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Everything I search for, no matter where or how I phrase the question, invariably results in answers for how to block IR light. That's not what I want to do.

I have a parabolic heat lamp that emits a fair amount of visible light. I'd like to put a sheet of something in front of this heat lamp so that only IR comes out and as little visible light as possible. Naturally this material should be very efficient at letting IR pass because it would get hot very quickly if it wasn't. If there is such a material (plastic, glass, etc) - does it have a name so I can find it on-line?

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  • $\begingroup$ NaCl wafers that are scratched and very difficult to see through are totally transparent to IR wavelengths. $\endgroup$ Commented Dec 17, 2020 at 4:38

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Isn't this an example of what you are looking for? https://www.eplastics.com/plexiglass/acrylic-sheets/ir-transmitting

If this is too fancy, a black garbage back is also transparent in IR and opaque in visible. Maybe not so sharp an absorption edge as the plastic in the link. But is used to do nice image "tricks" with an infrared camera. Why is a plastic bag transparent in infrared light?.

Edit In answer to the comment, you should be aware that no matter how transparent is the material in infrared, by absorbing the visible part of the radiation produced by the lamp it will still get hot. So your condition, transparent in IR, absorbant in visible, is not sufficient for your application if you want to avoid the shade getting hot. You could, at most, look for a material that can safelly get hot and also filter the visible light. If you don't mind some visible light getting through you can use some blue glass sheet. What is your specific purpose for filtering out the visible?

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  • $\begingroup$ I don't know if plexiglass will work on a heat lamp, but using a plastic bag as a lampshade for your heat lamp sounds really dangerous. $\endgroup$ Commented Dec 17, 2020 at 4:15

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