Lumen (unit)

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Unit system SI
Unit of luminous flux
1 lm in ...... is equal to ...
    SI base units     cdsr [lower-alpha 1]

The lumen (symbol: lm) is the unit of luminous flux, a measure of the total quantity of visible light emitted by a source per unit of time, in the International System of Units (SI). Luminous flux differs from power (radiant flux) in that radiant flux includes all electromagnetic waves emitted, while luminous flux is weighted according to a model (a "luminosity function") of the human eye's sensitivity to various wavelengths. One lux is one lumen per square metre.


The lumen is defined in relation to the candela as

1 lm = 1 cd·sr.

A full sphere has a solid angle of 4π steradians, [2] so a light source that uniformly radiates one candela in all directions has a total luminous flux of

1 cd × 4π sr = 4π cd⋅sr ≈ 12.57 lm. [3]


If a light source emits one candela of luminous intensity uniformly across a solid angle of one steradian, the total luminous flux emitted into that angle is one lumen (1 cd·1 sr = 1 lm). Alternatively, an isotropic one-candela light-source emits a total luminous flux of exactly 4π lumens. If the source were partly covered by an ideal absorbing hemisphere, that system would radiate half as much luminous fluxonly 2π lumens. The luminous intensity would still be one candela in those directions that are not obscured.

The lumen can be thought of casually as a measure of the total amount of visible light in some defined beam or angle, or emitted from some source. The number of candelas or lumens from a source also depends on its spectrum, via the nominal response of the human eye as represented in the luminosity function.

The difference between the units lumen and lux is that the lux takes into account the area over which the luminous flux is spread. A flux of 1,000 lumens, concentrated into an area of one square metre, lights up that square metre with an illuminance of 1,000 lux. The same 1,000 lumens, spread out over ten square metres, produces a dimmer illuminance of only 100 lux. Mathematically, 1 lx = 1 lm/m2.

A source radiating a power of one watt of light in the color for which the eye is most efficient (a wavelength of 555 nm, in the green region of the optical spectrum) has luminous flux of 683 lumens. So a lumen represents at least 1/683 watts of visible light power, depending on the spectral distribution.


An LED lamp capable of producing 470 lumens. It consumes about one sixth the energy of an incandescent light bulb producing the same light. V-LIGHT A314.JPG
An LED lamp capable of producing 470 lumens. It consumes about one sixth the energy of an incandescent light bulb producing the same light.

Lamps used for lighting are commonly labelled with their light output in lumens and, in many jurisdictions, that is required by law.

A 23 W spiral compact fluorescent lamp emits about 1,400–1,600 lm. [4] [5] Many compact fluorescent lamps and other alternative light sources are labelled as being equivalent to an incandescent bulb with a specific power. Below is a table that shows typical luminous flux for common incandescent bulbs and their equivalents.

Electrical power equivalents for different lamps [6] [7] [8]
Minimum light output (lumens)Electrical power consumption (watts)
Incandescent Compact fluorescent LED
2,40015030–5224–30 [9]
3,10020049–7532 [10]
4,00030075–10040.5 [11]

On 1 September 2010, European Union legislation came into force mandating that lighting equipment must be labelled primarily in terms of luminous flux (lm), instead of electric power (W). [12] That change is a result of the EU's Eco-design Directive for Energy-using Products (EuP). [13] For example, according to the European Union standard, an energy-efficient bulb that claims to be the equivalent of a 60 W tungsten bulb must have a minimum light output of 700–750 lm. [14]

Projector output

ANSI lumens

The light output of projectors (including video projectors) is typically measured in lumens. A standardized procedure for testing projectors has been established by the American National Standards Institute, which involves averaging together several measurements taken at different positions. [15] For marketing purposes, the luminous flux of projectors that have been tested according to this procedure may be quoted in "ANSI lumens", to distinguish them from those tested by other methods. ANSI lumen measurements are in general more accurate than the other measurement techniques used in the projector industry. [16] This allows projectors to be more easily compared on the basis of their brightness specifications.

