Why discuss lumens for their effectiveness on aquarium plants? Lumens is meaningless for plants. Plants do not utilize green light for photosynthesis. A higher lumen rating at the same wattage often means greener light. Lumen is a rating weighted entirely towards human perception. It has little to do with the value of a light for either growing or viewing plants.
Agreed. The utility of comparing Lumens in the context of the above post is that the number is generally printed on the bulb or easy to find. So if the question is how much better is Brand X T12 at Y color from Brand X T8 at Y color and if the assumption that the specturm is close enough holds, we can say that in terms of OUTPUT T5>T8>T12. This is important for the person who thinks a 40W T12 is 25% brighter than a 32 W T8!
Where I get into serious trouble is the comparison of incandescent, fluorescent, and halide technologies. The Lumen number just does not fully capture the output and does not serve as a decent comparison.
I would be more than happy to retest various light combos with a power meter. All I need is the hardware