Luminous Efficacy of a Lamp
Compare lamps by lumens per watt and efficiency relative to the theoretical maximum of 683 lm/W, free online.
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Luminous efficacy = luminous flux (lumens) ÷ electrical power (watts). A perfect green-light source would give 683 lm/W. Incandescent bulbs manage about 12–17 lm/W, fluorescent 60–90 lm/W and modern LEDs 90–200 lm/W.