Definition of illuminance
Illuminance – symbol E, unit lux (lx)
Illuminance is the quantity of light, or luminous flux, falling on unit area of a surface (Fig). It is designated by the symbol E. The unit is the lux (lx). One lux equals one lumen per square metre (lm/m2).
Illuminance can be defined as:
The ratio of the luminous flux incident on a surface to the area of that surface.
The Illuminance is independent of the direction from which the luminous flux reaches the surface.
Some practical examples:
|Summer, at noon, under a cloudiness sky||100 000 lux|
|Ditto, but in the shade||10000 lux|
|In the open under a heavily-overcast sky||5000 lux|
|Artificial light, in a well-lit office||1000 lux|
|Artificial light, average living-room||100lux|
|Street lighting||5-30 lux|
|Full moon, on a clear night||0,25 lux|