The lumen (symbol: lm) is the
SI derived unit
1 lm = 1 cd ⋅ sr.
A full sphere has a solid angle of 4π steradians, so a light source that uniformly radiates one candela in all directions has a total luminous flux of 1 cd × 4π sr = 4π cd⋅sr ≈ 12.57 lumens.
1 Explanation 2 Lighting 3 Projector output
3.1 ANSI lumens 3.2 Peak lumens 3.3 Color light output
4 SI photometry units 5 See also 6 Notes and references
If a light source emits one candela of luminous intensity uniformly
across a solid angle of one steradian, the total luminous flux emitted
into that angle is one lumen (1 cd·1 sr = 1 lm). Alternatively,
an isotropic one-candela light-source emits a total luminous flux of
exactly 4π lumens. If the source were partly covered by an ideal
absorbing hemisphere, that system would radiate half as much luminous
flux—only 2π lumens. The luminous intensity would still be one
candela in those directions that are not obscured.
The lumen can be thought of casually as a measure of the total
"amount" of visible light in some defined beam or angle, or emitted
from some source. The number of candelas or lumens from a source also
depends on its spectrum, via the nominal response of the human eye as
represented in the luminosity function.
The difference between the units lumen and lux is that the lux takes
into account the area over which the luminous flux is spread. A flux
of 1000 lumens, concentrated into an area of one square metre, lights
up that square metre with an illuminance of 1000 lux. The same 1000
lumens, spread out over ten square metres, produces a dimmer
illuminance of only 100 lux. Mathematically, 1 lx = 1 lm/m2.
A source radiating a power of one watt of light in the color for which
the eye is most efficient (a wavelength of 555 nm, in the green
region of the optical spectrum) has luminous flux of 683 lumens. So a
lumen represents at least 1/683 watts of visible light power,
depending on the spectral distribution.
Lamps used for lighting are commonly labelled with their light output in lumens; in many jurisdictions this is required by law. A 23 W spiral compact fluorescent lamp emits about 1,400–1,600 lm. Many compact fluorescent lamps and other alternative light sources are labelled as being equivalent to an incandescent bulb with a specific wattage. Below is a table that shows typical luminous flux for common incandescent bulbs and their equivalents.
Electrical power equivalents for differing lamps
Minimum light output (lumens) Electrical power consumption (watts)
Incandescent (non-halogen) Compact fluorescent LED
200 25 3-5 3
450 40 9–11 5–8
800 60 13–15 8–12
1,100 75 18–20 10–16
1,600 100 24–28 14–17
2,400 150 30–52 24-30
3,100 200 49–75 32
4,000 300 75–100 40.5
On September 1, 2010,
SI photometry quantities
v t e
Quantity Unit Dimension Notes
Name Symbol[nb 1] Name Symbol Symbol[nb 2]
Luminous energy Qv [nb 3] lumen second lm⋅s T⋅J The lumen second is sometimes called the talbot.
candela (= lm/sr)
candela per square metre
lux (= lm/m2)
Luminous exposure Hv lux second lx⋅s L−2⋅T⋅J Time-integrated illuminance
Luminous energy density ωv lumen second per cubic metre lm⋅s⋅m−3 L−3⋅T⋅J
Luminous efficacy η [nb 3] lumen per watt lm/W M−1⋅L−2⋅T3⋅J Ratio of luminous flux to radiant flux or power consumption, depending on context
See also: SI · Photometry · Radiometry
^ Standards organizations recommend that photometric quantities be denoted with a suffix "v" (for "visual") to avoid confusion with radiometric or photon quantities. For example: USA Standard Letter Symbols for Illuminating Engineering USAS Z7.1-1967, Y10.18-1967 ^ The symbols in this column denote dimensions; "L", "T" and "J" are for length, time and luminous intensity respectively, not the symbols for the units litre, tesla and joule. ^ a b c Alternative symbols sometimes seen: W for luminous energy, P or F for luminous flux, and ρ or K for luminous efficacy.
Brightness Luminous efficacy Lux Nit (unit)
Notes and references
^ "Lesson introduction to solid angles". Retrieved Oct 4, 2010.
^ Bryant, Robert H. "Lumens, Illuminance, Foot-candles and bright
shiny beads…". The LED Light. Retrieved Oct 4, 2010.
^ "OSRAM Dulux Energisparepærer" (PDF). Osram.dk. Archived from the
original (pdf) on March 1, 2012. Retrieved May 25, 2013.
^ "Conventional CFLs". Energy Federation Incorporated. Archived from
the original on 2008-10-14. Retrieved Dec 23, 2008.
^ "Learn About Light Output : ENERGY STAR". Energystar.gov.
^ "LED Light Bulb Buying Guide". The Lightbulb Company. 11 August
2016. Retrieved 16 November 2017.
^ for example:
^ one example has been found, at URL:
^ Philips Luxeon CoB emitters, specifically LHC1-4090-1208 datasheet
^ Philips Luxeon CoB emitters, specifically LHC1-4090-1211 datasheet
v t e
International System of Units
ampere candela kelvin kilogram metre mole second
Derived units with special names
becquerel coulomb degree Celsius farad gray henry hertz joule katal lumen lux newton ohm pascal radian siemens sievert steradian tesla volt watt weber
Other accepted units
astronomical unit bar dalton day decibel degree of arc electronvolt hectare hour litre minute minute of arc neper second of arc tonne atomic units natural units
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