Luminous flux

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Photopic (black) and scotopic [1] (green) luminosity functions. The photopic includes the CIE 1931 standard [2] (solid), the Judd-Vos 1978 modified data [3] (dashed), and the Sharpe, Stockman, Jagla & Jägle 2005 data [4] (dotted). The horizontal axis is wavelength in nm.
Integrating sphere used for measuring the luminous flux of a light source.

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.

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 is often used as an objective measure of the useful power emitted by a light source, and is typically reported on the packaging for light bulbs, although it is not always prominent. Energy conscious 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.

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[edit]

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 units
Quantity Unit Dimension Notes
Name Symbol[nb 1] Name Symbol Symbol
Luminous energy Qv [nb 2] lumen second lm⋅s TJ [nb 3] units are sometimes called talbots
Luminous flux Φv [nb 2] lumen (= cd⋅sr) lm J [nb 3] also called luminous power
Luminous intensity Iv candela (= lm/sr) cd J [nb 3] an SI base unit, luminous flux per unit solid angle
Luminance Lv candela per square metre cd/m2 L−2J units are sometimes called nits
Illuminance Ev lux (= lm/m2) lx L−2J used for light incident on a surface
Luminous emittance Mv lux (= lm/m2) lx L−2J used for light emitted from a surface
Luminous exposure Hv lux second lx⋅s L−2TJ
Luminous energy density ωv lumen second per metre3 lm⋅sm−3 L−3TJ
Luminous efficacy η [nb 2] lumen per watt lm/W M−1L−2T3J ratio of luminous flux to radiant flux
Luminous efficiency V 1 also called luminous coefficient
See also: SI · Photometry · Radiometry
  1. ^ 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
  2. ^ a b c Alternative symbols sometimes seen: W for luminous energy, P or F for luminous flux, and ρ or K for luminous efficacy.
  3. ^ a b c "J" here is the symbol for the dimension of luminous intensity, not the symbol for the unit joules.

Examples[edit]

Table of comparative luminous flux of several light sources[5][6][7]
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
Kerosene lantern 100
40 W incandescent lamp 325
7 W high-output white LED 450
100 W incandescent lamp 1700
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.

References[edit]