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Luminous flux

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Luminous flux

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.

Contents

  • Units 1
  • Weighting 2
  • Context 3
    • Relationship to luminous intensity 3.1
  • Examples 4
  • References 5

Units

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

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

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.

SI photometry quantities
Quantity Unit Dimension Notes
Name Symbol[nb 1]r Name Symbol Symbol
Luminous energy Qv [nb 2] lumen second lm⋅s TJ [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−2J Luminous power per unit solid angle per unit projected source area. Units are sometimes called nits.
Illuminance Ev lux (= lm/m2) lx L−2J Luminous power incident on a surface.
Luminous exitance / Luminous emittance Mv lux lx L−2J Luminous power emitted from a surface.
Luminous exposure Hv lux second lx⋅s L−2TJ
Luminous energy density ωv lumen second per cubic metre lm⋅s⋅m−3 L−3TJ
Luminous efficacy η [nb 2] lumen per watt lm/W M−1L−2T3J Ratio of luminous flux to radiant flux or power consumption, depending on context.
Luminous efficiency / Luminous coefficient V 1
See also: SI · Photometry · Radiometry
  1. ^ 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

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
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.

References

  1. ^ http://www.cvrl.org/database/text/lum/scvl.htm
  2. ^ http://www.cvrl.org/database/text/cmfs/ciexyz31.htm
  3. ^ http://www.cvrl.org/database/text/lum/vljv.htm
  4. ^ http://www.cvrl.org/database/text/lum/ssvl2.htm
  5. ^ Szokolay, S. V. (2008). Introduction to Architectural Science: The Basis of Sustainable Design (Second ed.). Routledge. p. 143.  
  6. ^ BeLight 3. Trendforce. 2010. pp. 10–12. 
  7. ^ Jahne, Bernd (2004). Practical Handbook on Image Processing for Scientific and Technical Applications (Second ed.). CRC. p. 111.  
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