The candela measures luminous intensity, the flux per steradian in a given direction; roughly the intensity of one ordinary candle. It is the SI base unit from which the lumen and lux are derived. Spotlights and track heads are often rated by center beam candlepower, the candela value on axis, which tells a designer far more about punch at a distance than total lumens do.
That 10 degree head puts out 18,000 candela on axis, so at 6 meters you're getting 500 lux on the artwork.
Also heard as
- cd
Related terms
Luminous intensity
How much light a source sends out in one specific direction, measured in candelas.
Lumen
The unit of total visible light leaving a source, with each wavelength counted by how bright it looks to the eye.
Inverse square law
The rule that illuminance from a point source falls with the square of distance, so double the distance gives a quarter of the light.
Beam angle
The angle across a light's beam at which intensity has fallen to 50 percent of its center value.
IES file
A standard text file of a luminaire's measured candela distribution, loaded into lighting calculation software.