Without going into gross detail, the primary wave spectrum for blue light growth is around 435 to 465 nm, and around 660-680 for red light growth. The luminous efficiency and lumen/watt conversion rates are similar for both. So lumens are NOT are perfect indicator of light efficiency overall. They are, however, pretty good for the wavelengths used in plant growth. All sorts of other formulas can be used to compute different sorts of things related to light. They're complicated formulas. They aren't necessarily better. For most people doing small to medium grows--and I am talking about people using 400W or lower HID lights and/or LCD lights--it can be a tremendous pain to work out information that may or may not be helpful. I have great respect for people who can work out formulas for luminous flux and PAR watt variations and things like that. If the information is available on the box for normal growers and there's a simple formula to figure out the efficiency...hey, I'm all for it. But it's my experience that this sort of information is not readily available, is not necessarily provably (or measurably) better, and is often difficult to compute for grow areas. Lumens, although imperfect, have the following things going for them 1) Usually listed for bulbs 2) Proven effectiveness 3) Easy to use formula On these boards, I think people look for those three factors.