How The Eye Works
The eye is sometimes likened to a camera. As light passes through the lens it is bent and transposed onto the eye’s film – the retina. The film is then ‘developed’ by the brain, becoming the image that we see.
As light enters the eye it first passes through the cornea – the clear ‘window’ to the eye. Because the cornea is curved, the light rays bend (refract). Light then passes through the pupil to the lens. (The iris – the colored portion of the eye – controls the amount of light that enters the eye with muscles that cause the pupil to contract if there is too much light or to dilate if there is too little light.) After passing through the pupil, the light hits the curved surface of the lens and is refracted, or bent even more, so that it focuses properly on the retina. The retina then turns the light into electrical energy, which passes through the optic nerve to the brain stem, and into the occipital lobe where it is converted into an image.
This process works perfectly in people with 20/20 vision. Imperfect vision occurs when the shape of the eye is irregular or when the light rays do not focus directly on the retina – these imperfections are collectively known as refractive errors.
Refractive Errors
The three most common refractive errors that can be surgically corrected with laser eye surgery are nearsightedness, farsightedness, and astigmatism.
Nearsightedness (Myopia):
Nearsightedness or myopia occurs due to an irregularly shaped cornea – when light enters the eye it focuses in front of the retina, rather than directly on the retina. If you are nearsighted you will see well up close, but distant objects will be fuzzy.
Farsightedness (Hyperopia):
Farsightedness or hyperopia also occurs due to an irregularly shaped cornea – when light enters the eye it focuses behind the retina, rather than directly on the retina. If you are farsighted you will have trouble seeing objects up close, but will see distant objects clearly.
Astigmatism:
The most common of all eye disorders, astigmatism occurs when the eye is shaped more like a football than a basketball. This odd shape causes the light to focus on two points of the retina, rather than one.
Astigmatism is often accompanied by either nearsightedness or farsightedness. If this is the case, LASIK will treat both refractive errors simultaneously.
LASIK, PRK, and other refractive procedures correct nearsightedness, farsightedness, and astigmatism by reshaping the cornea so that the focal point is on the retina.
What is LASIK?
LASIK is a surgical procedure intended to reduce a person's dependency on glasses or contact lenses. The goal of this Web site is to provide objective information to the public about LASIK surgery. See other sections of this site to learn about what you should know before surgery, what will happen during the surgery, and what you should expect after surgery. There is a glossary of terms and a checklist of issues for you to consider, practices to follow, and questions to ask your doctor before undergoing LASIK surgery.
LASIK stands for Laser-Assisted In Situ Keratomileusis and is a procedure that permanently changes the shape of the cornea, the clear covering of the front of the eye, using an excimer laser. A knife, called a microkeratome, is used to cut a flap in the cornea. A hinge is left at one end of this flap. The flap is folded back revealing the stroma, the middlesection of the cornea. Pulses from a computer-controlled laser vaporize a portion of the stroma and the flap is replaced. There are other techniques and many new terms related to LASIK that you may hear about.