# Do metallic meshes work in practice as thermal radiation shields?

The other day I was thinking about the wire mesh cover used in microwaves to keep the microwave radiation inside cooking your food. The idea is that the spacing of the mesh is much smaller than the wavelength of light used for microwave ovens (typically 120mm, compared to the mm size mesh).

This has gotten me thinking, in cryogenic applications you typically have to use shielding to block out thermal blackbody radiation. Usually you have an object at Liquid helium temperatures or below (<10 Kelvin), and are fighting off radiation from ambient temperature objects (300 Kelvin). Converting 300 Kelvin to a wavelength of light using $$k_B T = \hbar \omega$$, we are talking about 50 micrometer wavelengths. So if we wanted a grid to block that out, we would consider a mesh with a spacing of about ten times smaller, or 5 micrometers.

Does this mean that rather than a bulk radiation shield, we could use a fine mesh grid to block thermal radiation? I imagine this would be pretty useful in the context of Vis/UV/X-ray detectors where you could allow line of sight detection while also blocking out thermal radiation which may mess up the detector. On the other hand, most applications wouldn't need this complication, but surely some experiments would benefit from it.

My question is: has such a design ever worked in practice? If so, can you provide a citation? If not, was the limiting factor the physics itself, or an engineering problem?

• this answer seems to get into the mathematics of it physics.stackexchange.com/questions/149607/… . – anna v Jun 30 '19 at 12:34
• sounds interesting; but 300 K thermal radiation will contain several wavelengths unlike a microwave oven, which is tuned to a single one; an engineering issue would be to cool the meshed shield itself (porous structure will cool poorly compared to a bulk) to keep heat load low on the lower stages; specifically to reduce 300 K radiation, one needs several thermal shields (multi layer insulation); there could be many more points to think about... – nole Jun 30 '19 at 13:27
• @cryonole thanks for the comment! The meshes in microwaves aren't tuned to the frequency, they are just typically much, much finer than the wavelength. I'm thinking of something similar here. Overall, I'm just curious if this effect has ever been utilized and worked out in practice. At least in the area directly on front of a detector/sample, I can see using this effect to be pretty useful. – KF Gauss Jun 30 '19 at 14:32