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:

Summer, at noon, under a cloudless sky 100 000 lux
Street lighting 5 – 30 lux
Full moon, on a clear night 0.25 lux

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.