I strongly believe against using Metal Halides.
I once vegged in a room with a metal halide and an hps(400ws) and the plants grew taller AND tighter closer to the HPS, the same can be noted for budding in room with mixed lighting.
if you bring up UVB, MHs emit little to no detectable UVB, theyre shielded from that because theyre intended for human use, but they can produce UVB if for isntance they get broken. Theres a certain bulb they use for UVB in large reptile enclosures, such a bulb simulates the suns UVB at 3 feet
(if you were to touch the bulb or get close to it you would be experiencing like 10x the highest uv ever recorded naturally on earth)
Sorry about my rant. I will look for the proper place to have this argument, but to answer your question, IMHO Metal halides are only of use when the odd colour of HPS lights might attract attention.
Go with a High Pressure sodium, paint the room flat white and have 40 - 80 watts per square foot of HPS lighting, and you'll be set
You're completely backwards with this... HPS are clearly rated for human use...
this is obvious in the fact that they are intended to achieve the highest LUMEN
rating. Lumens are a measurement for the Human eye and the Human eye
only.. Therefore it means nothing to a plant. HPS are basically street lamps
repackaged and labeled for Growing, notice how the orange spectrum is the same?
Metal Halides have full balanced light and are great for vegetative growth, I
use them in Flowering and have nothing but great results. They have a lower
lumen rating because the spectrum spikes which they achieve are not worth
as many Lumens as the spikes in the HPS. But like I said, the spikes in the HPS
don't mean as much to a plant as people think. Go with a 600-1000 Watt MH
with a Digital MH/HPS switchable ballast and you won't be disappointed, the good
thing is Bulbs are inexpensive and if you for some reason don't achieve good
results with the MH you can always buy an HPS and flower with that.