The lux (symbol: lx) is the SI unit of illuminance and luminous emittance, measuring luminous flux per unit area. It is used in photometry as a measure of the intensity, as perceived by the human eye, of light that hits or passes through a surface. It is analogous to the radiometric unit watts per square metre, but with the power at each wavelength weighted according to the luminosity function, a standardized model of human visual brightness perception. In English, "lux" is used in both singular and plural
Illuminance is a measure of how much luminous flux is spread over a given area. One can think of luminous flux (measured in lumens) as a measure of the total "amount" of visible light present, and the illuminance as a measure of the intensity of illumination on a surface. A given amount of light will illuminate a surface more dimly if it is spread over a larger area, so illuminance is inversely proportional to area.
One lux is equal to one lumen per square metre:
1 lx = 1 lm/m2 = 1 cd·sr·m–2. A flux of 1,000 lumens, concentrated into an area of one square metre, lights up that square metre with an illuminance of 1,000 lux. However, the same 1,000 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 12,000 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 to the same level of lux requires a greater number of lumens.
As with other SI units, SI prefixes can be used, for example a kilolux (klx) is 1,000 lux.
Lux can be used as a measure of the brightness of a light source, for instance of a star or a distant light bulb. A star with apparent magnitude 0 is 2.54 microlux outside the earth's atmosphere, and 82% of that (2.08 microlux) under clear skies. A magnitude 6 star (just barely visible) would be 8.3 nanolux. A standard candle (one candela) at a kilometre would be 1 microlux, about the same as a magnitude 1 star.
Here are some examples of the illuminance provided under various conditions, or by heavenly sources:
|10-4||Total starlight sky|
|0.002 Lux||Moonless clear night sky with airglow.|
|0.01 Lux||Quarter Moon|
|0.27 Lux||Full moon on clear night|
|1 Lux||Full moon overhead at tropical lattitudes|
|3.4 Lux||Dark limit of civil twilight under clear sky|
|50 Lux||Family living room lights|
|80 Lux||Office building lights in hallway|
|100 Lux||Very dark overcast day|
|320-500 Lux||Office Lighting|
|400 Lux||Sunrise or Sunset on a clear day|
|1000 Lux||Overcast day, typical TV Studio lighting|
|10,000 - 25,000 Lux||Full daylight (not direct sun)|
|32,000 - 130,000 Lux||Direct Sunlight|