This is defined as the luminous flux arriving at a surface that is perpendicular to it per unit area. The metric unit is the lumen m-2 or abbreviated to lux or lx.

In the British system this was defined as the lumen ft-2 or known as the foot candle or fc. 1 foot candle ~ 10.7639 lux

Sunny day10000 lux
Overcast day1000 lux
Precision mechanical workshops, operating theatres1000 lux
Supermarkets, workshops750 lux
Normal office, library500 lux
Classes250 lux
Warehouses, home, theatre150 lux
Very dark day100 lux
Twilight10 lux
Full moon0.1 lux
Quarter moon0.01 lux
Starlight0.001 lux
Overcast night0.0001 lux

For very detailed precision work the light level may need to be greater than 1500lux.

Photograph courtesy of

Note: It is very difficult to measure illuminance more accurately than ±10%.

See also: Apostilb, Foot Candle, Lamberts Laws, Light Loss Factor, Light Reflectance, Lumen, Phot.

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Subjects: Optics Physics

PCE Group Your advisor in measuring and weighing proficient.