The candela (symbol: cd, Latin for candle) is one of the seven SI base units. It is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 × 10¹² Hz and that has a radiant intensity in that direction of (1/683) W per steradian.

The frequency chosen is in the visible spectrum near green. 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 (called the V-lambda curve in the reference below).

Historically, the candela was a fundamental unit of the SI. It was defined in terms of the black-body radiation emitted by 1/60 of 1 cm² of platinum at its melting point. The modern definition is no longer fundamental because it is based on another SI unit of power, the watt. Traces of its history remain, however. The arbitrary (1/683) term was chosen such that the new definition would exactly match the old definition.

SI light units

SI light units edit  (http://www.wordiq.com/definition/Template:SI_light_units)
Quantity	SI unit	Symbol	Notes
Luminous energy	joule	J
Luminous flux	lumen or ( candela · steradian )	lm	also called Luminous power
Luminous intensity	candela	cd
Luminance	candela / square metre	cd/m2	also called Luminosity
Illuminance	lux or (lumen / square metre)	lx
Luminous efficacy	lumens per watt	lm/w

Reference

• "Unit of Luminous Intensity: Candela (http://www.electro-optical.com/whitepapers/candela.htm) (cd)". Electro Optical Industries, Inc.

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