Candela
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[[Image:Luminosity.png|thumb|right|400px|[[Photopic vision|Photopic]] (black) and [[scotopic vision|scotopic]]<ref>[http://www.cvrl.org/database/text/lum/scvl.htm CIE Scotopic luminosity curve (1951)]</ref> (green) luminosity functions. The photopic includes the CIE 1931 standard <ref>[http://www.cvrl.org/database/text/cmfs/ciexyz31.htm CIE (1931) 2-deg color matching functions]</ref> (solid), the Judd-Vos 1978 modified data <ref>[http://www.cvrl.org/database/text/lum/vljv.htm Judd-Vos modified CIE 2-deg photopic luminosity curve (1978)]</ref> (dashed), and the Sharpe, Stockman, Jagla & Jägle 2005 data <ref name=Sharpe>[http://www.cvrl.org/database/text/lum/ssvl2.htm Sharpe, Stockman, Jagla & Jägle (2005) 2-deg V*(l) luminous efficiency function]</ref> (dotted). The horizontal axis is wavelength in nm.]]
[[Image:Spectrum4websiteEval.png|thumb|right|400px|]]
{{Otheruses1|the unit of luminous intensity}}
The '''candela''' ({{IPAEng|kanˈdɛlə}}, /-ˈdiːlə/, symbol: '''cd''') is the [[SI base unit]] of [[luminous intensity]]; that is, power emitted by a light source in a particular direction, weighted by the [[luminosity function]] (a standardized model of the sensitivity of the human eye to different [[wavelength]]s, also known as the ''luminous efficiency function''<ref name=Sharpe>[http://www.cvrl.org/database/text/lum/ssvl2.htm Sharpe, Stockman, Jagla & Jägle (2005) 2-deg V*(l) luminous efficiency function]</ref><ref>{{cite book |last=Wyzecki |first=G. |coauthors=Stiles, W.S. |year=1982 |edition=2nd ed. |title=Color Science: Concepts and Methods, Quantitative Data and Formulae |isbn=0471021067 |publisher=Wiley-Interscience}}</ref>). A common [[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.
==Definition==
Like other SI base units, the candela has an [[operational definition]]—it is defined by a description of a physical process that will produce one candela of luminous intensity. Since the 16th [[General Conference on Weights and Measures]] (CGPM) in 1979, the candela has been defined as:<ref>{{cite web
| title = Base unit definitions: Candela
| work = The NIST Reference on Constants, Units, and Uncertainty
| url = http://physics.nist.gov/cuu/Units/candela.html
| accessdate = 2008-02-08
}}</ref>
<blockquote>
The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540{{e|12}} [[hertz]] and that has a [[radiant intensity]] in that direction of 1/683 [[watt]] per [[steradian]].
</blockquote>
The definition describes how to produce a light source that (by definition) emits one candela. Such a source could then be used to calibrate instruments designed to measure luminous intensity, for example.
The candela is sometimes still called by the old name ''candle'' [http://m-w.com/dictionary/candela], such as in ''[[foot-candle]]'' and the modern definition of ''[[candlepower]]''.
==Explanation==
The frequency chosen is in the [[visible light|visible spectrum]] near [[green]], corresponding to a [[wavelength]] of about 555 nanometers. The [[human eye]] is most sensitive to this frequency, when [[Adaptation (eye)|adapted]] for bright conditions. At other frequencies, more [[radiant intensity]] is required to achieve the same [[luminous intensity]], according to the frequency response of the human eye. The luminous intensity for light of a particular wavelength <math>\lambda</math> is given by
:<math>I_v(\lambda)= 683.002\,\overline{y}(\lambda) I(\lambda)</math>
where <math>I_v(\lambda)</math> is the luminous intensity in candelas, <math>I(\lambda)</math> is the radiant intensity in W/sr and <math>\overline{y}(\lambda)</math> is the standard [[luminosity function]]. If more than one wavelength is present (as is usually the case), one must sum or integrate over the [[spectrum]] of wavelengths present to get the total luminous intensity.
A common [[candle]] emits roughly 1 cd. A 100 W incandescent [[lightbulb]] emits about 120 cd.
