Refraction test
The refraction test is an eye exam that measures a person's prescription for eyeglasses or contact lenses.
How the Test is Performed
This test is performed by an ophthalmologist or optometrist. Both of these professionals are often called "eye doctor."
You
sit in a chair that has a special device (called a phoroptor or
refractor) attached to it. You look through the device and focus on an
eye chart 20 feet away. The device contains lenses of different
strengths that can be moved into your view. The test is performed one
eye at a time.
The eye doctor performing the test will ask if the chart appears more or less clear when different lenses are in place.
How to Prepare for the Test
If you wear contact lenses, ask the doctor if you need to remove them before the test and for how long.
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 exam. The purpose is to
determine whether you have a refractive error (a need for glasses or
contact lenses).
For people over age 40 who have normal distance
vision but difficulty with near vision, a refraction test can determine
the right power of reading glasses.
Normal Results
If
your uncorrected vision (without glasses or contact lenses) is normal,
then the refractive error is zero (plano) and your vision should be
20/20.
A value of 20/20 is perfect vision, meaning you are 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
You
have a refractive error if you need a combination of lenses to see
20/20. Glasses or contact lenses should give you good vision.
If your final vision is less than 20/20, even with lenses, then there is probably another, non-optical problem with your eye.
The vision level you achieve during the refraction test is called the best-corrected visual acuity (BCVA).
Abnormal results may be due to:
- Astigmatism
- Hyperopia (farsightedness)
- Myopia (nearsightedness)
- Presbyopia (inability to focus on near objects that develops with age)
Other conditions under which the test may be performed:
- Corneal ulcers and infections
- Macular degeneration
- Retinal detachment
- Retinal vessel occlusion
- Retinitis pigmentosa
Risks
There are no risks.
Considerations
You
should have a complete eye examination every 3 - 5 years if you have no
problems. If your vision becomes blurry, worsens, or if there are other
noticeable changes, schedule an eye examination immediately.
After age 40 (or for people with a family history of glaucoma),
eye examinations should be scheduled at least once a year to test for
glaucoma. Anyone with diabetes should also have an eye exam at least
once a year.
People with a refractive error should have an eye examination every 1 -2 years, or whenever their vision changes.
References
Katz M, Kruger PB. The human eye as an optical system. In: Tasman W, Jaeger Ea, eds. Duane's Ophthalmology. 15th ed. Philadelphia, Pa: Lippincott Williams & Wilkins; 2009:chap 33.
Kaufman JH. Subjective refraction: fogging and use of the astigmatic dials. In: Tasman W, Jaeger Ea, eds. Duane's Ophthalmology. 15th ed. Philadelphia, Pa: Lippincott Williams & Wilkins; 2009:chap 39.
Scott CA. Testing of refraction. In: Yanoff M, Duker JS, eds. Ophthalmology. 3rd ed. St. Louis, MO: Mosby Elsevier;2008:chap 2.8.
Olitsky SE, Hug D, Smith LP. Examination of the eye. In: Kliegman RM, Behrman RE, Jenson HB, Stanton BF, eds. Nelson Textbook of Pediatrics. 18th ed. Philadelphia, Pa: Saunders Elsevier;2007:chap 618.
American
Academy of Ophthalmology Refractive Management/Intervention Panel.
Preferred Practice Pattern Guidelines. Refractive Errors &
Refractive Surgery. San Francisco, CA. American Academy of
Ophthalmology, 2010. Accessed January 17, 2011.
American Academy
of Ophthalmology Preferred Practice Patterns Committee. Preferred
Practice Pattern Guidelines. Comprehensive Adult Medical Eye Evaluation.
San Francisco, CA: American Academy of Ophthalmology, 2010. Accessed
January 17, 2011.
source:http://www.nlm.nih.gov/medlineplus/ency/article/003844.htm
