Candela (from Latin candela - candle) - unit of luminosity of a light source; basic unit in the SI system, denoted cd.
It is the luminosity with which a source emitting monochromatic radiation with a frequency of 5.4-1014 Hz and an energy yield in that direction equal to 1/683 W/sr. shines in a certain direction. An older definition defined a candela as the luminosity of 1/600,000 m² of the surface of a blackbody at the solidification temperature of platinum under a pressure of 1 physical atmosphere. However, due to difficulties in making the measurement system and low measurement accuracy (on the order of 0.1-0.2%), this definition was abandoned in 1979 and was replaced by a new definition. The candela was introduced in 1948 by the International Committee on Weights and Measures. Previously, the unit of luminosity was the candle.