Luminous efficacy, quantified in lumens per watt, is a measure of the ability of a light source to produce a visual response from its power. In the photopic region, luminous efficacy peaks at 683 lumens per watt at 555 nm. In fact, the lumen is defined in terms of the power at 555 nm (frequency of 540 × 1012 Hz). Specifically, the definition (adopted in 1979) is in terms of the candela (lumen per steradian).
|The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation at a frequency of 540 × 1012 Hz and that has a radiant intensity in that direction of 1/683 watt per steradian.|
It is usually clear from the context whether the power is the radiated power (as in the discussion above) or, often for lamps, the "wall-plug" power.