What causes refractive errors?
Since you need light in order to see, your eyes are constantly working from the moment you open them in the morning until you close them at night to sleep.
Clear vision is only achievable in an emmetropic eye, one that is perfectly formed so that when light that enters the eye it is refracted correctly and objects appear in focus.
When vision is blurred be it objects that are near or far away, it is usually because there is a refractive error present. Any eye with a refractive error is considered to be ametropic.
Laser Eye Surgery can correct refractive errors by removing a pre-determined amount of cells from the cornea, which is located at the front of the eye and is responsible for the majority of your eye’s refractive power.
How does the eye bend incoming light?
Refraction or the bending of light in the eye is made possible because of the shape of the cornea.
The cornea is designed to work with light passing from the air, which has a refractive index of 1.00. If this was any lower, then we wouldn’t be able to bend light clearly which is why if you open your eyes underwater everything is slightly out of focus because water has a lower refractive index.
In an emmetropic eye, the cornea is a perfect dome-shape so that light bends correctly.
In an ametropic eye, the curve of the cornea can be too steep, too flat or is oval-shaped instead of round.
Common refractive errors
Your ability to refract light correctly depends on four key characteristics that can often change over time as the eye grows and matures (especially during adolescence).
- The shape of the cornea and crystalline lens
- The number of irregularities on the corneal surface
- Depth of the anterior eye chamber
- Axial length of the eye (front to back)
Myopia: The eye is too long or the curve of the cornea is too steep, which results in too much optical power and light is focused in front of the retina rather than on it.
Hyperopia: If the eye doesn’t have enough refractive power because it is too short or the surface of the cornea is too flat, light lands beyond the retina to create a distorted image.
Astigmatism: Light rays that are refracted must focus on a single point on the retina known as the fovea. If the cornea is irregularly shaped, light falls on multiple focal points at the back of the eye instead of just one causing blurred vision.
All of the above can be effectively treated with Laser Eye Surgery by reshaping the cornea so that light is refracted and pinpointed directly onto the retina to create clear, sharp images.