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How the Eye Works
Clear vision in the normal eye is the result of light rays passing through
the cornea and focusing on the retina. |
Nearsightedness occurs when the cornea is too steep or the eye is too long. This
causes the light to focus in front of the retina, causing blurry vision. |
Farsightedness occurs when the cornea is too flat. This causes the light to focus
behind the retina, resulting in blurry near and distance vision. |
Astigmatism occurs when the cornea is shaped like a football; curved more in one
direction than the other. This causes light to focus on more than
one point, resulting in blurry and distored vision. |
The goal of Laser Vision Correction is to reduce or eliminate corrective lenses.
There's more to good vision than "20/20".
If you're reading this, you're probably familiar with the vision chart that hangs in every optometrist's examining room.
The one with a big "E" at the top. It's officially known as a Snellen chart and it's been the basis for measuring what's
known as visual acuity
since the late 1800s. Think of visual acuity as a quantitative measure of your vision, establishing where your
vision is placed on a numeric scale. There are also qualitative measures of your vision, such as your ability to
perceive subtle contrast changes, especially in low-light situations. Taken together, these quantitative and qualitative
measures determine the overall performance of your vision.
Visual acuity: your vision, by the numbers.
If your visual acuity is determined to be 20/20, you see at 20 feet what a person with normal vision sees at 20 feet. If
you have 20/40 vision, you see at 20 feet what a person with 20/20 vision would see at 40 feet. And, if you're one of
the lucky ones (e.g., 20/15), you can see at 20 feet what others would have to move closer (15 feet) to see.
Your visual acuity is an important measure of your vision. Many common problems can be identified by this measurement
method, including the ones you've most likely heard of before
— like nearsightedness and farsightedness.
Beyond the numbers: your vision's quality
There are also qualitative factors affecting your vision. A number of considerations — like contrast sensitivity
— also contribute to your vision's performance. Contrast sensitivity is a measure of the ability to discern subtle changes
in a visual image. Many routine visual tasks — recognizing a face, for example — rely more upon contrast
sensitivity than visual acuity. Problems believed to be associated with poor visual quality are normally most apparent in
low-light situations.
Understanding your vision requires taking both the quantitative and qualitative measures into consideration. This is because
it's possible to possess very good visual acuity yet have poor visual quality, and vice versa. It's important to consider both
as you make your decision about laser vision correction.
Diagnosing problems with your vision's quality
Through a process called wavefront mapping, it's possible to identify problems with your vision that experts believe contribute to these and other visual quality problems. Wavefront maps are plotted by passing a narrow ray of eye-safe light through the optical system and measuring the optical distortions as the light exits the eye. These patterns are then compared with the flat wavefront maps associated with normal vision.
A full listing of terms is available in the glossary of the Ladarvision Website.
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Gailey Eye Clinic
1008 N. Main St., Bloomington, IL 61701
309.829.5311 - 800.325.7706
1008 N. Main St., Bloomington, IL 61701
309.829.5311 - 800.325.7706 |
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Copyright © * 2008 Gailey Eye Clinic, LTD All rights reserved.
* Laser eye graphic, Dr. Lockhart
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