# spectral luminous efficacy of radiant flux

[3.10] $K(\lambda )=\Phi _{v\lambda }/\Phi _{e\lambda }$

The quotient of the luminous flux at a given wavelength by the radiant flux at that wavelength. It is expressed in lumens per watt.

Note: This term was formerly called luminosity factor. The reciprocal of the maximum luminous efficacy of radiant flux is sometimes called “mechanical equivalent of light”; that is, the ratio between radiant and luminous flux at the wavelength of maximum luminous efficacy. The most probable value is 0.00146 W/lm, corresponding to 683 lm/W as the maximum possible luminous efficacy. For scotopic vision values (see Table A.7), the maximum luminous efficacy is 1,700 “scotopic lumens” per watt.