If in an area that gets little bright, natural light, I too would burn the lights about 16 hrs./day. In greenhouses, with lots of light, I still burn my fixtures 6 hrs./day, from around 3:00 PM till 9:00 PM. I'm simply supplementing light while the sun is very low in the sky and during early evening.
You are correct,
@weedwhacker. You can position plants extremely close to T12 and T8 tubes. T5 tubes produce some heat so you would want to keep the plants further away. T5 HO tubes produce a good bit of heat (as well as intense light) so plant position would have to be altered pretty drastically. The number of tubes/fixture is very important too. Just place your hand near the fixtures tubes and you'll immediately know what heat is produced by what.
