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I am trying to take a thermal image of a glass plate inside vacuum. The back of the plate will be heated using a heater. A thermal camera, outside of vacuum, is used to take the picture of the glass plate through a small ZnSe windows.

The problem is that the glass plate surface is smooth that I can see some components of the chambers and the viewing port on the image. This affect the temperature reading.

Is there anyway to eliminate this problem? I only want to know the surface temperature of the plate. If glass is a bad material to use, which material would be give me no reflection?

The camera used is FLIR T420. The glass is at a much higher temperature than the chambers.

I include a picture to give a better presentation of the set-up


Are you seeing these components when the glass is hot? You could sandblast the surface, but the material reflectivity is still technically there. AR coating the front side would get you only part of the way there, assuming that you're not going to want to spend $$$$ on the coating.

This is an inherent problem with pyrometry, and typically one would make radiometric corrections to compensate for that. For example, if you take a hot image and cold image and take the difference, the resultant would be the emissivity times the radiometric change. Note that this approach would still have inaccuracies due to the change in reflectivity vs. temperature of the glass. What kind of accuracy are you trying to achieve? Your camera is only calibrated to ±2% accuracy for high temperatures, but I suspect there are unstated caveats with that.

Alternately, you could surround the glass with foil, leaving only a small opening to look through. Tilt the glass so that you do not have perpendicular line of sight to the glass and that the reflection off the glass is from the inside surface of the foil. This, in effect, creates a crude cavity blackbody; cavity blackbodies are the gold standard for achieving near unity emissivity.

However, I'm wondering what the intent is, given that you could easily attach a thermocouple or PRT and get better than 0.5°C accuracy on the measurement

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