Visual Acuity Tests
Visual acuity tests are the most common tests used to evaluate eyesight. They measure the eye’s ability to see details at near and far distances. The tests usually involve reading letters or looking at symbols of different sizes on an eye chart. Usually, each eye is tested individually and then both eyes are tested together with and without corrective lenses (if you wear them).
Further testing may be done if you cannot identify the largest letter on an eye chart.
Sometimes an improvement in visual acuity is tested by looking through a small hole (pinhole test). The small hole limits the light rays to only those that are reflected back directly from the object being viewed. Improved visual acuity while looking through the pinhole suggests that vision can be improved with corrective lenses.
After a visual acuity measurement, a test that measures the eye’s need for corrective lens (refractive error) is usually done. Refractive errors, such as nearsightedness or farsightedness, occur when light rays entering the eye do not focus exactly on the nerve layer (retina) at the back of the eye. This leads to blurred vision. Refraction is done as a routine part of an eye examination for people who already wear glasses or contact lenses, but it will also be done if the results of the other visual acuity tests show that your eyesight is below normal and can be corrected by glasses.
Visual field tests
Your complete visual field is the entire area seen when your gaze is fixed in one direction. The complete visual field is seen by both eyes simultaneously, and it includes the central visual field—which detects the highest degree of detail—and the side (peripheral) visual fields.
Visual field tests 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.
- Confrontation test. This is a simple screening method that allows the health professional to estimate your visual field. You will be asked to stare at a specific point while your health professional moves an object (such as a finger) within your visual field.
- Perimetry. This test identifies defects in your visual field (usually of one eye at a time) by having you sit in front of a machine that flashes lights randomly at various points in the visual field. Alternatively, your health professional may move objects toward your central vision and note at what point you see them.
- Tangent screen. Tangent screen is a form of perimetry that helps identify defects in the visual field by determining where a white target object can be seen against a black background.
- Amsler grid test. This is a simple screening test for macular degeneration, a disease that causes the loss of vision in the central area of the visual field. You are asked to focus on a small area with grid lines that form boxes. Distortions or defects in the lines suggest retinal damage or fluid under the retina.
Why It is Done
Visual acuity tests may be done:
- As part of a routine eye examination to screen for vision problems. Vision acuity tests may also be done on an ongoing basis to monitor a person who has a medical condition that affects the eyes, such as diabetic retinopathy.
- To determine a person’s need for glasses or contact lenses to improve vision.
- After an injury to the eye.
- When a person obtains or renews a driver’s license.
Refraction is done:
- To determine the correct prescription for a person’s corrective lenses (eyeglasses or contact lenses).
Visual field tests may be done:
- To detect loss of vision in the entire visual field or in one part of it.
- To screen for eye diseases, such as macular degeneration and glaucoma, which cause defects in the visual field.
- As part of a neurologic examination to help determine the location of a damaged area in the brain following a stroke, head injury, or other condition that causes reduced blood flow to the brain.