Illuminance – is the quantitative expression for the luminous flux incident on a unit area of a surface. A more familiar term would be lighting level. Illuminance is expressed in lux (lx), where one lux equals one lumen per square metre (lm/m²). Other units are metrecandle, phot, and nox. In Imperial units the unit is the foot-candle which equals lumen per square foot (lm/ft²).
