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The lumen (symbol: lm) is the
SI derived unit SI derived units are units of measurement derived from the seven SI base unit, base units specified by the International System of Units (SI). They are either dimensionless quantity, dimensionless or can be expressed as a product of one or more o ...
of
luminous flux In photometryPhotometry can refer to: * Photometry (optics), the science of measurement of visible light in terms of its perceived brightness to human vision * Photometry (astronomy), the measurement of the flux or intensity of an astronomical ...

luminous flux
, a measure of the total quantity of
visible light Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that is visual perception, perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nano ...
emitted by a source per unit of time. Luminous flux differs from
power Power typically refers to: * Power (physics) In physics, power is the amount of energy transferred or converted per unit time. In the International System of Units, the unit of power is the watt, equal to one joule per second. In older works, p ...
(
radiant flux In radiometry Radiometry is a set of techniques for measurement, measuring electromagnetic radiation, including visible light. Radiometric techniques in optics characterize the distribution of the radiation's power (physics), power in space, as op ...
) in that radiant flux includes all electromagnetic waves emitted, while luminous flux is
weighted A weight function is a mathematical device used when performing a sum, integral, or average to give some elements more "weight" or influence on the result than other elements in the same set. The result of this application of a weight function is a ...

weighted
according to a model (a "
luminosity function (black) and scotopic (green) luminous efficiency functions. The photopic includes the CIE 1931 standard (solid), the Judd–Vos 1978 modified data (dashed), and the Sharpe, Stockman, Jagla & Jägle 2005 data (dotted). The horizontal axis is wavel ...
") of the human eye's sensitivity to various
wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase (waves), phase on the wave, such as two adja ...

wavelength
s. One
lux The lux (symbol: lx) is the SI derived unit SI derived units are units of measurement ' Measurement is the number, numerical quantification (science), quantification of the variable and attribute (research), attributes of an object or event, wh ...

lux
is one lumen per square metre.


Definition

The 26th
General Conference on Weights and Measures The General Conference on Weights and Measures (GCWM; french: Conférence Générale des Poids et Mesures, CGPM) is the supreme authority of the International Bureau of Weights and Measures The International Bureau of Weights and Measures (fr ...
(CGPM) redefined the photometric units in 2018. With the new definition, which took effect on 20 May 2019, the lumen
..is defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency 540 × 1012 Hz, ''K''cd, to be 683 when expressed in the unit lm W–1 ../blockquote> Prior to 2019, the definition was based on the
candela The candela ( or ; symbol: cd) is the of in the (SI); that is, luminous power per unit emitted by a point light source in a particular direction. Luminous intensity is analogous to , but instead of simply adding up the contributions of ever ...

candela
. The lumen is related to the candela as : 1 lm = 1 . A full sphere has a
solid angle In geometry, a solid angle (symbol: ) is a measure of the amount of the field of view from some particular point that a given object covers. That is, it is a measure of how large the object appears to an observer looking from that point. The poi ...

solid angle
of 4π
steradian The steradian (symbol: sr) or square radian is the SI unit of solid angle. It is used in three-dimension thumb , 236px , The first four spatial dimensions, represented in a two-dimensional picture. In physics Physics (from gr ...

steradian
s, so a light source that uniformly radiates one
candela The candela ( or ; symbol: cd) is the of in the (SI); that is, luminous power per unit emitted by a point light source in a particular direction. Luminous intensity is analogous to , but instead of simply adding up the contributions of ever ...

candela
in all directions has a total luminous flux of :.


Explanation

If a light source emits one
candela The candela ( or ; symbol: cd) is the of in the (SI); that is, luminous power per unit emitted by a point light source in a particular direction. Luminous intensity is analogous to , but instead of simply adding up the contributions of ever ...

candela
of luminous intensity uniformly across a
solid angle In geometry, a solid angle (symbol: ) is a measure of the amount of the field of view from some particular point that a given object covers. That is, it is a measure of how large the object appears to an observer looking from that point. The poi ...

solid angle
of one
steradian The steradian (symbol: sr) or square radian is the SI unit of solid angle. It is used in three-dimension thumb , 236px , The first four spatial dimensions, represented in a two-dimensional picture. In physics Physics (from gr ...

steradian
, the total
luminous flux In photometryPhotometry can refer to: * Photometry (optics), the science of measurement of visible light in terms of its perceived brightness to human vision * Photometry (astronomy), the measurement of the flux or intensity of an astronomical ...

