Thanks guys for all the rep. By popular request, here's a graph of mh, hps, and incandescent all plotted against the absorption spectra of chlorophylls a-c (in gray). If it looks a little shabby it's because I edited it in MS Paint.
As you can see, all the lights emit a lot of infrared (heat) radiation. Incandescent lights give off so much heat they are virtually worthless for growing, but you didn't need a graph to tell you that.
The HPS is generally weaker than MH in this graph because I screwed up; the HPS light I picked is of a lower wattage than the MH light. Anyway, you can still tell from the graph that
MH is stronger in the blue end of the spectrum and that
HPS is stronger in the red end. But notice that both MH and HPS spike at some useless wavelengths: MH spikes in green and HPS spikes in IR, neither of which marijuana wants.
My tentative conclusion: Although MH and HPS are both a little inefficient they each cover most of chlorophylls' absorption spectra so, if you are willing to pay for the watts,
they will both be adequate grow lights. This is all that matters to most people, and if it's all that matters to you, go ahead. Just know that a similar effect could be achieved using
much less energy with some carefully selected fluoros.
And there are other benefits to fluoros. Here are a few:
(1) They are harder to detect because they emit less heat
(2) They can be put closer to the plants without burning them
(3) They are cheaper than MH and HPS lights
My graphs were based on data from
this site. If you want to know more I suggest you check it out. Thanks everyone for your input!