candela

The candela (symbol: cd) is the SI base unit of luminous intensity, a photometric quantity that describes how brightly a light source appears in a particular direction. It is defined by fixing the numerical value of the luminous efficacy of monochromatic radiation of frequency 540 × 10^12 hertz to be 683 lumens per watt.

More precisely, one candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation at 540 THz and has radiant intensity of 1/683 watt per steradian. This definition ties luminous intensity to human visual response, using the standard luminous efficiency function peaking at 555 nm to set the maximum luminous efficacy at 683 lm/W.

The candela is related to other photometric units: luminous flux, measured in lumens, equals luminous intensity times the solid angle (lm = cd × sr). Illuminance, measured in lux, equals lumens per square meter (lx = lm/m^2). Luminous intensity therefore describes directional brightness, while lumen and lux describe total flux and surface illumination, respectively.

The candela is one of the seven SI base units, alongside the meter, kilogram, second, ampere, kelvin, and mole. Its practical use spans lighting design, displays, and safety standards, where precise measurement of brightness in a direction is required. The historical basis in candlepower has evolved into a precise, physics-based definition anchored to fixed constants.