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Myths vs. Reality in Thermal Imaging - Part I

Thermal imagers (TIs) are revolutionizing the fire industry, quickly becoming standard equipment sought by every department in the country. Every day, another fire department accepts the value of TIs and puts a camera into service. Despite the quick adoption of thermal imaging, related technological and training issues have caused some resistance and confusion in the fire service. This multi-part article addresses some of the common misperceptions about thermal imaging technology and its uses, helping firefighters to better understand and apply it on the job.

Myth #1-Anything White on the TI Means a Fire or a Hotspot

Photo Courtesy Jonathan Bastian
In a 70?F room, a 50?F can of soda appears black because it is relatively cold, while a person with a skin temperature of 90?F appears bright white. The table and chairs are gray.

A fire department was called to a house that had been struck by lightning. On arrival, firefighters found an outlet with scorch marks around it. On their TI display, the FD members saw a white line running up the wall from the outlet. Taking this for an overheated electrical line behind the wall, the members quickly opened the wall from floor to ceiling, attempting to prevent damage from the hidden fire and to demonstrate the true value of their new tool. After opening the wall completely, the members discovered that the white line was caused by the hot water pipe running to the upstairs radiator. The FD mistook "white hot" to automatically mean "fire".

All thermal imagers detect and display relative differences in surface temperature. Modern TIs are extremely sensitive, recognizing temperature differences as little as 1/20th of a degree Celsius. As a result, "white" does not always mean "fire" or "hotspot." Image 1 shows a stable 70?F room. In it, a 50?F can of soda appears black because it is relatively cold, while a person with a skin temperature of 90?F appears bright white. The table and chairs are gray.

In a 100?F room, the can would still be black, but the table and chairs would be warmer than the person. As a result, the person would be light gray, and the table and chairs would display as white on the TI screen. The temperature of the can and the person would not change, but the person's relative temperature would change, thus the TI would display him differently.

Practical Application: When using the TI, remember that all temperature differences are relative. When looking for hotspots or investigating a possible fire, use additional techniques to verify questionable images. The back of a hand will help identify significant heat behind a wall, while a small inspection hole may help rule out smoldering wires. If verification is difficult, try comparing the item in question to similar items in the vicinity. For example, a "hot" roof may be caused by a smoldering attic fire or by the dark shingles that have been in the sun all day. Compare the questionable roof to a similar one next door to see if the heat patterns are alike.

Myth #2-Fire Victims Will Be Displayed In White

Photo Courtesy Jonathan Bastian
In this thermal image, firefighters are approaching a very hot door at a structure fire, which displays as white, while firefighters are displayed as dark gray.

This myth originated because many firefighters primarily operate thermal imagers in standard room temperature environments. As shown in Image 1, in normal environments, people are displayed as white because they are relatively hot. Unfortunately, many fire departments do not have regular access to live-fire environments. Consequently, much of their training occurs at room temperature, which gives the false impression that people will always be white on the TI display. In Image 2, firefighters are approaching a very hot door at a structure fire. Because the door is relatively hot, the firefighters in full turn out gear are relatively cool by comparison. Despite the fact it is 92?F outside, the firefighters are displayed in dark gray.

Practical Application: When training to use the TI for search and rescue efforts, make sure the "victim" is not the warmest object in the environment. In safe environments, heavily clothe the human victim to reduce the heat signature. When using rescue dummies, leave them in the ambient environment for a few hours before the drill, allowing them to reach nearly the same temperature as the environment. Of course, high heat environments will give the greatest realism. Remember that in live-fire drills, NFPA 1403 demands the use of rescue dummies exclusively.

Myth #3-TIs Will See Through Windows and Walls to Find Victims Faster

In some respects, the detector in the TI is not much different from the human eye. The TIs detector (called a focal plane array, or FPA) and the eye are both receivers. They receive electromagnetic energy and convert it into an image for our brains to interpret. The eye receives wavelengths of energy called "visible light" while the FPA receives wavelengths of heat energy called "infrared". As with radio frequencies, different wavelengths have different properties.

