Errors of Refraction

What is Refraction?

By definition, refraction is a phenomenon that takes place when light travels from one medium to another. Due to the different refractive indices (the different composition) of each medium, light is refracted (bent) along its pathway. The human eye consists of different parts (media with different refractive indices) that lead to light refraction upon reaching its final
destination, the retina. In a normal eye with no refractive errors, a condition known as “emetropia”, the light undergoes multiple refractions till it reaches the retina, where it is focused; resulting in a clear image formation.

However, when changes occur in the eye medias; the cornea and the lens alongside the axial length of the globe (the size of the globe), the formed image becomes unfocused, leading to what is perceived as “blurry vision”. Whether it’s a change in the shape of the cornea, the lens or a change in the axial length of the globe (the total size of the eye), the end result is an error of refraction, namely hypermetropia or long-sightedness, myopia or short-sightedness and astigmatism.

What are the errors of refraction?

Refractive errors refer to conditions in which light entering the eye is not properly focused on the retina, resulting in blurry vision. These errors usually occur due to changes in the shape of the cornea or the size of the eye.

Types of error of refraction:

Myopia (= short-sightedness)

In myopic eyes, the light rays of an image are formed in front of the retina, resulting in a blurred image, however, clarity is achieved the closer the objects get, hence, the term “short-sightedness”; the image is more focused on the retina when the object is viewed from a shorter distance.

Hypermetropia (= long sightedness)

In hyperopic eyes, the light rays of an image are formed behind the retina, resulting in a blurred image, however, clarity is achieved the farther the objects get, hence, the term “far-sightedness”; the image is more focused on the retina when the object is viewed from a longer distance.

Astigmatism

In an astigmatic eye, where the corneal surface obtains a rather oval shape, the light rays coming from certain directions will focus just in front of or just behind the retina causing a blurred image. Astigmatism along the horizontal and vertical axes are the least visually disabling.

How to correct errors of refraction:

Errors of refraction can be addressed in a myriad of ways, ranging from prescribing glasses, Contact lenses to refractive surgery.

There are different types of refractive surgery, whether it’s LASIK, Femto LASIK, SMILE or PRK, all procedures aim at vision improvement.

What is LASIK?

In LASIK (Laser Assisted In-Situ Keratomieulsis), a specific type of laser is used to change the shape of the cornea to improve the way light is refracted or bent upon reaching the retina, resulting in a focused image. A flap in the cornea is created allowing the laser to fall on the corneal layers and make changes in its overall thickness.

LASIK can be used to treat myopia, hyperopia and astigmatism.

What is Femto LASIK?

In Femtosecond LASIK the flap is created with a laser instead of the Microkeratome which is used in the conventional LASIK, leading to more precision in the creation of the flap and less complications associated with flap creation via the conventional way.