The candela (symbol: cd) is the SI base unit of luminous intensity. It is defined as the luminous intensity in a given direction of a source that emits monochromatic radiation with a frequency of 540 terahertz (THz) and has a radiant intensity in that direction of $\frac{1}{683}$ watt per steradian. In simpler terms, the candela measures how much light is emitted in a particular direction.