Light passes easily through windows, but does not pass through dark plastic bags (high in silicone content). A person looking through a window will see what is behind the window, while a person looking at a plastic bag will see only the color of the bag, not what is behind the bag. Silicone is somewhat transparent to infrared, so a TI will identify surface temperatures behind the plastic bag. Glass, however, is not transparent to infrared. In fact, glass acts mostly as a mirror to infrared, reflecting thermal signatures nearby. Images 3 and 4 demonstrate how light and infrared behave differently.

Photo Courtesy Jonathan Bastian
Silicone is somewhat transparent to infrared, so a thermal image will identify surface temperature behind a plastic bag.

Photo Courtesy Jonathan Bastian
Glass is not transparent to infrared, but it is transparent to light. The thermal image shows the reflection of the user.

Despite the rare transparent material, the human eye and the TI do not "see" through most materials. Drywall, plaster, concrete, steel, wood, paneling, down comforters, doors, sofas and the like are not transparent to visible light or infrared. They "see" only what is on the surface: colors for the eye, temperature differences for the TI.

While TIs do not see temperatures through glass, they can see temperatures of glass. Most TIs will differentiate windows that are hotter than other windows in the same structure. Keep in mind, this may mean:

  • Maybe there is a fire behind the window;
  • Maybe the boiler room is behind the window;
  • Maybe the window is reflecting a large heat source across the street;
  • Maybe the window is hot from being in the sun all day.

A "hot window" can mean any one of the above; use other resources (such as your eyes, your hands or comparisons) to verify your suspicions.

Some TIs have a feature that incorporates a basic video camera, which the user can choose to "overlay" on the thermal image. It is important that firefighters understand that the thermal energy of a fire or victim will not penetrate glass, and this feature does not change this fact. See image 4 for evidence on how thermal energy is reflected by glass, while visible light passes through it. The human eye and the video camera receive the same energy. As a result, if the human eye cannot detect anything in the room due to darkness or smoke, neither will the video camera. In short, if a firefighter's eyes cannot see through a window, neither will any thermal imager, no matter who manufacturers it and what features may be included.

Practical Applications: The TI cannot be used from the exterior to locate victims in the interior of a structure. A human body cannot generate enough heat to create a temperature difference on the exterior of the structure. Depending on the building's construction, the fire may generate enough heat that the exterior walls and windows will show greater surface temperatures near the seat of the fire. However, the TI will not "see" the actual fire. Similarly, TIs will not "see through" windows, though the temperature of the window may serve as an indicator. Just as firefighters can use visible exterior smoke conditions to estimate fire conditions, they can use exterior thermal signatures to estimate fire conditions. Always remember that the TI is displaying only relative surface temperature differences.

Myth #4-Temperature Readout is Accurate

By using what is known as non-contact temperature measurement, thermal imagers can employ radiometry or pyrometers to measure the surface temperature of an object. In a controlled environment, such as a laboratory, pyrometers can be accurate. The crux of the matter is that emergency incidents, by their very nature, are never controlled environments.

Calculating the temperature of an object is relatively easy using the equation:

P = s * e * T4 P

One problem for firefighters is the small "e" in the equation: emissivity. Emissivity is the measurement of how much thermal energy a material absorbs and then releases. Thermal energy that is not absorbed either passes through (like light through glass) or reflects off the surface (like light off a mirror). For a pyrometer to work, a figure must be included for "e." In a laboratory, a technician can manually adjust "e" based on the known material he is attempting to measure. Firefighters do not have that luxury for two reasons.

First, it is completely impractical to ask firefighters to memorize a list of emissivity factors for dozens of materials. Even if they could memorize the factors, it would be burdensome for teams in the middle of an event to stop and recalibrate their TIs based on the proper emissivity factors. Second, fire service TI manufacturers have recognized Reason One, and therefore preset the emissivity for all pyrometers. The figure is locked into the equation at a specific value, usually 0.95, and thus unable to be changed.

