What happens during a visual field test?
There are several types of visual field tests, but they all have one thing in common: the patient looks straight ahead at one point and signals when an object or a light is seen somewhere off to the side.
If the patient turns the eye to look directly at the object or the light, only the very center of the visual field will be tested. The tester will explain to the patient exactly where to look so that the test is accurate.
The two most basic types of visual field tests are very simple:
- Amsler grid: The Amsler grid is a pattern of straight lines that make perfect squares. The patient looks at a large dot in the middle of the grid and describes any areas where the lines look blurry, wavy, or broken. The Amsler grid is a quick test that measures only the middle of the visual field and provides your doctor with only a small amount of information.
- Confrontation visual field: The term "confrontation" in this test just means that the person giving the test sits facing the patient, about 3 or 4 feet away. The tester holds his or her arms straight out to the sides. The patient looks straight ahead, and the tester moves one hand or the other inward. The patient gives a signal as soon as the hand is seen.
The confrontation visual field test measures only the outer edge of the visual field, and it is not very exact.
What kind of test does the doctor order for more detailed information?
Computerized instruments are available to perform visual field tests and calculate results. These instruments give more reproducible and accurate results because:
- The head is always in the same place during the test.
- The instrument has a large central "target" for the patient to look at, so the center of the visual field can be kept steady.
- The instrument uses tiny spots of light to test vision, and the brightness and color of the light can be changed to measure the sensitivity of vision at each location.
- The tests have been given to thousands of healthy people, so the "normal" results are fairly well known. The instrument can compare each new test to these standards.