Editing Luminous intensity (section)

==Units==
Like other SI base units, the candela has an [[operational definition]]&mdash;it is defined by the description of a physical process that will produce one candela of luminous intensity. By definition, if one constructs a light source that emits monochromatic green light with a frequency of 540&nbsp;THz, and that has a radiant intensity of 1/683&nbsp;[[watt]]s per [[steradian]] in a given direction, that light source will emit one candela in the specified direction.<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 = 8 February 2008
 }}</ref>

The frequency of light used in the definition corresponds to a wavelength in a vacuum of {{val|555|u=nm}}, which is near the peak of the eye's response to light. If the {{val|1|u=candela}} source emitted uniformly in all directions, the total [[radiant flux]] would be about {{val|18.40|u=mW}}, since there are 4{{pi}}&nbsp;steradians in a sphere.  A typical modern [[candle]] produces very roughly one candela while releasing heat at roughly {{val|80|u=W}}.

Prior to the definition of the candela, a variety of units for luminous intensity were used 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&nbsp;[[Grain (mass)|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> In 1881, [[Jules Violle]] proposed the ''[[Violle]]'' as a unit of luminous intensity, and it was notable as the first unit of light intensity that did not depend on the properties of a particular lamp. All of these units were superseded by the definition of the candela.