In photometry, luminous intensity is a measure of the wavelength-weighted power emitted by a light source in a particular direction per unit solid angle, based on the luminosity function, a standardized model of the sensitivity of the human eye.

Photometry deals with the measurement of visible light as perceived by human eyes.

When adapted for bright conditions (photopic vision), the eye is most sensitive to yellow-green light at 555 nm.

The curve which represents the response of the human eye to light is a defined standard function y(λ) or V(λ) established by the International Commission on Illumination (CIE, for Commission Internationale de l'Éclairage) and standardized in collaboration with the ISO.

[1] Luminous intensity of artificial light sources is typically measured using and a goniophotometer outfitted with a photometer or a spectroradiometer.

If the optics were changed to concentrate the beam into 1/2 steradian then the source would have a luminous intensity of 2 candela.

If the 1 candela source emitted uniformly in all directions, the total radiant flux would be about 18.40 mW, since there are 4π steradians in a sphere.

One candlepower was the light produced by a pure spermaceti candle weighing one sixth of a pound and burning at a rate of 120 grains per hour.

Germany, Austria, and Scandinavia used the Hefnerkerze, a unit based on the output of a Hefner lamp.