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 don’t 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 lens of your eye. The refraction test will tell your doctor what prescription lens you should use in order to have 20/20 vision.
Why is this test used?
This test tells your doctor if you need prescription lenses, as well as what prescription lens you need to see properly.
The results of the test are used to diagnose the following conditions:
- astigmatism, a refractive problem with the eye related to the shape of the lens, which causes blurry vision
- hyperopia, which is also known as farsightedness
- myopia, which is also known as nearsightedness
- presbyopia, a condition related to aging that causes lens of the eye to have trouble focusing
The results of the test can help diagnose the following conditions:
- 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
What happens during the test?
Your doctor will first assess how light bends as it moves through your cornea and the lens of your eyes. This test will help your eye doctor determine whether you need corrective lenses and, if so, what type of prescription you need. Your doctor may use a computerized refractor for this part of the test, or they may simply shine a light into your eyes.
In the computerized test, you look through a machine that 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, they will shine a light into each of your eyes and look at the amount of light that is bouncing off your retina to measure your refractive score.
Afterward, your doctor will determine exactly what prescription you need. For this part of the test, you’ll 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.