luminous flux
emitted into that angle is one lumen (1 ·1 = 1 lm). Alternatively, an
isotropic Isotropy is uniformity in all orientations; it is derived from the Greek ''isos'' (ἴσος, "equal") and ''tropos'' (τρόπος, "way"). Precise definitions depend on the subject area. Exceptions, or inequalities, are frequently indicated by t ...
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 flux—only 2π lumens. The
luminous intensity In photometryPhotometry can refer to: * Photometry (optics), the science of measurement of visible light in terms of its perceived brightness to human vision * Photometry (astronomy), the measurement of the flux or intensity of an astronomical o ...
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 (black) and scotopic (green) luminous efficiency functions. The photopic includes the CIE 1931 standard (solid), the Judd–Vos 1978 modified data (dashed), and the Sharpe, Stockman, Jagla & Jägle 2005 data (dotted). The horizontal axis is wavel ...
. The difference between the units ''lumen'' and ''
lux The lux (symbol: lx) is the SI derived unit SI derived units are units of measurement ' Measurement is the number, numerical quantification (science), quantification of the variable and attribute (research), attributes of an object or event, wh ...

lux
'' is that the lux takes into account the area over which the luminous flux is spread. A flux of 1000 lumens, concentrated into an area of one square metre, lights up that square metre with an illuminance of 1000 lux. The same 1000 lumens, spread out over ten square metres, produces a dimmer illuminance of only 100 lux. Mathematically, . 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.


Lighting

Lamps Lamp, Lamps or LAMP may refer to: Lighting * Oil lamp, using an oil-based fuel source * Kerosene lamp, using kerosene as a fuel * Electric lamp, or light bulb, a replaceable component that produces light from electricity * Light fixture, or light ...
used for
lighting Lighting or illumination is the deliberate use of light Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that can be visual perception, perceived by the human eye. Visible light is u ...

lighting
are commonly labelled with their light output in lumens; in many jurisdictions this is required by law. A 23 W spiral emits about 1,400–1,600 lm. Many compact fluorescent lamps and other alternative light sources are labelled as being equivalent to an incandescent bulb with a specific
power Power typically refers to: * Power (physics) In physics, power is the amount of energy transferred or converted per unit time. In the International System of Units, the unit of power is the watt, equal to one joule per second. In older works, p ...
. Below is a table that shows typical luminous flux for common incandescent bulbs and their equivalents. On 1 September 2010,
European Union The European Union (EU) is a political and economic union of member states that are located primarily in Europe Europe is a which is also recognised as part of , located entirely in the and mostly in the . It comprises the wester ...

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). This change is a result of the EU's Eco-design Directive for Energy-using Products (EuP). 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.


Projector output


ANSI lumens

The light output of
projectors 200px, Acer projector, 2012 A projector or image projector is an optical Optics is the branch of physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), ...
(including
video projector 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 (a special mercury arc lamp) ...
s) is typically measured in lumens. A standardized procedure for testing projectors has been established by the
American National Standards Institute The American National Standards Institute (ANSI ) is a private non-profit organization that oversees the development of Standardization, voluntary consensus standards for products, services, processes, systems, and personnel in the United Sta ...
, which involves averaging together several measurements taken at different positions. 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. 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 . 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".


Peak lumens

Peak lumens is a measure of light output normally used with
CRT video projectors 150px, A Zenith 1200 CRT Projector based home theater. Circa 2006. Visible extending from the ceiling are 3 lenses, one each for red, green and blue A CRT projector is a video projector A video projector is an image projector that receives a vi ...
. 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.


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


See also

*
André Blondel André-Eugène Blondel (28 August 1863 – 15 November 1938) was a French engineer and physicist A physicist is a scientist A scientist is a person who conducts Scientific method, scientific research to advance knowledge in an Branches of s ...
*
Brightness Brightness is an attribute of visual perception in which a source appears to be radiating or reflecting light. In other words, brightness is the perception elicited by the luminance of a visual target. It is not necessarily proportional to lumina ...

Brightness
*
Luminous efficacy Luminous efficacy is a measure of how well a light source produces visible light. It is the ratio of luminous flux up Integrating sphere used for measuring the luminous flux of a light source. In photometry (optics), photometry, luminous flux or ...
*
Lux The lux (symbol: lx) is the SI derived unit SI derived units are units of measurement ' Measurement is the number, numerical quantification (science), quantification of the variable and attribute (research), attributes of an object or event, wh ...

Lux
*
Nit (unit) Nit, NIT or NiTs may refer to: Education * Narula Institute of Technology, West Bengal, India * National Institutes of Technology, India * Naval Institute of Technology, Biliran, Philippines * Northern Institute of Technology Management, German ...


Notes and references


External links


International Lighting Vocabulary 2nd Edition
(online searchable version of international standard CIE S 017:2020 {{DEFAULTSORT:Lumen (Unit) Units of luminous flux SI derived units