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

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

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

More specifically, if I have 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, is there any class of materials or thin film to cover the box with that would assure no air passed in or out, but that would not impede the rock radiating skyward its long wave IR?

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 aluminum.

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

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

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

Such windows are used to alloy inspection and temperature measurement of high voltage equipment inside safe enclosures.
It really depends on the wavelength involved.
If going to 5 micron works then sapphire is a good choice.
If you need to go longer, then you need to decide if it matters if the window is water soluble.

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P.E. Metallurgy, Plymouth Tube

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

Fascinating EdStainless, thank you, I've got a lot of great reading for tonight!

Expensive materials, no doubt. Any clue what price range these films might go for?

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, too.

- Shane

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

I saw 1.5" diameter sapphire windows for about $150 ea. A lot less than I thought.

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P.E. Metallurgy, Plymouth Tube

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

see: thread334-280328: Follow-up to the IR thread

Much depends on what "assure no air passed in or out " quantitatively means. Is the box hermetic? Saran wrap is still a plausible choice, as are a number of polyethylenes and plastics.


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

That first thread link is great, I've got plenty there to explore and digest tonight, thanks!

Box is sealed, and hermetically, if top film can be found that will let pass long wave far infrared.

Most common plastic sheeting do well passing short wave IR, like from direct sun, but as in a greenhouse,
they then largely trap and won't let pass back out the long wave far IR radiant heat emitted from objects
inside [Update: Not so sure about that now after testing reported in my post, 4th one down below.] that the
short wave IR sunlight entering has so easily and originally heated up. (Also, of course, air gets heated
up big-time via conduction/convection transfer between air and those hotter objects inside, but that's not
my concern here.)

Or, at least, that's how I understand it all. But, am eager to be corrected if not exactly the case.

Bottom Line; just seeing what's out there that will transmit and let pass long wave far infrared, what's
on the right side of chart below.

- Shane

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

You need good transmittance and low reflectivity.
There are a number of plastic films used for this regularly also.
But at real long wavelengths the semiconductors are best.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

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

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

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

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!

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