicke equations de rived from the irregularities revealed in the contour maps.

A wave front is an imaginary surface formed by a beam of parallel rays of light that propagates through a medium. This surface will be flat when all the rays travel at the same speed, and in electrodynamic terms, it is defined as an “equal phase surface”. In a defect-free human eye, this imaginary surface is curved because parallel rays have to travel over different distances and propagate at different speeds through the optics in the eye.

The ideal image of a spot light beam, emitted by an object in space, is a spot projected on the retina (macula). In a defect-free eye, only the rays that pass through the center of the pupil do so in a regular manner. Those that pass more towards the periphery of the pupil are directionally deviated in an irregular or asymmetric way. The effect of these directional deviations of the image on the retina is called an Aberration because it degrades the quality of the image and limits visual acuity.

Aberrations may also result from small irregularities of the optical system, originating in the cornea, the lens, the vitreous and/or the retina.

To this date, only two optical aberrations can be corrected: incorrect focusing (myopia and hypermetropia) and astigmatism. Those alterations that could not be corrected with conventional spectacles were called Higher Order Aberrations.

Wave front technology was originally developed 50 years ago in the field of astronomy where it was used to measure the distortions of the wave front of light coming from the atmosphere at the point of entry into the optical telescope. When adaptive optic controls were developed it was possible to improve the sharpness of images coming from outer space. Most of this technology was developed during the 70’s as part of the research on anti-missile defense systems.

In ophthalmology, the term has been used to describe all the concepts, instruments and applications developed to follow the light wave that enters the globe and measure distortions as it travels inside the eye.