Illuminance is the quantity of light, or luminous flux, falling on a unit area of a surface. It is designated by the symbol E. The unit is the lux (lx). One lux equals one lumen per square metre (lm/m²). The illuminance is independent of the direction from which the luminous flux reaches the surface.
Some practical examples of common lux levels are given below:
Full moon, on a clear night
The illuminance is measured by the instrument called illuminance meter or luxmeter. It uses a photocell that must be corrected by a special filter in order to match the V(λ) sensitivity curve of the human eye.