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 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.