Luminous flux describes the total light power emitted by a light source. As a rule, this radiant power could be expressed as emitted energy in the unit of watts. However, this method is inadequate for describing the optical effect of a light source, since the emitted radiation is recorded without discrimination over the entire frequency range and the different spectral sensitivity of the eye is not considered. The inclusion of the spectral sensitivity of the eye results in the quantity termed lumen. A radiant flux of 1W emitted at the maximum extent of spectral optical sensitivity (photopic, 555 nm) gives a luminous flux of 683 lm. Conversely, the same radiant flux emitted at frequency ranges of lower sensitivity (as per the V λ curve) results in correspondingly smaller luminous fluxes.