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

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

Explanation

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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 $\backslash lambda$ is given by

$I\_v(\backslash lambda)=\; 683.002\backslash ,\backslash overline\{y\}(\backslash lambda)\; I(\backslash lambda)$

where $I\_v(\backslash lambda)$ is the luminous intensity in candelas, $I(\backslash lambda)$ is the radiant intensity in W/sr and $\backslash overline\{y\}(\backslash lambda)$ 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 wavlengths present to get the total luminous intensity.

A common candle emits roughly 1 cd. A 100 W lightbulb emits about 120 cd.

Origin

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The candela was based on an older unit, the candlepower, which was referenced to the luminous intensity of a "standard candle" of known composition.

This was later replaced by a new unit, the candela, defined in terms of the black-body radiation emitted by 1/60 of 1 cm² of platinum at its melting point.

The candela was given its modern definition (above) in 1979. The arbitrary (1/683) term was chosen such 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

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References

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External links

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• Candela: The SI Unit of Luminous Intensity

SI base units | Photometry | Units of luminous intensity

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