♣  The Evolution of the Definition of the Candela
    The candela (cd), unit of luminous intensity, is the only psychophysical quantity of the SI units. Methods for realizing this unit changed from a candle to a pentane gas lamp and then to a platinum blackbody. The current definition of the candela, adopted in 1979 at the 16th International Conference of Weights and Measures, is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 10¹² hertz and that has a radiant intensity in that direction of (1/683) watt per steradian.