The method for measuring ANSI lumens is defined in the IT7.215 document which was created in 1992. First the projector is set up to display an image in a room at a temperature of 25 °C (77 °F). The brightness and contrast of the projector are adjusted so that on a full white field, it is possible to distinguish between a 5% screen area block of 95% peak white, and two identically sized 100% and 90% peak white boxes at the center of the white field. The light output is then measured on a full white field at nine specific locations around the screen and averaged. This average is then multiplied by the screen area to give the brightness of the projector in "ANSI lumens". [17]

Peak lumens

Peak lumens is a measure of light output normally used with CRT video projectors. The testing uses a test pattern typically at either 10 and 20 percent of the image area as white at the center of the screen, the rest as black. The light output is measured just in this center area. Limitations with CRT video projectors result in them producing greater brightness when just a fraction of the image content is at peak brightness. For example, the Sony VPH-G70Q CRT video projector produces 1200 "peak" lumens but just 200 ANSI lumens. [18]

Color light output

Brightness (white light output) measures the total amount of light projected in lumens. The color brightness specification Color Light Output measures red, green, and blue each on a nine-point grid, using the same approach as that used to measure brightness.

SI photometric units

QuantityUnit Dimension Notes
NameSymbol [nb 1] NameSymbolSymbol [nb 2]
Luminous energy Qv [nb 3] lumen second lm⋅sTJThe lumen second is sometimes called the talbot.
Luminous flux, luminous powerΦ v [nb 3] lumen (= candela steradian)lm (= cd⋅sr)JLuminous energy per unit time
Luminous intensity Iv candela (= lumen per steradian) cd (= lm/sr)JLuminous flux per unit solid angle
Luminance Lv candela per square metre cd/m2 (= lm/(sr⋅m2))L−2JLuminous flux per unit solid angle per unit projected source area. The candela per square metre is sometimes called the nit .
Illuminance Ev lux (= lumen per square metre) lx (= lm/m2)L−2JLuminous flux incident on a surface
Luminous exitance, luminous emittanceMvlumen per square metrelm/m2L−2JLuminous flux emitted from a surface
Luminous exposure Hv lux second lx⋅sL−2TJTime-integrated illuminance
Luminous energy densityωvlumen second per cubic metrelm⋅s/m3L−3TJ
Luminous efficacy (of radiation)Klumen per watt lm/W M−1L−2T3JRatio of luminous flux to radiant flux
Luminous efficacy (of a source)η [nb 3] lumen per watt lm/W M−1L−2T3JRatio of luminous flux to power consumption
Luminous efficiency, luminous coefficientV1Luminous efficacy normalized by the maximum possible efficacy
See also: SI  · Photometry  · Radiometry
  1. Standards organizations recommend that photometric quantities be denoted with a subscript "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. 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.
  3. 1 2 3 Alternative symbols sometimes seen: W for luminous energy, P or F for luminous flux, and ρ for luminous efficacy of a source.

See also


  1. The 9th edition of the SI Brochure gives cd⋅sr as the lumen expressed in terms of base units, although the steradian (sr) is itself listed as a derived unit. [1]

Related Research Articles

<span class="mw-page-title-main">Candela</span> SI unit of luminous intensity

The candela is the unit of luminous intensity in the International System of Units (SI). It measures luminous power per unit solid angle emitted by a light source in a particular direction. Luminous intensity is analogous to radiant intensity, but instead of simply adding up the contributions of every wavelength of light in the source's spectrum, the contribution of each wavelength is weighted by the standard luminosity function. A common wax candle emits light with a luminous intensity of roughly one candela. If emission in some directions is blocked by an opaque barrier, the emission would still be approximately one candela in the directions that are not obscured.

<span class="mw-page-title-main">Luminance</span> Photometric measure

Luminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through, is emitted from, or is reflected from a particular area, and falls within a given solid angle.

In optics, Lambert's cosine law says that the radiant intensity or luminous intensity observed from an ideal diffusely reflecting surface or ideal diffuse radiator is directly proportional to the cosine of the angle θ between the direction of the incident light and the surface normal; I = I0cos(θ). The law is also known as the cosine emission law or Lambert's emission law. It is named after Johann Heinrich Lambert, from his Photometria, published in 1760.

