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I’ve got some disappointing news to share. It will be tough to hear, but I don’t know any other way to convey it other than to just come right out and tell you: Your thermal imager isn’t very smart. In fact, it is rather dumb.
A thermal imager is just barely smart enough to generate an image. It can’t really think about that image a whole lot. It cannot reason with the image or try to make sense of it. It simply receives incoming heat and displays it. No thinking, no judging, no guessing.
Heat in through the lens; light out through the display. It’s a simple job. So simple, in fact, that it leaves little room for error. Many attempts have been made to improve the intelligence of thermal imagers, but this often introduces the opportunity for mistakes, so it seems best to leave well enough alone. In fact, the reliability and consistency shown by your thermal imager is one of its greatest features. It does not interpret; it merely displays. Interpretation therefore, is in the hands of the firefighter. The imager displays; the firefighter interprets, at which point the first opportunity for error enters the chain of events.
Most interpretation errors are quickly resolved and, many times, never even noticed; however, some interpretation errors can lead to poor decision-making and negative outcomes. To get the most out of your thermal imager, you must become excellent at image interpretation. The ability to “read” the thermal image must become second nature. This will require a little bit of knowledge and a whole lot of practice to get it right. For now, let’s stick to the little bit of knowledge.
Radiation, conduction, convection. No, I’m not going to lecture on the basic definitions of heat movement or even the nuances of BTU output and absorption; rather, I’m going to tell you that you must understand one thing very clearly in order to understand what your thermal imager is displaying, and that is this: “Your thermal imager can see only radiated heat!” That’s it. It cannot see conducted heat and it cannot see convected heat. It can only see radiated heat.
Why does this matter? Because, if you are using a thermal imager, the decisions you make based on your interpretation of the image depend on you understanding this key issue. Over the years, the imaging capability of thermal imagers has become so good that it is easy to forget that what the thermal imager is displaying is heat and not light.
Webster’s Dictionary defines radiated in the following way: “to send out rays.” Radiated heat is energy loss by process of radiation, or the sending out of rays. Conduction and convection do not send out rays. Only radiation sends out rays. It is these rays that the thermal imager receives and translates into light for the firefighter to interpret.
I am sure that many of you are thinking, “Of course it sees conduction! I’ve seen it happen before!” But you’re wrong. You have never seen conduction on a thermal imager. What you have seen is the radiation that occurs after the conductive heat works its way to the surface. Once the surface gets warm, there is nowhere else for the heat to go except to leave the surface in rays, which are visible to the thermal imager.
The practical difference here is this: just because you don’t see something doesn’t mean it is not there!
Think about a bedroom in winter. You arrive at a house fire in the middle of the night and are assigned to victim search. You enter a bedroom and scan it with a thermal imager. Although the smoke is thick, the image on the thermal imager is crisp and clear. You can see everything in that room down to the hair clips lying on the carpet. What you don’t see is a victim. You move to the next room, thinking there might be a victim there. What did you miss?