Europa,
I'm not sure, but I believe that most of the far IR is absorbed after leaving the glass shroud that encases the actual light emitting part of the bulb... I believe that most glasses will absorb frequencies on a similar basis that the glass shroud of HID bulbs does... Or maybe it's special glass? I think all that matters is getting as much light as close, and as directly from the light source as possible, that's my only problem with the glass sheath "bake a round" idea. So if maintaining spectra is the focus, then I think you should aim for nothing but straight MH/HPS bulbs, as in no glass tubes...
here's a neat article on Halides:
http://www.gaash.com/html/articles.htm
As for why Halides have more radiative heat, I would guess it's because of it's broader spectrum than HPS? Does that make sense? Especially since it has more blue energy, it's frequencies are longer, and would penetrate further. I mean that's why alot of deep water corals have adapted to blue light, because that's the only part of the spectrum that will make it that far down in the water. That's just my guess, but to me that's inconsequential, just that If I need the spectrum of a halide, I've given myself upto all their faults. I wonder if Borosilicate glass (pyrex) absorbs different frequencies than "normal" glass, like a hurrican candle lamp?
I've attached a diagram which I think is as suitable alternative to the glass tube idea. This way the lights can move up and down, assuming that you put the inner box on some sort of pulley system, and the heat goes right up past the bulbs. This is not feasible if you're planning on using CO2, because it assumes the influx of outside air for cooling.
I think to me the question is heat containment. Certainly that to me is the biggest problem, when you start talking about enclosed spaces. If you're using CO2 you have to contain the CO2, but if you're doing without CO2, then probably it would make sense to take advantage of the exchange of outside air.
That is a question I wonder, what would the requirement for air exchange be if you could keep the temperature and humidity down?
And Europa or Sputnick whomever is the chemistry wiz, maybe you can answer this one for me. I've always considered the bubbler idea a very simple effective way to do hydroponics. I like the idea, but recognize through my aquarium hobby that most of the exchange of oxygen and dissolved gases happens at the surface, and that the air stone bubbles themselves do nothing except to agitate the surface. What if you could do some oxygen saturation directly into the nutrient solution? I realize that this would raise the ORP significantly, and probably lock out many nutrients due to oxidation. Is that correct? I'm just thinking about an ideal nutrient solution, would it be possible to dissolve pure O2 into a nutrient solution for plants? I know that there's a big market for Oxygen regulating equipment, just curious as to the chemical aspects, What's your thoughts on that? Could you theoretically do away with external venting if you could add CO2 and O2 to the chamber? Thanks, Ocelaris