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Luminous flux

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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 light emitted, 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 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 even when comparing different technologies, such as incandescent and compact fluorescent bulbs. For example, a typical 100 watt incandescent light bulb emits about 1700 lumens. Roughly the same amount of light can be produced at a quarter the power (although at greater initial cost) by a 25 watt compact fluorescent light bulb. The article on luminous efficacy has more information on the efficiency of different light sources.

It is wrong or at least imprecise to refer to luminous flux as a measure of "brightness" since, for example, a nearby source of light may appear much brighter than a faraway source with much higher luminous flux. This is because not all of the emitted flux is received by your eye, and you receive less flux from sources that are farther away. In general, brightness has no unambiguous scientific meaning, and should be used only for nonquantitative references to physiological sensations and perceptions of light.

See alsoEdit

SI photometry units


Quantity Symbol SI unit Abbr. Notes
Luminous energy Qv lumen second lm·s units are sometimes called talbots
Luminous flux F lumen (= cd·sr) lm also called luminous power
Luminous intensity Iv candela (= lm/sr) cd an SI base unit
Luminance Lv candela per square metre cd/m2 units are sometimes called nits
Illuminance Ev lux (= lm/m2) lx Used for light incident on a surface
Luminous emittance Mv lux (= lm/m2) lx Used for light emitted from a surface
Luminous efficacy   lumen per watt lm/W ratio of luminous flux to radiant flux; maximum possible is 683.002
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