Health Encyclopedia

Refraction test

Definition

The refraction test is an eye exam that measures a person's ability to see an object at a specific distance.

Alternative Names

Eye test - refraction; Vision test - refraction

How the test is performed

This test is performed by an ophthalmologist or optometrist (eye doctor). You sit in a chair that has a special device (phoroptor or refractor) attached to it, and look through the phoroptor at an eye chart approximately 20 feet away. The phoroptor contains lenses of different strengths that can be moved into view.

The eye doctor will ask if the chart appears more or less clear with the lenses that are in place.

The eye doctor can determine if you have nearsightedness, farsightedness, astigmatism (asymmetrical cornea), or presbyopia (inability to focus on objects that are close to you). The extent of vision difficulty can be determined. The information obtained from a refraction test helps provide the correct prescription for eyeglasses or contact lenses for each person. It also will determine if you need bifocals.

How to prepare for the test

If you wear contact lenses, ask the doctor how long they should be left out before the test.

How the test will feel

There is no discomfort.

Why the test is performed

This test can be done as part of a routine eye test to determine if a person has normal vision. When a person complains of blurred vision, this test can help determine the extent of poor vision. It can also be performed to help follow the progress of treatments or diseases of the eye.

The test is used to prescribe glasses if needed.

Normal Values

A normal value is 20/20 vision (perfect vision - able to read 3/8 inch letters at 20 feet). A small type size is also used to determine normal near vision.

What abnormal results mean

Myopia (nearsightedness)
Hyperopia (farsightedness)
Astigmatism (asymmetrical cornea)
Presbyopia (inability to focus on near objects that develops with age)

Additional conditions under which the test may be performed:

What the risks are

There are no risks.

Special considerations

A complete eye examination should be done every 3 - 5 years if there are no problems. If vision becomes blurry, worsens, or if there are other noticeable changes, an eye examination should be scheduled immediately.

After age 40 (or for people with a family history of glaucoma), eye examinations should be scheduled more frequently to test for glaucoma. Anyone with diabetes should have an eye exam at least once a year.

People with refraction problems should have an eye examination every 2 - 3 years.

References

Yanoff M, Duker JS, Augsburger JJ, et al. Ophthalmology. 2nd ed. St. Louis, Mo: Mosby; 2004:71-77.

Behrman RE. Nelson Textbook of Pediatrics. 17th ed. Philadelphia, Pa: WB Saunders; 2004; 2084-2085.

Review Date:2/22/2007
Reviewed By:Manju Subramanian, MD, Assistant Professor in Ophthalmology, Vitreoretinal Disease and Surgery, Boston University Eye Associates, Boston, MA. Review provided by VeriMed Healthcare Network.

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