In photometry, luminous flux or luminous power is the measure of the perceived power of light. It differs from radiant flux, the measure of the total power of electromagnetic radiation (including infrared, ultraviolet, and visible light), in that luminous flux is adjusted to reflect the varying sensitivity of the human eye to different wavelengths of light.
The SI unit of luminous flux is the lumen (lm). One lumen is defined as the luminous flux of light produced by a light source that emits one candela of luminous intensity over a solid angle of one steradian. In other systems of units, luminous flux may have units of power.
The luminous flux accounts for the sensitivity of the eye by weighting the power at each wavelength with the luminosity function, which represents the eye's response to different wavelengths. The luminous flux is a weighted sum of the power at all wavelengths in the visible band. Light outside the visible band does not contribute. The ratio of the total luminous flux to the radiant flux is called the luminous efficacy.
Luminous flux is often used as an objective measure of the useful light emitted by a light source, and is typically reported on the packaging for light bulbs, although it is not always prominent. Consumers commonly compare the luminous flux of different light bulbs since it provides an estimate of the apparent amount of light the bulb will produce, and is useful when comparing the luminous efficacy of incandescent and compact fluorescent bulbs.
Energy conscious consumers can compare the luminous flux and the power usage (watts) of different light bulbs. The ratio of luminous flux to Power (watts) is useful for comparing the efficiencies of various light bulbs.
Luminous flux is not used to compare brightness, as this is a subjective perception which varies according to the distance from the light source.
Relationship to luminous intensity
Luminous flux (in lumens) is a measure of the total amount of light a lamp puts out. The luminous intensity (in candelas) is a measure of how bright the beam in a particular direction is. If a lamp has a 1 lumen bulb and the optics of the lamp are set up to focus the light evenly into a 1 steradian beam, then the beam would have a luminous intensity of 1 candela. If the optics were changed to concentrate the beam into 1/2 steradian then the source would have a luminous intensity of 2 candela. The resulting beam is narrower and brighter, however the luminous flux remains the same.
|Luminous energy||Qv [nb 2]||lumen second||lm⋅s||T⋅J [nb 3]||Units are sometimes called talbots.|
|Luminous flux / luminous power||Φv [nb 2]||lumen (= cd⋅sr)||lm||J [nb 3]||Luminous energy per unit time.|
|Luminous intensity||Iv||candela (= lm/sr)||cd||J [nb 3]||Luminous power per unit solid angle.|
|Luminance||Lv||candela per square metre||cd/m2||L−2⋅J||Luminous power per unit solid angle per unit projected source area. Units are sometimes called nits.|
|Illuminance||Ev||lux (= lm/m2)||lx||L−2⋅J||Luminous power incident on a surface.|
|Luminous exitance / luminous emittance||Mv||lux||lx||L−2⋅J||Luminous power emitted from a surface.|
|Luminous exposure||Hv||lux second||lx⋅s||L−2⋅T⋅J|
|Luminous energy density||ωv||lumen second per cubic metre||lm⋅s⋅m−3||L−3⋅T⋅J|
|Luminous efficacy||η [nb 2]||lumen per watt||lm/W||M−1⋅L−2⋅T3⋅J||Ratio of luminous flux to radiant flux or power consumption, depending on context.|
|Luminous efficiency / luminous coefficient||V||1|
|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
- Alternative symbols sometimes seen: W for luminous energy, P or F for luminous flux, and ρ or K for luminous efficacy.
- "J" here is the symbol for the dimension of luminous intensity, not the symbol for the unit joules.
|Source||Luminous flux (lumens)|
|37 mW "Superbright" white LED||0.20|
|15 mW green laser (532 nm wavelength)||8.4|
|1 W high-output white LED||25–120|
|40 W incandescent lamp||325|
|7 W high-output white LED||450|
|18 W fluorescent lamp||1250|
|100 W incandescent lamp||1750|
|40 W fluorescent lamp||2800|
|35 W xenon bulb||2200–3200|
|100 W fluorescent lamp||8000|
|127 W low pressure sodium vapor lamp||25000|
|400 W metal-halide lamp||40000|
|Values are given for newly manufactured sources. The output from many sources decreases significantly over their lifetime.|
- Szokolay, S. V. (2008). Introduction to Architectural Science: The Basis of Sustainable Design (Second ed.). Routledge. p. 143. ISBN 9780750687041.
- BeLight 3. Trendforce. 2010. pp. 10–12.
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