Editing Candela (section)

== History ==
Prior to 1948, various standards for luminous intensity were in use in a number of 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 was the English standard of 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&nbsp;[[Grain (unit)|grains]] per hour. Germany, Austria and Scandinavia used the [[Hefnerkerze]], a unit based on the output of a [[Hefner lamp]].<ref>{{cite web |url=http://www.sizes.com/units/hefner.htm |title=Hefner unit, or Hefner candle |work=Sizes.com |date=30 May 2007 |access-date=25 February 2009}}</ref>

A better standard for luminous intensity was needed. In 1884, [[Jules Violle]] had proposed a standard based on the light emitted by 1&nbsp;cm<sup>2</sup> of [[platinum]] at its melting point (or freezing point). The resulting unit of intensity, called the "violle", was roughly equal to 60 English candlepower. Platinum was convenient for this purpose because it had a high enough melting point, was not prone to [[oxidation]], and could be obtained in pure form.<ref name=Cottington>{{cite journal |journal=Platinum Metals Review |year=1986 |volume=30 |issue=2 |page=84 |title=Platinum and the Standard of Light: A Selective Review of Proposals Which Led to an International Unit of Luminous Intensity |first=Ian E. |last=Cottington |url=https://technology.matthey.com/article/30/2/84-95/}}</ref> Violle showed that the intensity emitted by pure platinum was strictly dependent on its temperature, and so platinum at its melting point should have a consistent luminous intensity. 

In practice, realizing a standard based on Violle's proposal turned out to be more difficult than expected.<ref name=Cottington/> Impurities on the surface of the platinum could directly affect its emissivity, and in addition impurities could affect the luminous intensity by altering the melting point. Over the following half century various scientists tried to make a practical intensity standard based on incandescent platinum. The successful approach was to suspend a hollow shell of [[thorium dioxide]] with a small hole in it in a bath of molten platinum. The shell (cavity) serves as a [[black body]], producing [[black-body radiation]] that depends on the temperature and is not sensitive to details of how the device is constructed.

In 1937, the ''[[Commission Internationale de l'Éclairage]]'' (International Commission on Illumination) and the CIPM proposed a "new candle" based on this concept, with value 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&nbsp;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 = 0-941375-74-9 |page = 18 |url = https://books.google.com/books?id=y2-BDaoBVnwC&pg=PA18 }} (NIST Special Publication 330, 1991 ed.)</ref></blockquote>

It was then ratified in 1948 by the 9th&nbsp;CGPM<ref>
[http://www.bipm.org/utils/common/pdf/CGPM/CGPM9.pdf#page=54 Proceedings of the 9th CGPM], 1948, page 54 (French)</ref> 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 {{nowrap|1&thinsp;/&thinsp;600&thinsp;000}} square&nbsp;metre of a black body at the temperature of freezing platinum under a pressure of {{nowrap|101&thinsp;325}}&nbsp;newtons per square&nbsp;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&nbsp;CGPM adopted a new definition of the candela:<ref>16th CGPM Resolution 3, CR, 100 (1979), and ''Metrologia'', '''16''', 56 (1980).</ref><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
|access-date =27 September 2010}}</ref>

<blockquote>
The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency {{val|540|e=12|ul=hertz}} and that has a [[radiant intensity]] in that direction of {{sfrac|683}}&nbsp;[[watt]] per [[steradian]].
</blockquote>

The definition describes how to produce a light source that (by definition) emits one candela, but does not specify the luminous efficiency function for weighting radiation at other frequencies.  Such a source could then be used to calibrate instruments designed to measure luminous intensity with reference to a specified luminous efficiency function. An appendix to the SI Brochure<ref>{{cite web |url=http://www.bipm.org/en/publications/mises-en-pratique |title=''Mise en pratique'' for the definition of the candela and associated derived units for photometric and radiometric quantities in the International System of Units (SI) |work=SI Brochure Appendix 2 |date=22 March 2021 |publisher=Bureau International des Poids et Mesures |access-date=2023-10-07 }}</ref> makes it clear that the luminous efficiency function is not uniquely specified, but must be selected to fully define the candela.

The arbitrary (1/683) term was chosen so that the new definition would precisely match the old definition. Although the candela is now defined in terms of the [[second]] (an SI base unit) and the watt (a derived SI unit), the candela remains a base unit of the SI system, by definition.<ref>{{cite web |url=https://www.bipm.org/en/publications/si-brochure |title=Units for photochemical and photobiological quantities |work=SI Brochure Appendix 3 |date=22 March 2021 |publisher=Bureau International des Poids et Mesures |access-date=2023-10-07 }}</ref>

The 26th CGPM approved the modern definition of the candela in 2018 as part of the [[2019 revision of the SI]], which redefined the SI base units in terms of fundamental physical constants.