What materials, if any, are transparent to Long Wave IR?

[shane2]

More specifically, if I have a 100 degree F hot rock in an insulated box with reflective sides and I want it to radiate it's heat away to cooler outer space at night through its open top, what type of material or film could I cover the box with that would assure no air passed in or out, but that would not impede the rock radiating its long wave IR?

 

[.Scott]

You're looking for transmission of mid-wavelength and long-wavelength infrared, 3 to 15 micrometers.

If you can keep things dry, a thin sheet of salt (Sodium Chloride) or Potassium Bromide would work.
But Zinc Selenide would be more practical.
$60 on ebay. http://www.ebay.com/itm/like/262478291062?lpid=82&chn=ps&ul_noapp=true

 

[shane2]

Very interesting, .Scott, thank you!

Expensive materials to cover more than just a few square inches to initially test it.

Has me wondering now if practical way radiant heat from rock inside reflective box could be effectively concentrated
to go through a smaller aperture holding such a material?

Of course, doing that would, by itself, greatly reduces air flow in/out of box which was the original need for it, too.

While I've got you, let me also ask two related questions, specific to long wave IR emission and reflection properties.

I know they are largely the same for both long and short waves, paint rock black, have sides of box silver or shiny aluminum.

But, to really max out both radiating emission and reflective sides, is there anything more specific to long waves that'd enhance effects?

I'm looking for best paint or material to maximize the hot rocks' emissions that are radiating at the long wave end of the scale?

And, best paint or material to maximize the internal sides of the box reflecting the hot rocks' long wave emissions?

Thanks for any insights.

- Shane

 

[shane2]

Question arises, if rock temperature will only ever be between 0-40C, does that mean it will also only ever be radiating at around the 9 microns wavelength?

Or, does a solid of a specific temperature radiate at all different wavelengths, even though one specific wavelength will be the peak or majority radiated?

I ask, because instead of my trying to find a plastic film that will pass and transmit all of the broad ranging long wave far IR, should I instead just be looking for one that will, at least, be transparent in that 9 micron neighborhood?

If so, would that then let maximum radiant heat emitting from hot rock of 0-40C pass and transmit through it?

Any clues where to find, if above makes sense?

- Shane

 

[shane2]

Scrounged around in home, office, lab and warehouse tonight and found 36 different kinds of flexible thin plastic films, mostly clear, some black, like trash bags, or white or with colored writing on both sides, everything imaginable really, even shipping tape, etc. Some super thin, like saran wrap, some really thick, like through both sides of a double layer of heavy black garbage bag and double layered plastic grocery bags of all kinds, some still even mildly scrunched up, and everything in between and imaginable, even three different kinds of bubble wrap!

Cranked up FLIR Scout PS, propped up my bare feet as bright white target and proceeded to drape one after another in front of lens. Three different hard plastic sections from product packaging, like what you have to cut with a blade to get into, blacked out the screen, nothing got through them.

Everything else did, some so well I had to double check it was really in front of the lens at all!

When it comes to identifying, selecting, and buying any favorite plastic film in quantity later, I'll also check it out at lower temps, too, not just my 95F feet.

Was above test likely fully valid, as far as comparable results being expected in the field to accomplish mission of OP, too, or might I be missing anything here?

If not, then plastic film covering to keep out wind and air is plenty doable and readily available, especially if best IR transparent plastic, that'll still hold up in environment, does not erode maximum radiating intensity anywhere near as much as air movement likely would have without it there covering apparatus top opening to the black night sky.

Appreciate any comments; good/bad/ugly to enhance goals here, thanks!