Interpreting Images: What Are You Looking At?

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.


Avoid mistakes

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.


Radiated heat

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?

We use blankets to stay warm. Why? We use them because they are excellent at trapping and retaining the heat from our bodies. The more efficient they are at retaining our body heat, the warmer they feel, but that retention efficiency is due to excellent insulative qualities. Excellent insulative qualities come from not allowing the body heat to conduct to the outside of the blanket, where that heat would be lost to radiation to the atmosphere. This lack of outward radiation would mean that, although your thermal imager can see every detail of the comforter on the bed, it cannot see the victim under the comforter. In the moment, when you are moving quickly, the environment is hostile and there are potential victims to save, your brain has a hard time realizing what you are not seeing. It only interprets what it sees. Mistakes happen.



What about convection? I am sure many of you have seen convected air currents on your thermal imager. Next time you are at a training fire, watch the ceiling in the burn room. You can definitely see heat being convected across the ceiling. Once again, you are seeing radiated heat and the difference here is just as critical as the search mentioned above.

Convection occurs when air is heated. As the air is heated, the molecules start moving around and bumping into each other. Every molecular connection transfers heat from one molecule to the next and also creates separation. This transfer and separation makes the air less dense, so it rises. As it rises, a low-pressure area is created and draws in denser, cooler air.

We all naturally know this, and the process is quite intuitive – so intuitive that we almost expect to see it. If we expect to see it, and then look at a thermal imager and see what we expect, it sets off no alarm bells at all – but it should. As gases are heated, their primary method of cooling themselves is through contact with other gas molecules. As the heat continues to build, this collision frequency increases; however, if the heat builds at a rate at which collision frequency alone cannot dissipate it, the gas molecules begin to emit heat in rays. In other words, they radiate. It is not until a gas radiates that it can be observed by the thermal imager, and at the point that the gas radiates, it can be considered superheated.

So, back to that house you were searching. During your search, you see currents of heat moving along the ceiling with your thermal imager, and it seems normal. Your brain mistakes it for what you would expect to see. Heat moves away from the fire and toward a vent point. Your brain would expect smoke and heat to be moving along the ceiling, so the thermal image looks normal; however, if your thermal imager can see the heat moving along the ceiling, you are not seeing the convected air currents. You are most likely observing the radiated heat from a column of superheated gas, and the implications are very different.


Train your brain

Practice, practice, practice. You should practice viewing the world through the thermal imager so that your brain gets better at recognizing how common objects look from a heat-based perspective; however, you should also practice in order to realize how things should not look.

Train your brain to realize that simply looking at a bed is not enough to rule out a victim. Only a hand search can do that. Train your brain to recognize that there is a good chance that the heat movement you can see in the air on the thermal imager is likely far hotter than you would think it is. No conduction, no convection; only radiation. The differences may be subtle, but the implications on interpretation and decision-making are not.


BRAD HARVEY is the Thermal Imaging Product Manager at Bullard. He is a veteran of public safety as a firefighter, police officer and paramedic and is certified through the Law Enforcement Thermographers’ Association (LETA) as a thermal imaging instructor. Harvey has worked as a high-angle rescue instructor and is a certified rescue technician and fire instructor. If you have questions about thermal imaging, you may email him at