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.