The lumens a source sends into a narrow cone around one direction, divided by the cone's solid angle in steradians. A luminaire's light distribution is a map of intensity versus angle, measured on a goniophotometer and stored in photometric files as a table of candela values. Intensity combined with distance gives illuminance through the inverse square law, which is how point-by-point lighting calculations work.
Luminous intensity drops off fast past 30 degrees on that optic, so the spacing has to be tighter than the old fixtures.
Also heard as
- intensity
- candlepower
Do not mix it up with
- Luminance: Luminance is intensity per unit of visible area, which is what the eye judges as brightness of a surface.
Related terms
Candela
The unit for how much light a source throws in one direction, and the SI base unit behind lumens and lux.
Luminous flux
The total amount of visible light a source emits in all directions, weighted for the eye's sensitivity and measured in lumens.
Goniophotometer
An instrument that measures a luminaire's light intensity in every direction by turning the luminaire, or a mirror around it, relative to a detector.
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.
IES file
A standard text file of a luminaire's measured candela distribution, loaded into lighting calculation software.