Lux: Meaning (information, definition, explanation, facts)

For other meanings, see Lux (disambiguation).

The lux (symbol: lx) is the SI derived unit of illuminance or illumination. It is equal to one lumen per square metre.

• sunlight on an average day ranges from 32 000 (32 klx) to 100 000 lux (100 klx)
• TV studios are lit at about 1 000 lux (1 klx)
• a bright office has about 400 lux of illumination
• moonlight represents about 1 lux
• starlight measures a mere 0.000 05 lux (= 50 μlx)

The footcandle is an older customary unit of illuminance, still used in the United States; for example, it is sometimes referenced in building codes. Footcandles and lux are both measuring the same thing—illuminance—and can be converted to each other. One footcandle = 10.76 lux, and one lux = .0929 footcandles.

Lux versus lumens

The difference between the lux and the lumen is that the lux takes into account the area over which the luminous flux is spread. 1000 lumens, concentrated into an area of one square meter, lights up that square meter with an illuminance of 1000 lux. The same 1000 lumens, spread out over ten square meters, produces a dimmer illuminance of only 100 lux.

Achieving an illuminance of 500 lux might be possible in a home kitchen with a single fluorescent light fixture with an output of 12000 lumens. To light a factory floor with dozens of times the area of the kitchen would require dozens of such fixtures. Thus, lighting a larger area with the same number of lux requires a larger number of lumens.