Luminous efficacy measures how much visible light a source produces per watt consumed, in lumens per watt, and it is where three generations of lighting technology stand furthest apart. A halogen lamp is a heater that happens to glow, converting most of its wattage to heat and yielding only 15–25 lumens from each watt. A fluorescent tube excites phosphors with a mercury-vapor discharge and roughly triples that — CFLs run about 55–70 lm/W and linear tri-phosphor tubes 80–100. An LED emits light directly from a semiconductor junction and, in current retail lamps, delivers on the order of 80–150 lumens per watt, with the best products higher.
Efficacy is what converts a lighting swap into an energy line item: across dozens of sockets running hours a day, the gap between 20 and 100+ lm/W compounds into the largest lighting saving available in a house. In design work the quantity shows up disaggregated — the lumen method sizes a room by N = (E × A)/(F × CU × MF) with F the lumens per luminaire, while the lighting power density check counts the watts per luminaire; efficacy is precisely the bridge between those two columns of the same layout.