Building Inspections, Windows Glass isn’t reflective, or is it?

In building thermography, there are many materials that appear thermally the way that you’d expect them to. Glass is the one exception. Consult any of the available long wavelength emissivity tables and they show glass emissivity to be fairly high; ~.90 except for fused quartz or Covex D glass, both of which are used in optics, not windows. Like almost all other materials, the emissivity of glass changes with an increase in temperature, but before you’d see it drop below .90 it would need to be hotter than 200°C. Why then do we see such sharp reflections when inspecting residential windows?

This is when it’s important to understand the difference between and the interactions of reflectivity and specularity. Generally speaking, when most people consider the concept of reflection, they think of specular reflection, as in a mirror. The Greco-Roman mathematician Ptolemy is credited with discovering the Law of Reflection, which describes the response to light energy when it meets a specular surface. With a specular surface, the angle of incidence and angle of reflection are equal. See the diagram below.

There are also what is known as defuse reflections, both visually and thermally. Diffusion is when light energy contacts a surface and scatters, instead of reflecting in a coplanar fashion as it does with a specular surface. This is the type of reflection that would be typical of most high emissivity surfaces, such as wood, paper, or paint.  Remember the RAT Law; R+A+T=1. What’s reflected, plus what’s absorbed (or emitted), plus what’s transmitted equals all the photon energy that contacts a surface at a point in time. Remember also that most surfaces we inspect are thermally opaque, meaning there is no transmission through them. Therefore, these surfaces are some part reflective and some part absorptive/emissive. So, every surface we inspect will have some reflection from it. In the case of most highly emissive surfaces what we’ll have are diffuse reflections.

In the thermal image below, we see examples of both. The window in the thermal foreground is very specular, so the reflection of the thermographer is very sharp. Look at the reflection of the person seated at the table. The table top is wood, approximately the same emissivity of the window, but much less specular. Therefore, the photons emitted from the seated person come in contact with the table and scatter. Glass, however, is microscopically smooth and flat which is why is behaves the way it does when wave energy contacts it. While there are other surfaces that can be emissive and specular, glass is the one we will most often see in buildings applications.

Hence, glass is in fact reflective, but in most cases only marginally so. It is however highly specular, so whatever reflections we do detect are intensely sharp due to their specularity.  This gives the untrained thermographer the impression that glass very reflective. If you attend one of our courses and continue to Think Thermally®, these conditions won’t fool you.