What is the SI unit of luminous intensity?

The SI unit of luminous intensity is the candela, with the symbol cd. Luminous intensity describes how much visible light a source emits in a particular direction per unit of solid angle.

The candela is one of the seven SI base units. Its modern definition is linked to monochromatic light with a frequency of 540 THz and a specified radiant intensity of 1/683 watt per steradian. That frequency corresponds to green light near 555 nanometres, where human daytime vision is especially sensitive.

A typical wax candle produces roughly one candela in a given direction, which explains the unit's name: candela is Latin for candle. Earlier standards included candlepower and the Hefner candle, but international measurement gradually replaced those less consistent references with the candela. The unit's definition was updated during the 2019 revision of the SI.

The candela is often confused with the lumen and lux. A lumen measures total perceived light output, while a lux measures luminous flux arriving on a square metre. Candela is directional: focusing the same light into a narrower beam can increase its intensity in that direction without necessarily increasing the total lumens.