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Emissivity puzzle
06-27-2012, 04:21 AM (This post was last modified: 06-27-2012 04:25 AM by Kostas Dalavouras.)
Post: #1
Emissivity puzzle
Hello all,
I have a question that have been puzzling me the last few days.
As we know glass has high emissivity usually 0.90 to 0.95. My question is why when we look from a thermal camera at glass surfaces we have so strong and well visable reflections while not from surfaces also with high emissivity like walls or concrete. I suspected the smooth surface of the glass but when I attached the black tape we use for the emissivity messurments (e=0.95) on a glass window still I could see very well my reflection on the window but not on the tape witch also had very smooth surface.
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06-27-2012, 09:06 AM (This post was last modified: 06-28-2012 09:00 AM by Gary Orlove.)
Post: #2
ITC RE: Emissivity puzzle
Great question! Actually, the emissivity of glass changes with wavelength. Since IR cameras essentially "average" the emissivity of a material within their wavelength sensitivity range we tend to assign this average number to a material such as glass.

Now glass does have a very high emissivity at wavelengths around 5 µm. So appropriately filtered Midwave (3-5 µm) cameras can measure glass easily. However, most of today' cameras are Longwave (8-12 µm) cameras. And the emissivity of typical glass falls somewhere between 0.8 and 0.85.

That means that about 20% of the incoming thermal radiation is reflected by the glass. There are two kinds of reflectors of IR energy, we classify them as diffuse and specular. Specular reflectors provide a sharp mirror type of reflection and diffuse reflectors give a very unfocused blurry reflection.

Glass is a specular reflector and black tape is a diffuse reflector, hence the difference you see between the two.

Here is a thermogram of a computer monitor. Note the reflections. They are very clear because glass is a “specular” reflector, in other words it produces a sharp reflection.
   

Here is an image of a “diffuse” reflector. Note how the image is very blurred. This surface is not hot, its just reflecting something warmer than itself.
   

Also see this thread for more about thermography of glass.
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06-28-2012, 04:14 AM (This post was last modified: 06-28-2012 04:24 AM by Kostas Dalavouras.)
Post: #3
RE: Emissivity puzzle
Thank you Gary, I really apreciate the info.
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08-03-2012, 09:21 AM
Post: #4
RE: Emissivity puzzle
This is an excellent answer, Gary. I always assumed that glass had a much higher reflectivity, but you go into detail about the nature of the reflectivity. Thanks for educating me!
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