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).
A common candle emits about 1 cd. A 100 W lightbulb about 120 cd.
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
Reference
- "Unit of Luminous Intensity: Candela (cd)". Electro Optical Industries, Inc.