When you look at something you not only see the object you are looking at, but you can see all around it.
The area that you can see – without moving your head or eye – is called your visual field. In each eye it extends from the object you are looking at, to about 95 degrees out to the sides, 60 degrees upwards, 75 degrees downwards and 60 degrees towards your nose.
The visual field test can be a good indication of the health of your eyes and indeed the whole of your visual system.
It is a useful part of the eye examination – for example, it is used in the diagnosis and monitoring of glaucoma, and it can show up many other problems as well.
The visual field test is conducted using a piece of equipment called a visual field screener (sometimes known as an auto-perimeter). During the test you have to look at a little spot in the centre of the machine (to keep your eye still) and respond when you see the target (usually a light that flashes on and off once), which will appear somewhere else on the screen.
There are two main ways of doing the test:
Static testing (when the target doesn’t move): The most common sort involves you responding when you see a quick flash of light. This light flashes very quickly because otherwise you would be tempted to look towards it. The flash of light can be anywhere in your field of vision but it is important that you keep looking at the central spot and not search for the target lights.
Sometimes you will be asked how many flashes of light you see and at other times you will only be asked whether or not you see a light. At the beginning of the test you may not be able to see any flashes of light at all. This is because the lights are too faint to see. The lights are then made brighter until you can see them and the level of brightness is recorded to show how sensitive your visual field is.
In another version of this test, parts of the visual field are made to ‘shimmer’ and you will be asked which areas shimmer.
Kinetic testing (when a moving target is used): This is when a spot of light is moved from outside your visual field towards the central spot and you will be asked to indicate immediately when you see the spot of light. This test is often repeated using different sized spots of light.
It can be difficult for you to keep looking at the central spot whilst being asked what you see around it and you might find the test quite tiring. It sometimes takes several attempts at the visual field test before you fully understand how to do the test. For these reasons your optometrist may ask you to come back on a different occasion to repeat the test. This does not necessarily mean that you have something wrong with your visual field; it shows that your optometrist wants to get the best possible results from the test.
Note: Your optometrist might also conduct non-automated perimetry tests, using hand-held targets.
