Luminous Efficacy of Radiant Power

the ratio of the luminous flux emitted by a light source to the radiant power, or radiant flux, emitted. The unit of luminous efficacy in the International System of Units is the lumen per watt (lm/W).

Special terms are used in the case of monochromatic light—that is, radiation of a single frequency or a single wavelength λ. In this case we speak of the spectral luminous efficacy K(λ), formerly called the luminosity. The spectral luminous efficiency is V(λ) = K(λ)/[K(λ)]_max; it has also been called the relative spectral luminous efficacy and the relative luminosity. The system of photometric quantities is based on the two quantities K(λ) and V(λ). The term “mechanical equivalent of light” is sometimes applied to the reciprocal of the maximum spectral luminous efficacy of radiant power.