Message Board Thread - "IR transmissive material for rotor"

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IR transmissive material for rotor daveh 5/8/2008
I'm trying to measure surface temperatures on a flat disc stator mounted below a rotor (spinning up to 3000rpm) using an IR camera. Clearly the rotor needs to be made of a material that will transmit IR, and from my calculations at the temperatures I'm interested in (say 50degC to 90degC) the spectrum of most interest is about 8-9microns wavelength, but anything from about 4-16microns would probably do.
The rotor also needs to be robust, which probably rules out some of the more brittle and delicate materials. I was hoping to use some kind of polymer (eg polysulfone?) but am struggling to find information and a supplier. Any advice would be much appreciated. My other option is to abandon the IR thermography and use thermocouples on the surface, but that has another set of challenges associated with it!
Re:IR transmissive material for rotor Doctir bob 5/8/2008
Measurement of these temperatures can be readily done with either a mid-wave or long-wave IR camera. Camera speed, thermal and spatial resolution are key metrics for this application. Robust IR window materials include for the mid-wave, sapphire and for the long-wave, zinc selenide. Google on ‘zinc selenide IR window mechanical properties’ for web sources of more information. These are hard materials and are normally supplied as disks of specific diameters. I don’t know what shape you need so if it isn’t standard, these materials could get expensive. Polymers that transmit IR pretty well include polyethylene, polypropylene, Teflon and others sold under brand names specifically for IR window applications. Usually though these are thin materials. Beer’s law says the transmittance decreases exponentially with increasing thickness, so polymers requiring good mechanical strength could become problematic. For best transmittance anti-reflective coatings are desired on your IR window material as well. You also want an IR window whose transmittance is ‘gray’, ie spectrally constant over the waveband of your IR camera. Here’s a reference to a publication that discusses non-gray windows: The materials discussed in the paper would not be optimal for your application as they are probably not mechanically strong enough. Otherwise, they would be great for mid-wave IR camera applications.

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