THERMOGRAPHY – Specular verses Diffuse

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THERMOGRAPHY COMFORT LEVEL – Specular verses Diffuse

The question comes up sometimes about the reflectivity of glass, since it has a high emissivity of .92.  If the emissivity is .92 the reflectivity is .08.  So why can I see my infrared reflection when pointing my infrared camera toward glass?  Two words the thermographer needs to add to his vocabulary if he haven’t already: specularity and diffusivity.  Specular reflector is defined by ASNT as: “Smooth reflecting surface that reflects all incident radiant energy at an angle complementary (equal around the normal) to the angle of incidence.  A mirror is a specular reflector.”  Diffuse reflector is defined by ASNT as: “Surface that reflects a portion of the incident radiation in such a manner that the reflected radiation is equal in all directions.  A mirror is not a diffuse reflector.”

What is glass made of?  Sand!  Both sand and glass have a high IR emissivity and a low IR reflectivity.  You can see your IR reflection in glass but not in sandpaper.  Why?  Glass is a good example of a specular surface and sandpaper is a good example of a diffuse surface.  Water has a very high emissivity – .98.  And yet you can see the IR reflection of the sun in the water using your infrared camera.

I took an image with my Fluke Ti32 infrared camera so that I could have both the visible light image and the infrared image.  These two images are really the same image.  One is blended down to 100% visible light and the other is blended to 100% infrared in the picture-in-picture mode.   I saved them as two different images and then exported them to the jpg file format.  In the first image (the 100% visible light image) you can see the glass window with a piece of sand paper being held up.   In the second image (with the 100% infrared) you can see my IR reflection in the glass but not in the sandpaper held below my head.  Although both the sandpaper and glass have a high emissivity you see a reflection only in the glass.  Why?  Again, the glass is specular and the sandpaper is diffuse.  On a hot day in the summer time you can get a “cold” reading on glass if you are standing at an angle where your infrared camera is catching a reflection of the sky.  Hopefully you will be more comfortable now when you run into these situations.



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