Luminous Efficacy & Efficiency Of Incandescent Bulbs

A lamp can waste power by emitting too much energy outside of the visible spectrum.

Only visible light is useful for illumination, some wavelengths are brighter than others.

Taking this into account, luminous efficacy is a ratio of the useful power emitted to the total radiant flux (power).

Luminous efficiency is the ratio of the luminous efficacy to this maximum possible value.