Luminous flux

It differs from radiant flux, the measure of the total power of electromagnetic radiation (including infrared, ultraviolet, and visible light), in that luminous flux is adjusted to reflect the varying sensitivity of the human eye to different wavelengths of light.

Consumers commonly compare the luminous flux of different light bulbs since it provides an estimate of the apparent amount of light the bulb will produce, and a lightbulb with a higher ratio of luminous flux to consumed power is more efficient.

Luminous flux of artificial light sources is typically measured using an integrating sphere, or a goniophotometer outfitted with a photometer or a spectroradiometer.

Luminous flux (in lumens) is a measure of the total amount of light a lamp puts out.

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

Photopic (black line) and scotopic [ 1 ] (green line) luminosity functions. The photopic includes the CIE 1931 standard (solid), [ 2 ] the Judd-Vos 1978 modified data (dashed), [ 3 ] and the Sharpe, Stockman, Jagla & Jägle 2005 data (dotted). [ 4 ] The horizontal axis is wavelength in nm.
Integrating sphere used for measuring the luminous flux of a light source
Comparison of photometric and radiometric quantities