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.