Luminous flux as a photometric quantity indicates the total light output emitted by a light source. In principle, this radiant flux could be specified as emitted energy in the unit watt (W). However, the optical effect of a light source is not adequately described because the emitted radiation is recorded in an indiscriminate way over the entire frequency range and the different spectral sensitivity of the eye is therefore not taken into account.
By including the spectral sensitivity of the eye, the luminous flux is obtained. A radiant flux of 1W emitted at the maximum of spectral eye sensitivity (photopic, 555nm) generates a luminous flux of 683lm. However, the same radiant flux in frequency ranges of lower sensitivity generates correspondingly smaller luminous fluxes according to the V λ curve.