Your visual field is the space or area in which objects are visible to your immobile eyes at a given point of time. Peripheral vision is useful to detect objects and to direct central vision to see the detected objects in detail.
Perimetry is the measurement of your visual field function. This helps in testing your peripheral or side vision. This test is easy and painless. Each eye is tested separately.
You will be asked to sit and look into an instrument which is shaped like a bowl and is called the perimeter. While you are looking at the centre of the bowl, lights will be flashed. You will be asked to press a button every time you see the flash of light.
A computer is used to make a record of the spot where the flash occurred and whether you noticed the flash in that spot by pressing the button.
At the end of this test, a result is generated if there were areas of your vision where the flashes of light were not seen by you. These are the areas of your vision loss.
This test can be done quickly, but it can take up to 30-45 minutes when done to test both your eyes.
Perimetry is done to identify certain patterns of vision loss which are present in diseases like glaucoma, retinitis pigmentosa, retinal detachment, brain damage due to strokes or injuries, head injuries, pressure of the optic nerve head, eye strokes etc. The severity of the loss of peripheral vision is comparable to the severity of damage to your optic nerve.