<span class="mw-page-title-main">Lux</span> SI derived unit of illuminance

The lux is the unit of illuminance, or luminous flux per unit area, in the International System of Units (SI). It is equal to one lumen per square metre. In photometry, this is used as a measure of the intensity, as perceived by the human eye, of light that hits or passes through a surface. It is analogous to the radiometric unit watt per square metre, but with the power at each wavelength weighted according to the luminosity function, a standardized model of human visual brightness perception. In English, "lux" is used as both the singular and plural form.

<span class="mw-page-title-main">Light meter</span> Device used to measure the amount of light

A light meter is a device used to measure the amount of light. In photography, a light meter is used to determine the proper exposure for a photograph. The meter will include either a digital or analog calculator which displays the correct shutter speed and f-number for optimum exposure, given a certain lighting situation and film speed. Similarly, exposure meters are also used in the fields of cinematography and scenic design, in order to determine the optimum light level for a scene.

<span class="mw-page-title-main">Lighting</span> Deliberate use of light to achieve practical or aesthetic effects

Lighting or illumination is the deliberate use of light to achieve practical or aesthetic effects. Lighting includes the use of both artificial light sources like lamps and light fixtures, as well as natural illumination by capturing daylight. Daylighting is sometimes used as the main source of light during daytime in buildings. This can save energy in place of using artificial lighting, which represents a major component of energy consumption in buildings. Proper lighting can enhance task performance, improve the appearance of an area, or have positive psychological effects on occupants.

<span class="mw-page-title-main">Flashlight</span> Portable hand-held electric light

A flashlight or torch is a portable hand-held electric lamp. Formerly, the light source typically was a miniature incandescent light bulb, but these have been displaced by light-emitting diodes (LEDs) since the mid-2000s. A typical flashlight consists of the light source mounted in a reflector, a transparent cover to protect the light source and reflector, a battery, and a switch, all enclosed in a case.

In photometry, luminous intensity is a measure of the wavelength-weighted power emitted by a light source in a particular direction per unit solid angle, based on the luminosity function, a standardized model of the sensitivity of the human eye. The SI unit of luminous intensity is the candela (cd), an SI base unit.

<span class="mw-page-title-main">Photometry (optics)</span> Science of the measurement of visible light

Photometry is the science of the measurement of light, in terms of its perceived brightness to the human eye. It is distinct from radiometry, which is the science of measurement of radiant energy in terms of absolute power. In modern photometry, the radiant power at each wavelength is weighted by a luminosity function that models human brightness sensitivity. Typically, this weighting function is the photopic sensitivity function, although the scotopic function or other functions may also be applied in the same way.

<span class="mw-page-title-main">Video projector</span> Device that projects video onto a surface

A video projector is an image projector that receives a video signal and projects the corresponding image on a projection screen using a lens system. Video projectors use a very bright ultra-high-performance lamp, Xenon arc lamp, LED or solid state blue, RB, RGB or remote fiber optic RGB lasers to provide the illumination required to project the image, and most modern ones can correct any curves, blurriness, and other inconsistencies through manual settings. If a blue laser is used, a phosphor wheel is used to turn blue light into white light, which is also the case with white LEDs. A wheel is used in order to prolong the lifespan of the phosphor, as it is degraded by the heat generated by the laser diode. Remote fiber optic RGB laser racks can be placed far away from the projector, and several racks can be housed in a single, central room. Each projector can use up to two racks, and several monochrome lasers are mounted on each rack, the light of which is mixed and transmitted to the projector booth using optical fibers. Projectors using RB lasers use a blue laser with a phosphor wheel in conjunction with a conventional solid state red laser.

<span class="mw-page-title-main">Foot-candle</span>

A foot-candle is a non-SI unit of illuminance or light intensity. The foot-candle is defined as one lumen per square foot. This unit is commonly used in lighting layouts in parts of the world where United States customary units are used, mainly the United States. Nearly all of the world uses the corresponding SI derived unit lux, defined as one lumen per square meter.

<span class="mw-page-title-main">Luminous flux</span> Perceived Luminous Power

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, in that luminous flux is adjusted to reflect the varying sensitivity of the human eye to different wavelengths of light.

