Candela
The candela (symbol: cd) is the base unit of luminous intensity in the International System of Units (SI). Luminous intensity is a measure of the wavelength-weighted power emitted by a light source in a particular direction per unit solid angle. It describes how bright a light source will appear to the human eye.
The candela is defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency 540 × 1012 Hz (corresponding to green light with a wavelength of approximately 555 nanometers), Kcd, to be 683 when expressed in the unit lm/W, which is equal to cd sr W−1, where the steradian (sr) is the SI unit of solid angle, W is the watt, and where the definition implies that the unit is also equal to cd sr kg−1 m−2 s3.
Before 1979, various national standards and definitions existed for luminous intensity, often based on the light emitted by a specific type of candle or filament lamp. The current SI definition, adopted in 1979, provides a more precise and reproducible standard based on physical constants.
The candela is one of the seven SI base units.