and one of the questions being deliberated here has to do with the reflectivity of a glass mirror. But let’s remember that the reflectivity is not of the glass but of the silvered back surface, so it’s irrelevant that the infra red or UV passes through the glass. It does, I’m sure, reflect from the metalic back of the mirror, presenting an observer with some sort of “mirror” image. xo C.
A lot of infrared “night vision” goggles use CCDs in conjunction with visible-light-blocking filters to capture near-infrared light. This is not thermal radiation like far-infrared, but the portion just outside the visible spectrum.
This is much more useful for some night vision applications because things look like what they really are (for the most part) instead of the false color reditions of some thermal imaging systems. It’s also more practical because the sensors are far cheaper and don’t have to be cooled to ridiculously low temperatures like thermal cameras do.
Since CCDs are naturally sensitive to near-IR, it’s easy to block out the visible light and let the IR through. This is the kind of system that is employed in camcorders that have a “night shot” mode, like the Sony Handycams that were infamous for seeing through clothing. I use an IR filter on my digital SLR to capture some very ineresting pics. (And no, no “x-ray” pics through clothes. I can’t get that to work for the life of me.) Just do a Google search for infrared photography and see how different things look in that spectrum.
Obviously, thermal imaging is used in other applications. Police and the military use it for acquiring targets based on heat signature. But most cheap and readily available night vision systems use near-IR.
Anyway, to my point. With near-IR, you can see in total darkness by simply illunating the area with an IR light source. I’ve seen plans for making an IR flashlight using LEDs like those used in TV remotes. I suppose you might call it a “flashdark.” This would be invisible to anyone still looking in visible light, but through the night vision system, things would be lit up like daylight.
For once Q.E.D. is mistaken - I am definitely among the “science-impaired”. 
Sorry … what happened was, I missed “This is how greenhouses work” and still had my (admittedly tiny) mind on mirrors. 
Hrmm… is it possible to get laser pointers in near-IR? If so, you could make a very funky version of an invisible sniper sight thing.
Certainly IR and near-IR diode lasers are possible - in fact I think they hit the market before visible laser diodes. IR laser diodes are used in most laser printers in CD players (although perhaps more so in earler models in the case of CD players).
IR and UV lasers are ultra dangerous. You can’t see the beam so you won’t blink your eyes. They can do heavy damage to the eye.
Here a fun little experiment to do.
Get a video camera. Hide video camera on a shelf, turned on. Make room near-perfectly dark. Get an IR light(even your everyday average remote control can light a room enough for a camera to pick it up) see what your friends do when they know nobody can see them. Blackmail friends.
Just so you folks know – the IR that’s used by IR goggles and by animals sensitive to IR light is near IR, not much longer than visible light and, for all practical purposes, acting like visible light. These wavelengths are transmitted by glass and reflected by aluminum and silver, so a mirror would act like a mirror. In addition, a room dark by visible light standards will also be pretty dark by the stands of such short wavelength IR, too. Your body doesn’t throw off a lot of ligght at 850 nm because of body heat.
There are IR systems that work at longer wavelengths (and animal recptors, too, like the organs of pit vipers) which can sense body heat, and for which some of the transmissions and reflections may vary a bit, but in most such cases mirrors will still work as mirrors, I think.