That 683 lumens per watt number is, as you say, for a monochromatic light. This is not useful for humans to see with[1]; the actual limit for human vision is a

That 683 lumens per watt number is, as you say, for a monochromatic light. This is not useful for humans to see with[1]; the actual limit for human vision is about 300 lumens per watt[2]. That's an upper bound even ignoring losses, and some LEDs are around 70 now, so the remaining efficiency gains we can make seem limited to about 3x. [1] http://www.cool.conservation-us.org/byorg/us-doe/color_quality_of_white_leds.pdf http://www.cool.conservation-us.org/byorg/us-doe/color_quali... [2] http://phys.org/news202453100.html http://phys.org/news202453100.html