The lux is the SI derived unit of illuminance or illumination. It is equal to one lumen per square metre. It is used in photometry as a measure of the intensity, as perceived by the human eye

Lux is a derived unit based on lumen, and lumen is a derived unit based on candela. One lux is equal to one lumen per square metre, where 4π lumens is the total luminous flux of a light source of one candela of luminous intensity

The purpose of lux is intended to inform the amount of lumens required for the area to be illuminated. 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 lluminance of only 100 lux.

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.

Following are some example of lux:

  • Direct sunlight - 32,000–130,000 lux
  • Office lighting - 320–500 lux
  • Full daylight (not direct sun) - 10,000–25,000 lux

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