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Encyclopedia > Lumen (unit)

The lumen (symbol: lm) is the SI unit of luminous flux. Cover of brochure The International System of Units. ... Luminous flux is a measure of the energy emitted by a light source in all directions. ...

## Contents

1 lm = 1 cd·sr = 1 cd·m2·m–2

## SI multiples

Multiple Name Symbol Multiple Name Symbol
100 lumen lm
101 decalumen dalm 10–1 decilumen dlm
102 hectolumen hlm 10–2 centilumen clm
103 kilolumen klm 10–3 millilumen mlm
106 megalumen Mlm 10–6 microlumen µlm
109 gigalumen Glm 10–9 nanolumen nlm
1012 teralumen Tlm 10–12 picolumen plm
1015 petalumen Plm 10–15 femtolumen flm
1018 exalumen Elm 10–18 attolumen alm
1021 zettalumen Zlm 10–21 zeptolumen zlm
1024 yottalumen Ylm 10–24 yoctolumen ylm

## Explanation

If a light source emits one candela of luminous intensity into a solid angle of one steradian, the total luminous flux emitted into that solid angle is one lumen. Alternatively, an isotropic one-candela light source emits a total luminous flux of exactly lumens. The lumen can be thought of casually as a measure of the total "amount" of visible light emitted. The candela (symbol: cd) is the SI base unit of luminous intensity (that is, power emitted by a light source in a particular direction, with wavelengths weighted by the luminosity function, a standardized model of the sensitivity of the human eye). ... Luminous intensity is a measure of the energy emitted by a light source in a particular direction. ... The steradian (ste from Greek stereos, solid) is the SI derived unit of solid angle, and the 3-dimensional equivalent of the radian. ... Luminous flux is a measure of the energy emitted by a light source in all directions. ... Isotropic means independent of direction. Isotropic radiation has the same intensity regardless of the direction of measurement, and an isotropic field exerts the same action regardless of how the test particle is oriented. ...

A standard 100 watt incandescent light bulb emits approximately 1700 lumens. See the article on incandescent light bulbs for the specific efficiency of various types of electric light sources. The watt (symbol: W) is the SI derived unit of power. ... An incandescent light bulb and its glowing filament. ...

## SI photometry units

SI photometry units
Quantity Symbol SI unit Abbr. Notes
Luminous energy Qv lumen second lm·s units are sometimes called Talbots
Luminous flux F lumen (= cd·sr) lm also called luminous power
Luminous intensity Iv candela (= lm/sr) cd an SI base unit
Luminance Lv candela per square metre cd/m2 units are sometimes called nits
Illuminance Ev lux (= lm/m2) lx Used for light incident on a surface
Luminous emittance Mv lux (= lm/m2) lx Used for light emitted from a surface
Luminous efficacy   lumen per watt lm/W ratio of luminous flux to radiant flux; maximum possible is 683.002

Results from FactBites:

 Essentials of the SI: Base & derived units (468 words) Examples of such SI derived units are given in Table 2, where it should be noted that the symbol 1 for quantities of dimension 1 such as mass fraction is generally omitted. In photometry, the unit name steradian and the unit symbol sr are usually retained in expressions for derived units. For a graphical illustration of how the 22 derived units with special names and symbols given in Table 3 are related to the seven SI base units, see relationships among SI units.
 Lumen (unit) - Wikipedia, the free encyclopedia (163 words) The lumen (symbol: lm) is the SI unit of luminous flux. If a light source emits one candela of luminous intensity into a solid angle of one steradian, the total luminous flux emitted into that solid angle is one lumen. The lumen can be thought of casually as a measure of the total "amount" of visible light emitted.
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