Visual Field Perimetry Testing Equipment are used by eye doctors to look for signs of vision loss in patients. During the non-invasive test, a patient sits and looks inside the instrument at a screen. Every time the patient sees a flash of light, they are instructed to press a button on a remote. In this way, the patient's entire vision -- including their peripheral or side vision -- is checked, as a loss of peripheral vision can be an early indicator of glaucoma.