Luminous Efficacy of a Light Source

the ratio of the luminous flux emitted by a light source to the power consumed by the source. It is measured in lumens per watt (lm/W) and characterizes the efficiency of the source. The luminous efficacy of modern general-purpose incandescent lamps is 8–20 lm/W, and that of flourescent lamps is 40–80 lm/W.