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What is your thermal imager telling you?

Consider these three scenarios

June 13, 2024 
By Manfred Kihn

The TI is showing an overhead gas heater found left on found during investigation.

Every time I pick up a thermal imager (TI), I am amazed at what it is telling me. Does my imager talk to me? NO, but it does tells me what it sees. In other words, I must interpret the image that is being displayed. Even if it is showing me in black/white/grayscale or colourization of yellow/orange/red, I need to understand the interpretation of the images being displayed.

Not all calls that a fire department responds to are actual fires or have working fire conditions but there are many calls that they respond to where a thermal imager (TI) should be used. The unknown is anything that can’t be seen by the visible eye and we know that heat signatures can come from passive emitters (inanimate objects), active emitters (living organisms) and direct emitters (energy sources). So, besides the task of fighting fires, other applications include size up, overhaul, search and rescue, hazmat, industrial and smells & bells calls. All these calls require some sort of investigation with a thermal imager. In this article I would like to share three scenarios and walk through what the thermal imager telling you.

Scenario #1
For an investigative purpose, we use a thermal imager to find the cause of the unknown in the location you have been called to. The public or homeowner looks upon the fire department as experts in their fields and to make a mistake or miscalculation is not an option. A TI is a valuable tool that often gets forgotten in the apparatus, which I consider under-utilized technology.

An exterior door with elevated temperature at the top level: What is the TI telling you?

Where there is fire there is always heat, but there can be heat and no fire, which the TI will detect but what caused this heat is what needs to be investigated. Using your senses plus a TI can help you mitigate any potential invisible situations. An automatic alarm response call for an unknown problem can take some time to investigate, but the aid of a TI can help shorten that process.

Frequently, a smells and bells call for electrical issues such as overheated light ballasts, electrical panel breakers and improperly wired dryer plugs or receptacles, etc., all take time to ensure we identify the issue. Calls to industrial locations where there are mechanical issues with overheated motors or bearings on conveyors is another example. See what your thermal imager is telling you!

Scenario #2
The National Fire Protection Association (NFPA) 1801-2021 Standard on Thermal Imagers for the Fire Service removed the digital spot temperature indicator that was in the lower left side of the display screen. The reasoning for this is that some situations occurred where firefighters were using the thermal imager as a thermometer and the temperature was misread.

No thermal imager on the market can give a 100 per cent accurate temperature as there are too many variables such as:

  • Emissivity, affected and changed by:
  • temperature of emitter
  • surface geometry
  • wavelength being measured
  • surface roughness
  • angle of view
  • atmospheric attenuation
  • particulate, gases, humidity/water particulate
  • atmospheric augmentation
  • focal point/area of measurement
  • optical transmission
  • background/reflected energy

For the sake of argument, I will say that the temperature the imager is giving me is a range. The photo of the door indicates a temperature range of around 77 F, but in my opinion and experience, that door is nowhere near that temperature. Think about what you have on the inside or your door jam — some plastic or rubber weather stripping that will be the first to melt away during high heat conditions. On your next on-scene arrival, when you are looking at the top and both sides of the door, what can you see and what is your thermal imager telling you?

Scenario #3
The importance of thermal imaging training is to understand what a thermal imager is showing you on the display screen. It cannot tell you anything — you must interpret what it is you are looking at. Let’s consider the same fire but two different temperatures. One is from a piece of sheet metal on the floor causing reflectance which is only inches away from the fire and the second is showing the temperature at the fire itself.

Emissivity is defined as the ratio of the energy radiated from a material’s surface to that radiated from a perfect emitter, known as a blackbody, at the same temperature and wavelength and under the same viewing conditions. It is a dimensionless number between 0 (for a perfect reflector) and 1 (for a perfect emitter).

Any material that has a shiny or glossy surface will cause reflections to a TI. Just to name a few, all stainless-steel appliance’s, mirrors, ceramic tile, hardwood floors and glass, etc., all have a low emissivity value. Extra or additional images can show up on your display screen. Understand what is real and what is not as the imager can’t tell you that. Even in a fire situation, temperature disruption can occur so be careful of what you are looking at.

If there was a concrete cinder block lying next to the fire, it would have absorbed the heat energy from the fire as it has a high emissivity value and would be glowing white.

Whatever the response or wherever it takes you, be sure to take your thermal imager with you as it is a valuable tool to aid you in investigating, but you need to understand what your thermal imager is telling you.

Until next time, stay safe and practice often.

Manfred Kihn is a 19-year veteran of the fire service, having served as an ambulance officer, emergency services specialist, firefighter, captain, and fire chief. He has been a member of Bullard’s Emergency Responder team since 2005 and is the company’s fire training specialist for thermal imaging technology. If you have questions about thermal imaging, you can e-mail him at

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