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The candela (symbol: cd) is the SI base unit of luminous intensity (that is, power emitted by a light source in a particular direction, with wavelengths weighted by the luminosity function, a standardized model of the sensitivity of the human eye).

Definition

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 540×10 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.

Explanation

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, when 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 ${\displaystyle \lambda }$ is given by

${\displaystyle I_{v}(\lambda )=683.002\,{\overline {y}}(\lambda )I(\lambda )}$

where ${\displaystyle I_{v}(\lambda )}$ is the luminous intensity in candelas, ${\displaystyle I(\lambda )}$ is the radiant intensity in W/sr and ${\displaystyle {\overline {y}}(\lambda )}$ 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

The units of luminous intensity based on flame or incandescent filament standards in use in various countries before 1948 were replaced initially by the “new candle” based on the luminance of a Planck radiator (a black body) at the temperature of freezing platinum. This modification had been prepared by the Commission internationale de l'éclairage (International Commission on Illumination) and by the CIPM before 1937, and the decision was promulgated by the CIPM in 1946:

The value of the new candle is such that the brightness of the full radiator at the temperature of platinum solidification is 60 new candles per square centimetre.

It was then ratified in 1948 by the 9th CGPM which adopted a new name for this unit, the candela. In 1967 the 13th CGPM (Resolution 5, CR, 104 and Metrologia, 1968, 4, 43-44) removed the term "new candle" and gave an amended version of the candela definition, specifying the atmospheric pressure applied to the freezing platinum:

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.

In 1979, because of the difficulties in realizing a Planck radiator at high temperatures, and the new possibilities offered by radiometry, 16th CGPM (1979, Resolution 3; CR, 100 and Metrologia, 1980, 16, 56) adopted the modern definition of the candela. 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 base unit of the SI system .

SI photometric light units

1. The symbols in this column denote dimensions; "L", "T" and "J" are for length, time and luminous intensity respectively, not the symbols for the units litre, tesla and joule.
2. Standards organizations recommend that photometric quantities be denoted with a subscript "v" (for "visual") to avoid confusion with radiometric or photon quantities. For example: USA Standard Letter Symbols for Illuminating Engineering USAS Z7.1-1967, Y10.18-1967
3. ^ Alternative symbols sometimes seen: W for luminous energy, P or F for luminous flux, and ρ for luminous efficacy of a source.

References

• "The Unit of Luminous Intensity: Candela (cd)". Retrieved 2006-02-08.
• "Base unit definitions: Candela". The NIST Reference on Constants, Units, and Uncertainty. Retrieved 2006-02-08.

External links

• Candela: The SI Unit of Luminous Intensity

• SI base units
• Photometry
• Units of luminous intensity