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Since the 16th General Conference on Weights and Measures in 1979, the candela has been defined as follows:
The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540×1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.
The candela was based on an older unit, the candlepower, which was referenced to the luminous intensity of a "standard candle" of known composition.
The frequency chosen is in the visible spectrum near green, corresponding to a wavelength of about 555 nanometers. The human eye is most sensitive to this frequency. At other frequencies, more radiant intensity is required to achieve the same luminous intensity, according to the frequency response of the human eye. (See luminosity function).
A common candle emits about 1 cd. A 100 W lightbulb emits about 120 cd.
Historically, the candela was defined in terms of the black-body radiation emitted by 1/60 of 1 cm2 of platinum at its melting point. The arbitrary (1/683) term was chosen such that the new definition would exactly match the old definition.
SI photometric light unitsEdit
|Luminous energy||Qv||lumen second||lm•s||units are sometimes called Talbots|
|Luminous flux||F||lumen (= cd•sr)||lm||also called luminous power|
|Luminous intensity||Iv||candela (= lm/sr)||cd|
|Luminance||Lv||candela / square meter||cd/m2||also called luminosity|
|Illuminance||Ev||lux (= lm/m2)||lx||Used for light incident on a surface|
|Luminous emittance||Mv||lux (= lm/m2)||lx||Used for light emitted from a surface|
|Luminous efficacy||lumens / watt||lm/W||ratio of luminous flux to radiant flux, maximum possible is 683|
- "The Unit of Luminous Intensity: Candela (cd)". 2006-02-08. http://www.electro-optical.com/whitepapers/candela.htm.
- "Base unit definitions: Candela". The NIST Reference on Constants, Units, and Uncertainty. 2006-02-08. http://physics.nist.gov/cuu/Units/candela.html.