Having some more fun with my thermal camera (after a cold kept me from it for a bit). I had noticed that my lunch did not show up while being microwaved, because long-wave IR generally doesn’t pass through glass — here’s another image confirming this
You can’t see the thermal signature of my hand that’s behind the window. This is, of course, responsible for the greenhouse effect — visible light goes in but thermal IR does not leave, making the greenhouse (or your car, or, with a more wavelength-selective effect, the planet) get hotter.
I wanted to see if I could find anything that was transparent. I checked the transmission of sapphire and discovered it was a possibility — the transmission cutoff is out at 5 microns. Alas, the windows were opaque, meaning either that the windows I looked at were antireflection-coated (a distinct possibility), or that the bolometer sensors aren’t sensitive at the shorter wavelengths. But the plastic bag this window is in transmits just fine!
It’s probably made of polyethylene, which has good transmittance in much of the thermal range — just a few absorption lines (at about 3.5, 6.5 and 13 microns). We happen to have some 1/16″ thick poly sheets, we use for our air sled to allow it to work over porous surfaces, and are opaque to visible light. But not to thermal IR!
You can see a couple of spots where I touched the sheet with my fingertips and warmed it up, but in the picture my hand is behind the sheet and not touching it.