Lumen (unit)

The lumen (symbol: lm) is the SI unit of luminous flux.

Contents 1 Definition 2 SI multiples 3 Explanation 4 SI photometry units

Definition

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 4π lumens. The lumen can be thought of casually as a measure of the total "amount" of visible light emitted.

A standard 100 watt incandescent light bulb emits approximately 1500 lumens. See the article on incandescent light bulbs for the specific efficiency of various types of electric light sources.

SI photometry units

SI photometry units edit
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
Luminance	Lv	candela / square metre	cd/m2	also called luminosity
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		lumens / watt	lm/W	ratio of luminous flux to radiant flux, maximum possible is 683

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