units of luminance
The candela (cd) is the base unit of luminous intensity in the SI system of units. One (cd) candela 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 watt per steradian.
The candlepower (cp) is an obsolete unit of luminous intensity. One candlepower (cp) was originally defined (1860) as the amount of light given off by a pure spermaceti candle (made from the fat of the sperm whale) weighing one sixth of a pound and burning at a rate of 120 grains per hour. Other definitions followed later. Today, if the term candlepower is used it is exactly equivalent to the candela.