n. an instrument for mapping the extent of the *visual field. The patient fixes his or her gaze on a target in the centre of the inner surface of the hemisphere. Objects are presented on this surface and the patient says if they can be seen. The area of the visual field can be defined and any gaps in the field can be detected. There are several types of perimeter. In the static perimeter the movable object is replaced by a system of tiny lights that can be flashed briefly. A patient with a field defect will fail to see the lights that flash in the area of the defect. Modern visual field testing uses computer-assisted automated perimeters to map out and analyse visual fields and thus detect very subtle field defects (computerized perimetry). Automated perimeters are commonly used in the diagnosis and follow-up of glaucoma. —perimetry n.
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