The “visual field” is defined as “the area or extent of physical space visible to an eye in a given position” by the Dictionary of Visual Science. Perhaps you have opened up this article because your optometrist has recently recommended that you complete a visual field test. It may also be the case that you have done this test for years and are curious to know what it is actually measuring.
There are a variety of reasons that a visual field test may have been ordered for you. Managing glaucoma is a common use of the test as glaucoma is a disease where an individual gradually loses their peripheral vision, often without realizing. This is because rather than seeing a black ring in their periphery, a patient with glaucoma would lose awareness of that area. Another reason may be if you have recently experienced trauma to your head, which may have affected the vision portions of your brain.
How Does A Visual Field Work?
The visual field testing machines work in a similar way. Before special technology was developed, visual field testing was accomplished by the optometrist manually, using a pen or marker. The basis was to map out the limits of a patient’s visual field and track changes over time or for defects after trauma. Each test mainly wants to discover the size of the visual field or if there are any spots of vision loss within the field. Their commonality is that the patient is to look at a specific point and to make sure NOT to move their eyes around. This is important as movement will allow someone to see something only because they shifted their view, not because their visual field is actually able to see that spot.
Frequency Doubling Technology (FDT) and Peripheral Vision
This test is often used as a fast and easy way to assess a patient’s visual field, taking only about two minutes per eye to complete. The target to look out for is a squiggly line that appears in random areas on the screen.
The test procedure is still the same: to focus on a central dot and press a button to indicate when a line is seen in the periphery without moving the focus of the eyes. This test is often completed with glasses on or off.
Autoperimetry Visual Fields (i.e. Humphrey, Octopus)
This is usually the longer visual field test, often completed on a device that holds a disc with flashing lights. The test time length can vary depending on which test is being completed but it usually takes around seven minutes per eye. The test consists of you looking at the center of the disc and pressing a remote to indicate you notice a light has flashed in the periphery.
The dome on the device is larger than the FDT testing and more precise, plotting more points on the visual field and getting a more accurate representation of visual field defects. The examiner will ask the patient to remove their glasses and may insert a lens in front of the patient to assist them in seeing the target better. Contact lenses can be worn for this procedure.