perimeter

1. A circumference, edge, or border. 2. An instrument, usually half a circle or sphere, used to measure the field of vision. [G. perimetros, circumference, fr. peri, around, + metron, measure]

- arc p. a p. consisting of a semicircular frame at the center of which the patient looks while a white object is moving along the arc, the exact point where it becomes visible or invisible being noted and recorded on a chart.

- Goldmann p. a projection p. that adds further precision by controlling the surrounding illumination.

- projection p. a p. that uses as target a spot of light that can be adjusted rapidly as to size, brightness, and color, and moves silently at any desired speed.

- Tübinger p. a bowl p. in which a static stimulus was increased in intensity until detected. [Tübingen, German city]

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pe·rim·e·ter pə-'rim-ət-ər n

1 a) the boundary of a closed plane figure

b) the length of a perimeter

2) an instrument for examining the discriminative powers of different parts of the retina often consisting of an adjustable semicircular arm with a fixation point for the eye and variable stations for the visual stimuli

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n.

an instrument for mapping the extent of the visual field. The patient fixes his 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 he sees them. 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, set in a black background, that can be flashed briefly. If the patient has a field defect he 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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pe·rim·e·ter (pə-rimґə-tər) [peri- + -meter] 1. a line forming the boundary of a plane figure. 2. an apparatus for determining the extent of the peripheral visual field on a curved surface.