One of the basic concepts discussed in a Level 1 Thermography course is emissivity. Some people read ahead before they come to the class, and the first question out of their mouth is, “What is emissivity?” In our Level 1 –Thermographic Applications course at The Snell Group, we make you wait to have that question answered. We cover emissivity and reflectivity extensively in our theory (kitchen physics) section of the course. As these topics are arguably the most important and misunderstood areas of thermography. After we discuss modes of heat transfer and such. Even after a Level 1 course, the idea of what emissivity really is remains cloudy to some people.

To refresh and sum it up, emissivity is the opposite of reflectivity. All materials that are thermally opaque are in some part emissive, and some part reflective. The relationship between R and E is one of inverse proportions. Think of a see-saw (where I live, in Georgia, we call it a teeter-totter), on one end is Reflectivity (R) and on the other end is Emissivity (E). The more you have of one, the less you have of the other. So, if R is up, E is down, and vice-versa. You get the idea.

Emissivity and reflectivity are both material properties. In the Level 1 course, we teach that bare metals are generally the most thermally reflective (and therefore the least emissive) of the materials you will encounter during an inspection. Many other materials that you will encounter, such as wood, rubber, plastic, and concrete are generally very emissive and rank low on the reflectivity scale. When you look at charts of estimated emissivity of materials, you’ll notice that most materials live at one end of the scale or the other. One notable exception is glass. Most glass is Emissive and can be quite specular, appearing to be reflective, dependent on the composition of each particular type of glass and the coatings that may be present (such as Low-E windows). Remember it is the last surface that emits / reflects. To exemplify this, point your thermal imager at a window that you’re standing in front of. You’ll see yourself.

However, keep in mind that the emissivity of glass can vary from sample to sample. Factors such as coatings, or the types of finish will impact how reflective a sample of glass is. Your angle of view will exacerbate this issue also, as we cover in our Level 1 course. In building applications, windows are one of the components that seem to be of great concern for heat loss and heat gain. Windows either preform as designed or they don’t, some of the high performance gas filled windows can lose containment and reduce the performance because of the gas loss. And therefore, are excellent candidates for inspection. However, to be able to inspect windows adequately and reliably, we have to overcome the emissivity/reflectivity hurdle.

Inspecting windows is one of the occasions where emissivity correction is easy to accomplish. As long as you can safely reach them ( recommend completing this procedure from the interior of the building IF quantitative data is required), applying some high emissivity material to the window surface should be relatively easy. Scotch 33 electrical tape is great for this. It’s inexpensive, portable, and easy to apply, with a known Emissivity value of .95. building inspections are typically qualitative.

One approach if quantitative data is required (such as in manufacturing performance testing), you can take an area of a windowpane and break it down into quadrants. In the center of each quadrant place a one-inch piece of electrical tape. Set the emissivity correction setting in your camera to .95 (which is the emissivity of Scotch 33 electrical tape) and measure the temperature of each piece of tape. In doing so, you would be getting a reliable measurement of the window surface beneath the tape. This method will yield much more reliable temperature measurements than attempting to determine the exact emissivity of the type of glass you’re inspecting.

Windows can be challenging, but remember this important tip to collect accurate data, and you can stay a step ahead.