Lux (lx) measures illuminance—the density of light arriving at a surface. It is the primary metric used in lighting standards worldwide (EN 12464-1, IESNA RP-1). Key reference values: full daylight = 10,000–25,000 lx, overcast day = 1,000 lx, office = 500 lx, warehouse = 200 lx, emergency lighting = 1 lx. Unlike lumens (total output), lux depends on distance and beam angle—the same lumens spread over a larger area produce fewer lux. Formula: Lux = Lumens ÷ Area (m²).