The Info List - Luminous Flux

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.


1 Units 2 Weighting 3 Context

3.1 Relationship to luminous intensity

4 Examples 5 References

Units[edit] 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. Weighting[edit] 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. Context[edit] Luminous flux
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 a lightbulb with a higher ratio of luminous flux to consumed power is more efficient. Luminous flux
Luminous flux
is not used to compare brightness, as this is a subjective perception which varies according to the distance from the light source and the angular spread of the light from the source. Relationship to luminous intensity[edit] Luminous flux
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.

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.

Luminous flux
Luminous flux
/ luminous power Φv [nb 3] lumen (= cd⋅sr) lm J Luminous energy per unit time

Luminous intensity Iv candela (= lm/sr) cd J Luminous flux
Luminous flux
per unit solid angle

Luminance Lv candela per square metre cd/m2 L−2⋅J Luminous flux
Luminous flux
per unit solid angle per unit projected source area. The candela per square metre is sometimes called the nit.

Illuminance Ev lux (= lm/m2) lx L−2⋅J Luminous flux
Luminous flux
incident on a surface

Luminous exitance
Luminous exitance
/ luminous emittance Mv lux lx L−2⋅J Luminous flux
Luminous flux
emitted from a surface

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

Luminous efficiency
Luminous efficiency
/ luminous coefficient V

1 Luminous efficacy
Luminous efficacy
normalized by the maximum possible efficacy

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.


Table of comparative luminous flux of several light sources[5][6][7]

Source Luminous flux
Luminous flux

37 mW "Superbright" white LED 0.20

15 mW green laser (532 nm wavelength) 8.4

1 W high-output white LED 25–120

Kerosene lantern 100

40 W incandescent lamp at 230 volts 325

7 W high-output white LED 450

6 W COB filament LED lamp 600

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.


^ http://www.cvrl.org/database/text/lum/scvl.htm ^ http://www.cvrl.org/database/text/cmfs/ciexyz31.htm ^ http://www.cvrl.org/database/text/lum/vljv.htm ^ "Sharpe, Stockman, Jagla & Jägle (2005) 2-deg V*(l) luminous efficiency function". Archived from the original on 2007-09-27. Retrieved 2007-05-10.  ^ 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.  ^ Jahne, Bernd (2004). Practical Handbook on Image Processing for Scientific and Technical Applications (Second ed.). CRC. p. 111. ISBN 97808493