Candela

See also: Candela (homonymy)

The candela is the unit of measurement of the international Système of the Luminous intensity, i.e. of the glare perceived by the human eye of a source of light. It replaced the old unit of luminous intensity the Bougie. Candela (Symbole Cd , of the Latin word which means candle ), is one of the seven basic units of the international system.

Definition

Since the October 12th 1979, the candela is in the following way defined:
the candela is the Luminous intensity, in a given direction, of a source which emits a radiation Monochromatique of Fréquence 540 Hertz and whose radiant intensity in this direction is 1/683 Watt by Stéradian .

Reference mark

  • the selected frequency belongs to the Visible spectrum, near to the Vert. The maximum of sensitivity of the human eye is around this frequency, in the yellow-green.
  • a Bougie standard emits 1 roughly Cd , a Ampoule with traditional incandescence emits approximately 120 Cd . It is precisely to stick to old definitions of the luminous intensity, realized with candles that standardization a little curious about the candela was selected.
  • One carries out the definition of the candela with a relative uncertainty of 3 \ times 10^ {- 3}

Multiples of the candela

History

  • the candela was defined the October 21st 1948 by the ninth General conference of the weights and measures. The definition given to this conference stipulated that one candela was the unit of luminous intensity, definite according to the model of the black Corps like the radiation emitted by 1/60e of cm of Platine to its not of solidification.
  • It was redefined with the sixteenth General conference of the weights and measures, on October 12th, 1979.
  • Before 1979, one measured the values of the lamps standards usually used by comparison with the brightness of the Radiateur of Planck (black Corps) at the temperature of congelation of platinum (1764°C). Since the new definition of the candela adopted in 1979, this measurement is carried out by comparison with the radiation Monochromatique specified in the definition. The lamps standards are lamps incandescent, supplied with a specified continuous Electric current.

See too

candela|candela

Simple: Candela

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