Thermographers operate in a different world than all other humans—maybe that explains something about us. Initially, it can be quite intimidating to learn how to Think Thermally™, but most thermographers I’ve known soon find it fun as well as challenging. Even after nearly 30 years of paying attention to the temperature relationships in the world, I’m still delighted by them and occasionally left smiling with surprise.
No matter the application—from buildings to aerospace to medical to industrial—we all have to deal with emissivity and reflectivity issues. As I’ve explained in a previous post about how to accurately measure radiometric measurements, our thermal confusion would be tolerable if it were only one or the other of these issues, but emissivity and reflectivity come as a package deal and that poses a challenge.
When we look at a bare metal object, for instance, it is both low-emissivity (it doesn’t tell us much about how hot or cold it really is) and also high-reflectivity (it appears to be something it isn’t).
Here are two thermal images of a marvelous stainless steel sculpture that is displayed on a street in downtown Atlanta. Looking at it with our naked eyes, our mind insists that the object must all be the same temperature—the same or close to what the surrounding air temperature is. The thermal images, however, challenge our normal thought processes with a potentially confusing and contradictory perception.
As we become more skilled at Thinking Thermally™, the presence of reflections, like above, is usually obvious. The clincher of course, is that as we move our position the objects being reflected typically change and, in the above case, the apparent temperature we see on the sculpture changes. In these images, we might recognize the cold areas as being a reflection of the clear sky and the warmer ones as those of surrounding buildings or even ourselves! With that in mind, what would the Marx Brothers say??
While we can understand reflections as “thermal lies,” what is often not so obvious about these reflective surfaces is that they aren’t telling us much of the “thermal truth”—they are inefficient emitters. Exactly why they are inefficient is a separate discussion, but for now, we can simply accept it as true. This is hard to see on the sculpture because it is all air temperature, but when we look at a window it becomes easier to see.
Windows are only about 20% reflective, but they are quite specular or mirror-like. While we often see different reflected backgrounds on windows (image 3), we can also usually see differences related to actual temperature and emissivity (image 4).
When we look at any low-emissivity surface, we need to remember it is also a high-reflectivity surface (and vice versa). It is a good idea to separate the two phenomena and appreciate the impact of both. If you are measuring radiometric temperatures, of course, you must correct for both emissivity and reflected background temperatures. However, even then expect errors—perhaps unacceptably high ones—if excessive corrections must be made. It is always a good idea to double-check your results against reality! If you want to reflect more on these ideas, consider taking one or more of our recorded webinars on emissivity and its impact on thermographers.
John Snell—The Snell Group, a Fluke Thermal Imaging Blog content partner