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Does a material exist that is transparent to infra-red light, but is opaque to microwaves (S-Band, about 2.5 GHz)? I am basically looking for a filter I can put in front of a infra-red sensor.

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    \$\begingroup\$ I don't know but, "infra-red" covers a lot of territory. You might get better answers if you can add some detail to your question. What wavelength(s) are you trying to detect? Are you trying to capture an image? Trying to receive and demodulate a signal of some kind? What's the source of the microwaves that interfere with your detector? How close? How powerful? etc. It may be that you can solve your problem with something very different from the "optical" filter that you are asking for. \$\endgroup\$ – Solomon Slow Jun 23 at 12:34
  • \$\begingroup\$ You could try a metallic mesh and then filter the mesh out of the recorded signal or you could try water depending on your relevant wavelengths. \$\endgroup\$ – Christian B. Jun 23 at 12:37
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    \$\begingroup\$ When you say ..."in front of"... you may be erroneously thinking that microwaves act similarly to visible light, and don't relect off other surfaces. You may have to completely cage your sensor to filter-out microwaves. \$\endgroup\$ – glen_geek Jun 23 at 12:40
  • \$\begingroup\$ Search for "cold mirror", though I'm not sure that will block your GHz wave. \$\endgroup\$ – Scott Seidman Jun 23 at 12:40
  • \$\begingroup\$ There is also a Materials Stack Exchange. \$\endgroup\$ – user1271772 Jun 24 at 1:30
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For bulk material, you might try a slab of germanium. It's not a metal, but a metalloid, resistivity about 1 Ωm in its pure state, but will improve dramatically with doping, which may not affect its IR transparency in the 8 to 13 µm range. However, germanium is quite expensive and hard to come by.

If it's not an imaging sensor, then you could simply use a metal mesh, with hole sizes much less than the wavelength of the RF. For 2.4 GHz with a wavelength of 125 mm, holes less than 5 mm should provide a reasonable attenuation.

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  • \$\begingroup\$ ` holes less than 5 mm should provide a reasonable attenuation.` This seems like the most practical solution for my problem. \$\endgroup\$ – Lucas Jun 23 at 16:46
  • \$\begingroup\$ You can buy polka-dot beasmplitters on CaF2 or ZnSe substrates, both of which are transparent over a wide IR range. The coating is aluminium, with 56um gaps between coated regions. \$\endgroup\$ – Chris H Jun 24 at 9:59
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I think the screens in the doors of microwave ovens block the microwaves, but obviously wouldn't block the IR completely.

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  • \$\begingroup\$ Amazingly practical answer. \$\endgroup\$ – Itsme2003 Jun 24 at 17:10
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Microwaves can't pass through conductors such as metals. Infrared transparent conductors exist (see : https://patents.google.com/patent/US6761986B2/en or https://www.spiedigitallibrary.org/conference-proceedings-of-spie/4375/0000/Infrared-transparent-conductive-oxides/10.1117/12.439187.short?SSO=1) But they're space and defense engineering so they will not be readily available.

If you're not taking images with that sensor you could consider a metal sheet with holes similar to those on a microwave oven door. Be careful not to make those holes too small or you will block the infrared too. From 3 to 8mm should be enough to be safe from microwaves while still letting infrared pass through

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if you want substantial attenuation, then plan to locate the sensor well behind the wire mesh ---- if the holes are 5mm square, then have the sensor at least that far behind/inside the shielding cage.

If a 1:1 ratio, you will get 6.28 nepers of attenuation. According to the lecture of Richard Feynman.

This stackX answer Why are many IR receivers in metal cages?

provides lots more details and theory on mesh-shields.

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There are a lot of possible solutions here. Without knowing more, my preference would be to run an optical fiber from wherever your mixture of microwaves and IR light is to a sensor either far from the microwave source or sealed in a metal box.

Diameter of an SMA or APC connector is <<< your microwave wavelength, so you will not have ingress if you put a hole in your enclosure for a fiber.

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