<span class="mw-page-title-main">Illuminance</span>

In photometry, illuminance is the total luminous flux incident on a surface, per unit area. It is a measure of how much the incident light illuminates the surface, wavelength-weighted by the luminosity function to correlate with human brightness perception. Similarly, luminous emittance is the luminous flux per unit area emitted from a surface. Luminous emittance is also known as luminous exitance.

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Bicycle lighting is illumination attached to bicycles whose purpose above all is, along with reflectors, to improve the visibility of the bicycle and its rider to other road users under circumstances of poor ambient illumination. A secondary purpose is to illuminate reflective materials such as cat's eyes and traffic signs. A third purpose may be to illuminate the roadway so that the rider can see the way ahead. Serving the latter purposes require much more luminous flux and thus more power.

<span class="mw-page-title-main">European Union energy label</span> Energy consumption labelling scheme

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<span class="mw-page-title-main">Guide number</span>

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Luminous efficacy is a measure of how well a light source produces visible light. It is the ratio of luminous flux to power, measured in lumens per watt in the International System of Units (SI). Depending on context, the power can be either the radiant flux of the source's output, or it can be the total power consumed by the source. Which sense of the term is intended must usually be inferred from the context, and is sometimes unclear. The former sense is sometimes called luminous efficacy of radiation, and the latter luminous efficacy of a light source or overall luminous efficacy.

A foot-lambert or footlambert is a unit of luminance in United States customary units and some other unit systems. A foot-lambert equals 1/π or 0.3183 candela per square foot, or 3.426 candela per square meter. The foot-lambert is named after Johann Heinrich Lambert (1728–1777), a Swiss-German mathematician, physicist and astronomer. It is rarely used by electrical and lighting engineers, who prefer the candela per square foot or candela per square meter units.

Several measures of light are commonly known as intensity:

Lumen maintenance is the most useful gauge to determine the lifetime or useful light output rating of an LED light source. Unlike traditional light sources such as incandescent lamps, LEDs rarely fail outright and instead continue to emit light, albeit at slowly diminishing rate over time. Lumen maintenance is the luminous flux remaining at any selected elapsed operating time. Lumen depreciation is the luminous flux lost over time, and thus the complement of lumen maintenance.


  1. The International System of Units (SI) (PDF) (9th ed.). Bureau International des Poids et Mesures. 2019. pp. 137–138.
  2. "Lesson introduction to solid angles" . Retrieved Oct 4, 2010.
  3. Bryant, Robert H. "Lumens, Illuminance, Foot-candles and bright shiny beads…". The LED Light. Retrieved Oct 4, 2010.
  4. "OSRAM Dulux Energisparepærer" (PDF). Archived from the original (PDF) on March 1, 2012. Retrieved May 25, 2013.
  5. "Conventional CFLs". Energy Federation Incorporated. Archived from the original on 2008-10-14. Retrieved Dec 23, 2008.
  6. "Learn About Light Output : ENERGY STAR". Retrieved 2012-07-15.
  7. "LED Light Bulb Buying Guide". The Lightbulb Company. 11 August 2016. Retrieved 16 November 2017.
  8. "Halogen lamps". Westinghouse. Retrieved 10 March 2020.
  9. one example has been found, at URL:
  10. Philips Luxeon CoB emitters, specifically LHC1-4090-1208 datasheet
  11. Philips Luxeon CoB emitters, specifically LHC1-4090-1211 datasheet
  12. "Lighting now labelled in lumens". England: National Physical Laboratory. Retrieved March 10, 2012.
  13. "Ecodesign for energy-using appliances". Summaries of EU legislation. the EU institutions. Feb 2, 2010. Retrieved March 10, 2012.
  14. "Energy-saving light bulbs: how to read the packaging". European Commission. Archived from the original on 28 June 2013. Retrieved March 10, 2012.
  15. "ANSI lumen article". PC Magazine Encyclopedia. Retrieved Dec 20, 2006.
  16. "Projector Guide". February 2004. Archived from the original on September 3, 2006. Retrieved Dec 20, 2006.
  17. "ANSI method of light output measurement". 1993. Archived from the original (doc) on February 26, 2012. Retrieved Jan 15, 2008.
  18. "Sony G70 Brochure" (PDF).