units of luminance


In photometry, the apostilb is an old unit of luminance, being that of a uniformly diffusing surface reflecting or emitting one lumen per square meter.



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.



The lumen (lm) is the derived unit of luminous flux in the SI system of units. One lumen equals the luminous flux emitted by a uniform point source of 1 candela in a solid angle of 1 steradian.



The lux (lx) is the derived unit of illuminance in the SI system of units. One lux equals the illumination produced by a luminous flux of 1 lumen distributed uniformly over an area of 1 square meter.