Candela

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.

Historically, the candela was a fundamental unit of the SI. It was defined in terms of the blackbody radiation emitted by 1cm³ 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.