A measure of the efficacy of a light source in terms of the light produced for the power consumed. For example, a 100-watt lamp producing 1750 lumens gives 17.5 lumens per watt. Examples: Edison's first lamp 1.4 lpw Incandescent lamps 10-40 lpw Fluorescent lamps 35-100 lpw Halogen lamps 20-45 lpw.