Lux (unit)

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Physical unit
Unit name lux
Unit symbol
Physical quantity (s) Illuminance
Formula symbol
system International system of units
In SI units
Named after Latin lux 'light'
Derived from Lumens , meters
See also: Phot

The lux (unit symbol lx ) is the SI unit of illuminance . The name is derived from the Latin word lux for "light".


A luminous flux of 1 lumen, which hits an area of ​​1 m 2 , illuminates this (averaged) with 1 lux

The unit of measurement lux is defined as the photometric lighting that produces a luminous flux of 1  lumen (lm) when it is evenly distributed over an area of ​​1  square meter (m 2 ):

The equivalent of this can also be formulated: 1 lux is the illuminance that receives an area whose points are 1 meter away from a point light source with a luminous intensity ofcandela (1 cd).

The designation “Lux” as a special name for lm / m 2 is only recommended for information on illuminance, i.e. the luminous flux density on the receiver side. The specific light emission as the equivalent on the transmitter side is also specified in lm / m 2 , but the unit name “Lux” should not be used here.


The unit lux (together with the lumen and other photometric units) was proposed by André-Eugène Blondel in 1894 and adopted at the International Congress of Electricians in Geneva in 1896. In 1948, the 9th  General Conference on Weights and Measures added this unit and the symbol to its recommended list of units.

The unit Phot (ph) from the CGS system of units with the definition 1 ph = 1 lm / cm 2 = 10 4  lx is no longer in use.

The unit foot-candle (fc; ftc) of the United States customary unit system with the definition 1 fc = 1 ftc = 1 lm / ft 2  = 10.76391 lx is still in use in the USA.

Web links

  • Photometry. Numerical description of light with numerous illustrations

Individual evidence

  1. An area of ​​1 m 2 occupies a solid angle of 1  sr at a distance of 1 m . Due to the relationship 1 cd = 1 lm / sr, both definitions are equivalent.
  2. DIN 5031 radiation physics in the optical field and lighting technology. Part 3: Quantities, symbols and units in lighting technology. Beuth, Berlin 1982
  3. ref. 845-01-48, Luminous exitance . International Electrotechnical Commission (IEC): International Electrotechnical Vocabulary ; accessed on February 10, 2015
  4. H.-J. Hentschel: Light and Lighting - Theory and Practice of Lighting Technology. 4th edition. Hüthig Buch, Heidelberg 1994, ISBN 3-7785-2184-5 , p. 33
  5. Resolution 7 of the 9th meeting of the CGPM (1948) ( online , accessed June 4, 2020), English
  6. ^ Jan Gyllenbok: Encyclopaedia of Historical Metrology, Weights and Measures: Volume 1 . In: Science Networks Historical Studies . tape 56 . Birkhäuser, Springer International Publishing AG part of Springer Nature, Cham 2018, ISBN 978-3-319-57596-4 , p. 102 , doi : 10.1007 / 978-3-319-57598-8 (English, [accessed on June 22, 2020]).