The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 × 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.
The frequency of 540 × 1012 hertz was chosen for the definition because it is the frequency of light to which the human eye is most sensitive (when adapted for high light levels).
The definition of the candela says nothing about the experimental set-up for measuring luminous intensity, and metrologists are free to choose any method they wish.
