${\displaystyle {\eta }_{\text{v}}=\frac{{\Phi }_{\text{v}}}{P}}$

where Φ_v is luminous flux and P is the power consumed by the light source

Note 1 to entry: It has to be specified whether or not the power dissipated by auxiliary equipment such as ballasts, if any, is included in the power consumed by the light source.

Note 2 to entry: The luminous efficacy of a light source is expressed in lumen per watt (lm·W⁻¹).

Note 3 to entry: This entry was numbered 845-01-55 in IEC 60050-845:1987.