The luminous flux of light source is the total quantity of visible light emitted by a source per second.  Its is measured in the SI units of lumens which have the symbol lm.  The greater the number of lm, the brighter the light source will appear to the human eye.

Luminous flux differs from radiant flux in that radiant flux includes all electromagnetic waves emitted and therefore represents the luminous power of the source.  However, some of this light will be at wavelengths (frequencies) that cannot be detected by the human eye.  Luminous flux is adjusted to account for the human eye’s sensitivity to various wavelengths.

Another measure of the amount of light often encountered in lighting designs is lux.  The units of lumens are related to lux in that one lux is one lumen per square metre.  However, due to the dispersal of light as it travels away from a source, the lux of a light source will normally reduce significantly as you get further away.  Meanwhile, the luminosity of the source (its luminous flux) is a property of the source and remains constant.  This make luminous flux more useful when comparing light sources and considering energy efficiency.