the amount of lumens it gives off and what spectrum are the most important, but you definitely don't want to have a light that gives off a lot of heat, it'll cause stress in the plants...for example, an incandescent bulb puts off too much heat, causes the plants to droop, and doesn't have the right wavelength, more than one problem at a time...

flourescents don't give off that much heat, so you can keep them pretty close to the plants to maximize the amount of light that reaches them...

from my understanding, i think the mh and hps lights may put off a little more heat, but that's just because there's more power in them, and the light is a lot brighter than flourescents, so they don't have to be so close to the plants, and their heat in turn won't fry the plants...
jadeius Reviewed by jadeius on . are they worth being free?? because a few events over the past few weeks I ended up with some lights and balast from a friend who use to grow. I've bought a hps and a mh 400w and have been trying to find a new cheap way to keep some mothers and some veg plants going when they are both filled and need more room. Anyways I got the free stuff and have no info about them, they are mercury vapor lights, one 175 and one 100w lights. Has anyone used them to grow and are they going to be worth even trying to put up?? I mean Rating: 5