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 get into emissivity at the end of the Theory section of the course, 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, but 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. Some types of glass can be quite reflective, dependent on the composition of each particular type. 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 are of great concern for heat loss and heat gain, 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.  In electrical applications, contact with the surfaces being inspected is often an issue when the need arises to increase emissivity.  With windows this is less of a concern.  As long as you can safely reach them, 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.

In one commonly used approach, you can take an area of a window pane 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 approximate emissivity of Scotch 33) 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.