A refraction test is usually given as part of a routine eye examination. It may also be called a vision test. This test tells your eye doctor exactly what prescription you need in your glasses or contact lenses.
Normally, a value of 20/20 is considered to be optimum, or perfect vision. Individuals who have 20/20 vision are able to read letters that are 3/8 of an inch tall from 20 feet away.
If you do not have 20/20 vision, you have what is called a refractive error. A refractive error means that the light is not bending properly when it passes through the cornea and retina of your eye. The refraction test will tell your doctor what prescription you should use in order to have 20/20 vision.
This test is normally given as part of a routine eye examination. It tells your doctor if you need prescription lenses, as well as what prescription you need to use in order to see properly.
The results of the test can also be used to diagnose, or help diagnose, the following conditions:
- astigmatism, which is an abnormal curvature in the cornea that causes blurry vision
- hyperopia, which is also known as farsightedness
- myopia, which is also known as nearsightedness
- presbyopia, which is a condition related to aging that causes lens of the eye to have trouble focusing
- an ulcer or infection on the cornea
- macular degeneration, a condition related to aging that affects your sharp, central vision
- retinal vessel occlusion, a condition that causes the small blood vessels near the retina to be blocked
- retinitis pigmentosa, a rare genetic condition that damages the retina
- retinal detachment, when the retina detaches from the rest of the eye
Healthy adults who are not experiencing vision problems should have a refraction test every three to five years. Children should have a refraction test every one to two years, starting at no later than three years of age.
If you currently wear prescription glasses or contact lenses, you should have a refraction test every one to two years. This will allow your doctor to figure out what prescription is necessary as your eyes change. If you are having problems with your vision between exams, you should see your eye doctor for another refraction test.
Individuals who have diabetes should have an eye examination every year. A number of eye conditions are associated with diabetes, such as diabetic retinopathy and glaucoma. According to the American Diabetes Association, people with diabetes are at a greater risk for blindness than other Americans. (ADA)
Individuals over 40 or those with a family history of glaucoma should also have a refraction test every year. Glaucoma occurs when pressure builds up in the eye, damaging the retina and the optic nerve. Regular exams will help your eye doctor to screen for glaucoma and other eye conditions associated with aging, and if possible, treat them early.
Your doctor will first assess how light bends as moves through your cornea and the lens of your eyes. This test will help your eye doctor to know what type of prescription you need, or it might determine that you do not need corrective lens. He or she may use a computerized refractor for this part of the test, or simply shine a light into your eyes.
In the computerized test, you look through a machine, and it measures the amount of light reflected by your retina.
Your doctor may also do this test without the help of a machine. In this case, he or she shines a light into each of your eyes and looks at the amount of light that is bouncing off your retina to measure your refractive score.
Afterwards, your doctor will need to determine exactly what prescription you need. For this part of the test, you will be seated in front of a piece of equipment called a Phoroptor. This looks like a large mask with holes for your eyes to look through. On a wall about 20 feet in front of you will be a chart of letters. For children who cannot yet identify letters, your doctor will use a chart with small pictures of common items.
Testing one eye at a time, your eye doctor will ask you to read the smallest row of letters that you can see. Your doctor will change out the lenses on the Phoroptor, asking you each time which lens is clearer. If you are unsure, ask your doctor to repeat the choices. When your eye doctor is finished testing one eye, he or she will repeat the procedure for the other eye. Finally, he or she will come up with the combination that most closely gives you 20/20 vision.