Lighting Design Glossary

Luminous Intensity
Candlepower (USA)

(Term of photometry)

The luminous intensity is the luminous flux emitted from a point per unit solid angle into a particular direction.

The luminous intensity is the official base unit for light. It was defined in 1979 by the "Conférence Générale des Poids et Mesures" (CGPM, the 11th General Conference on Weights and Measures) as follows:

The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 10¹² hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.

Together with the CIE photometric curve, the luminous intensity provides the weighting factor needed to convert between radiometric and photometric measurements.

Standard unit of luminous intensity is Candela (cd),
also expressed as Lumen per Steradian (lm/sr)

There are several older units of luminous intensity: