The SI unit of Luminous Intensity
The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540x1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.
The candela is abbreviated cd.
Originally: the Candela represented the intensity of an actual candle, assumed to be burning whale tallow at a specified rate in grains per hour.
The new definition of a light source producing single-frequency light at a frequency of 540 terahertz (THz) with a power of 1/683 watt per steradian or 18.3988 milliwatts over a complete sphere centred at the light source.