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)

Trivia: Unicode has a symbol for "lx": (㏓), but this is just a legacy code to accomodate old code pages in certain Asian languages, and it is not recommended for use in any language today.

Lux versus lumen

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 metre, lights up that square metre with an illuminance of 1000 lux. The same 1000 lumens, spread out over ten square metres, 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.

Relationship between illuminance and power

Illuminance is not a direct measure of the energy of light, but rather of the illumination it provides as perceived by the human eye. Thus, the conversion factor varies with the wavelength composition or color temperature of light. At 555 nm, the middle of the visible spectrum, one lx is equal to 1.46 mW/m².

SI units of light

Non-SI units of illuminance

• foot-candle