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Concept
Lux
Summary
The lux (symbol: lx) is the unit of illuminance, or luminous flux per unit area, in the International System of Units (SI). It is equal to one lumen per square metre. In photometry, this is used 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 watt per square metre, but with the power at each wavelength weighted according to the luminosity function, a model of human visual brightness perception, standardized by the CIE and ISO. In English, "lux" is used as both the singular and plural form.
The word is derived from the Latin word for "light", lux.
Illuminance is a measure of how much luminous flux is spread over a given area. One can think of luminous flux (with the unit lumen) 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 when the luminous flux is held constant.
One lux is equal to one lumen per square metre:
1 lx = 1 lm/m2 = 1 cd·sr/m2.
A flux of 1000 lumens, spread uniformly over an area of 1 square metre, lights up that square metre with an illuminance of 1000 lux. However, the same 1000 lumens spread out over 10 square metres produces a dimmer illuminance of only 100 lux.
Achieving an illuminance of 500 lx might be possible in a home kitchen with a single fluorescent light fixture with an output of 12000lumens. 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 illuminance (lux) requires a greater luminous flux (lumen).
As with other named SI units, SI prefixes can be used. For example, 1 kilolux (klx) is 1000 lx.
Here are some examples of the illuminance provided under various conditions:
The illuminance provided by a light source on a surface perpendicular to the direction to the source is a measure of the strength of that source as perceived from that location.
Official source
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