Luminous Flux (Lumen)
The fundamental measure for electromagnetic radiation emitted by a source is the radiant flux (W) - a measure of the rate of flow of energy emitted by the source.
In lighting, the radiant flux is multiplied wavelength by wavelength by the human photopic (daytime) spectral sensitivity to visible light (380nm ~780nm) forming the metric Luminous Flux.
The SI (System Internationale) unit for luminous flux is the lumen. The lumen (Lm) is therefore a measure of the total amount of light (luminous flux) emitted per second by a light source and can therefore be used to compare the performance of different light sources and forms the basis for lighting calculations.