How It Is Done continued...
If you cannot read any of the letters or print on these charts because of poor vision, your visual acuity will be tested by other techniques, such as counting fingers, detecting hand movements, or distinguishing the direction or perception of light sources (such as room light or a penlight held up close to the face).
Visual acuity tests usually take about 5 to 10 minutes. They may be performed by a nurse, a medical assistant, an ophthalmologist, an optometrist, a teacher, or some other trained person. Testing may be done at a doctor's office, school, workplace, health fair, or elsewhere.
Refraction is a test that measures the eye's need for a corrective lens (refractive error). For this test, you will be asked to describe the effects of looking at an eye chart through various corrective lenses.
Your doctor may use eyedrops to widen (dilate) your pupils before you start this test. The eyedrops take about 15 to 20 minutes to dilate the pupil fully.
The doctor may put a device (called a phoropter) in front of your eyes. The device contains many different lenses. Testing one eye at a time, the doctor will ask you to compare the effects of two lenses (first one lens, then the other). You should state which lens of each pair gives you better vision. The doctor will continue to test your eyes with different lenses until it is determined which lenses correct your vision the best. Your doctor may use a hand-held device (retinoscope) to shine light into your eyes. A series of trial lenses will be placed in front of your eyes and adjusted until the light rays are properly focused on your retina.
Visual field tests
Visual field tests are used to check for gaps in your range of vision. They can help detect eye diseases or nervous system problems that limit your ability to see objects clearly in the entire visual field or in one part of it. Several tests are commonly done to evaluate a person's visual field.
- The confrontation test. Your doctor sits or stands 2 ft (0.6 m) to 3 ft (1 m) in front of you. You cover one eye while fixing your gaze on his or her nose. He or she slowly moves a finger or hand from the outer edge of your visual field toward the center and from the center toward the edge through all areas of your visual field. You will focus your eye on your doctor's nose and signal when you first see his or her finger or hand. The test is then repeated for the other eye.
- The Amsler grid test checks for macular degeneration, a disease that causes loss of vision in the center of your visual field. The test uses a 4 in. (10 cm)square chart with straight lines that form boxes. The grid has a black dot at the center. The chart is held about 14 in. (36 cm) from your face. You cover one eye while focusing your other eye on the black dot. The test is then repeated on the other eye. Tell your doctor if:
- You cannot see the black dot.
- You see a blank or dark spot (other than the center dot).
- The lines in the grid look wavy, blurred, or curved instead of straight. You will be asked to point to the specific abnormal area of the grid.
Perimetry testing uses a machine that flashes lights randomly at various points in the visual field. You look inside a bowl-shaped instrument called a perimeter. While you stare at the center, lights will flash, and you press a button each time you see a flash. A computer records the location of each flash and whether you pressed the button when the light flashed in that location. At the end of the test, a printout shows any areas of your visual field where you did not see the flashes of light. In an alternative manual perimetry test, your doctor moves a light target and notes your visual field on paper.
- The tangent screen test uses a black screen with concentric circles and lines leading out from a center point (like a bull's-eye). Sitting 3 ft (1 m) to 6 ft (2 m) away from the screen, you cover one eye while fixing your gaze on a target point marked on the screen. Test objects of various sizes at the tip of a wand are then moved inward from the outer edge of the screen toward the center. You will signal when you can see the object, and that point is then marked on the screen. The points on the screen where you see the objects are connected to provide an outline of your visual field. The test is then repeated for the other eye. An alternative manual tangent screen test uses a white object against a black background. If you wear glasses, you will keep them on for this test.
Color vision tests