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.
How’s your comfort level in thermography?
(The above comments represent my opinion).
Rod Hoff / Restoration Consultants Inc
Thermography Instructor / IR camera sales
3284 Ramos Circle, Sacramento CA 95827
toll free 888-617-3266 ext 301
Provider of Fluke TiS, TiR, TiR1, Ti, Ti25, TiR27, Ti27, TiR29, Ti29, TiR32, Ti32, TiR3, and TiR4 infrared cameras.
See DEMO www.moistureview.com/demo.html