==Origin==
Prior to 1948, there existed a variety of standards for luminous intensity in use in various countries. These were typically based on the brightness of the flame from a "standard candle" of defined composition, or the brightness of an incandescent filament of specific design. One of the best-known of these standards was the [[England|English]] standard: [[candlepower]]. One candlepower was the light produced by a pure [[spermaceti]] candle weighing one sixth of a pound and burning at a rate of 120 [[Grain (mass)|grains]] per hour.
It became clear that a better-defined unit was needed. The [[Commission internationale de l'éclairage]] (International Commission on Illumination) and the CIPM proposed a “new candle” based on the luminance of a [[Planck radiator]] (a [[black body]]) at the temperature of freezing [[platinum]]. The value of the new unit was chosen to make it similar to the earlier unit candlepower. The decision was promulgated by the CIPM in 1946:
<blockquote>The value of the '''new candle''' is such that the brightness of the full radiator at the temperature of solidification of platinum is 60 new candles per [[square centimetre]].<ref>{{cite book | title = The Metric System: The International System of Units (SI) | author = Barry N. Taylor | publisher = U. S. Department of Commerce | year = 1992 | isbn = 0941375749 | pages = p.18 | url = http://books.google.com/books?id=y2-BDaoBVnwC&pg=PA18&dq=%22value+of+the+new+candle+is+such+that+the+brightness+of+the+full+radiator%22&as_brr=3&ei=elatR_S1FofgswPvu430BQ&sig=yl2AU7A-R1O9e5ZuEzuLwekiM2E }} (NIST Special Publication 330, 1991 ed.)</ref></blockquote>
It was then ratified in 1948 by the 9th [[General Conference on Weights and Measures|CGPM]] which adopted a new name for this unit, the ''candela''. In 1967 the 13th CGPM removed the term "new candle" and gave an amended version of the candela definition, specifying the atmospheric pressure applied to the freezing platinum:
<blockquote>The candela is the luminous intensity, in the perpendicular direction, of a surface of 1/600 000 square metre of a black body at the temperature of freezing platinum under a pressure of 101 325 newtons per square metre.<ref>13th CGPM Resolution 5, CR, 104 (1967), and ''Metrologia'', '''4''', 43–44 (1968).</ref></blockquote>
In 1979, because of the difficulties in realizing a Planck radiator at high temperatures and the new possibilities offered by [[radiometry]], the 16th CGPM adopted the modern definition of the candela.<ref>16th CGPM Resolution 3, CR, 100 (1979), and ''Metrologia'', '''16''', 56 (1980).</ref> The arbitrary (1/683) term was chosen so that the new definition would exactly match the old definition. Although the candela is now defined partly in terms of the watt, which is a derived SI unit of power, the candela remains a [[SI base unit|base unit]] of the SI system, by definition.[http://www1.bipm.org/en/si/si_brochure/appendix2/photometric.html#history]
==SI photometric light units==
{{SI light units}}
===Relationship between luminous intensity and luminous flux===
If a source emits a known intensity (in candelas) in a well-defined cone, the total luminous flux in [[Lumen (unit)|lumen]]s can be calculated by taking the number of candelas, and dividing it by the number in the table below that corresponds to the "radiation angle" of the lamp (the full vertex angle of the emission cone). See [[MR16]] for emission angles of some common lamps.
[http://www.theledlight.com/lumens.html Theory]
[http://forums.linear1.org/index.php/topic,113.0.html Formulas]
[http://led.linear1.org/lumen.wiz Online converter]
* Example: A lamp that emits 590 cd with a radiation angle of 40°: 590/2.64 = approximately 223 lumens.
{| class="prettytable"
! Radiation angle !! Divide by
|-
| 5° ||167.22
|-
| 10° || 41.82
|-
| 15° ||18.50
|-
| 20° || 10.48
|-
| 25° || 6.71
|-
| 30° || 4.67
|-
| 35° || 3.44
|-
| 40° || 2.64
|-
| 45° || 2.09
|}
If the source emits light uniformly in all directions, the flux can be found by multiplying the intensity by 4π: a uniform 1 candela source emits 12.6 lumens.
==References==
<references/>
==External links==
* [http://www.jracademy.com/~ewotawa/CandelaM.html Candela: The SI Unit of Luminous Intensity]
* {{cite web
| title = The Unit of Luminous Intensity: Candela (cd)
| url = http://www.electro-optical.com/html/whitepapers/candela.asp
| accessdate = 2008-02-08
}}
[[Category:SI base units]]
[[Category:Photometry]]
[[Category:Units of luminous intensity]]
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