Plain English Meaning

A device beside the runway that shines a light beam across a fixed distance and measures how much of the light gets through. The clearer the air, the more light arrives. That reading is turned into a visibility value pilots see as RVR.

Full Definition

An instrument system used at airports to measure the transparency of the atmosphere along a fixed horizontal path, providing the data used to calculate Runway Visual Range (RVR). It works by projecting a light beam from a transmitter to a receiver a known distance away and measuring how much of that light reaches the receiver.

Context Anchor

Seen in airport weather reporting, especially at airports that report runway visual range during low-visibility conditions.