Lux (lx)

Lux is the SI unit of illuminance: one lumen of luminous flux spread over one square metre of surface. It is the number lighting specifications are written in — a warehouse aisle may require 200 lx maintained, a road surface 20–30 lx average. Because lux measures light arriving at a surface, it depends on distance and geometry: doubling mounting height roughly quarters the illuminance from the same fixture. Field verification uses a lux meter at the working plane. Related but distinct: luminance (cd/m²) measures light reflected toward the eye and is the metric used for motorway lighting classes under EN 13201.