Perimetron

Part of speech: noun

Definitions

1. An instrument used to measure peripheral vision and the visual field
2. a device that detects the extent of one’s sight by mapping reactions to light stimuli
3. a specialized tool employed in ophthalmology to assess visual acuity and field loss

Etymology: The word is built from two Greek elements: "peri", meaning "around", and "metron", meaning "measure". Together they gave Greek geometers "perimetros", the distance around a figure. The sense of a boundary carried over into the instrument: a perimeter maps the outer edge of what an eye can see, tracing the field of vision the way a geometer traces a shape's outline. The device works by presenting small points of light at different positions and recording where the patient reports seeing them. Each response marks a point on the boundary, and the connected points produce a map of the visual field. The name therefore describes the output as much as the method: the result is a measured outline, a perimeter, of sight itself.