Dim light and Live Plants

redchigh

Plus, if I recall correctly a "peak" in yellow or green (both colors are useless to plants) will skew the lumans higher because of the way human eyes percieve light.

Geomancer

Lumens are a measure of the total amount of visible light passing through a particular area, so doesn't mater where the peaks are as long as it is visible light.

PAR is a rating of how much useful light energy there is for plants, but a meter for this is very expensive and next to no light manufacturer specifies this. Which is really too bad, they should at least for aquarium lights. Give a number based on set tank depth, or put a graph on like they do for the color temperature.

Quantum

Lux measures light per unit area, lumen is a measure of light emitted. But what redchigh is referring to is that these photometric measures are weighted to account for human vision, which is not equally sensitive to different wavelengths. So different light sources emitting the same amount of light, but at different wavelengths can have different lumen ratings.

I agree that PAR would be better, but when given as number of photons/area/time as this is what is most relevant to the photosynthetic process, though it says little about wavelength other than it is between 400 and 700 nm. Ultimately, though it's is just a number, and not of much use in my opinion.