0.95 is the common emissivity because it approximates the value for a number of common building materials, such as concrete, drywall, wood, etc. While a wooden door can have an emissivity of 0.90, a painted steel door may have an emissivity of 0.50. The result is a dramatic difference in whether the TI shows the firefighter a completely inaccurate 90?F door or an approximately accurate 200?F door. A shiny aluminum container could have an emissivity as low as 0.05. Flat paint can be 0.70 while gloss paint can be 0.40. The variations are endless, and therefore so are the potential problems and complications.

Emissivity is not the only challenge to accurate fireground use of pyrometers. Pyrometer effectiveness is measured commonly by a ratio called distance-to-spot (DTS). The DTS ratio measures how large an area the pyrometer "sees" at a specific distance. For example, a pyrometer with a DTS ratio of 20:1 is attempting to read the temperature of one square foot of area at a range of 20 feet. If the firefighter varies from this distance, the accuracy level changes.

Additionally, the pyrometer is receiving information from a "cone." Picture a cone extending from the ?-inch lens of the pyrometer all the way to the one square foot area on the wall 20 feet away. While the pyrometer is attempting to get as much information as possible from the wall itself, it is actually receiving information from the air and any objects that are within the receiving cone. As a result, air temperature and anything between the pyrometer and the wall (such as smoke and water vapor) will influence the accuracy. Therefore, even if the firefighter is looking at the correct material from the correct distance, the temperature measurement can still be inaccurate.

Practical Application: First and foremost, avoid making critical decisions based on the temperature indicator on your thermal imager. It should be used only as a guide to verify information gathered from other sources. Remember that the readout attempts to measure surface temperatures; the pyrometer does not measure air temperatures. If a team is monitoring the temperature of a pressurized metal container, realize that the temperature quite likely is inaccurate. However, if the reading on the container doubles from 100?F to 200?F, the temperature of the container surface has doubled. It may have doubled from 300?F to 600?F or from 220?F to 440?F; the team will not know the exact temperature. Nevertheless, it has doubled.

Myth #5-Thermal Imagers Indicate Air Temperature

This myth is tied closely to the misunderstandings about pyrometers. As mentioned above, pyrometers (and thus thermal imagers that use them) measure relative surface temperatures. Thermal imagers detect differences in surface temperature. There is no commercially available thermal imager in the fire service that detects environmental air temperatures.

The temperature gauge on any TI will not help predict a flashover. Flashovers occur because of air temperatures, not surface temperatures. While air temperatures may affect the accuracy of pyrometers, thermal imagers will not actually measure air temperature. TIs can be used, however, to identify pre-flashover conditions through proper image interpretation.

Practical Application: Ensure that firefighters are regularly reminded about the limitations of temperature measurement and that they never confuse surface temperature measurement with measurement of surrounding air temperatures. Currently, the only ways to monitor air temperatures are the old-fashioned ways: firefighter training and firefighters observing the environment. Encourage firefighters to regularly practice image interpretation, involving all aspects of firefighter safety and accountability.


TIs are important tools, and firefighters need to be as informed and prepared as possible to use the technology effectively. These first five myths are some of the most common and dangerous misperceptions about thermal imaging. However, this list is not exhaustive. It is hoped that dispelling some of these myths will contribute to overall firefighter safety. Installment Two of this series will address myths regarding when a TI should or should not be used, as well as which technology is "newest."


Jonathan Bastian is a Thermal Imaging Specialist for Bullard. He is certified as a thermal imaging instructor by the Law Enforcement Thermographers' Association (LETA). He is also the author of the FD Training Network "FireNotes" book, Thermal Imaging for the Fire Service. Bastian served 12 years on the North Park, IL, Fire Department, including the last three as a captain. He has taught classes on thermal imaging, rapid intervention teams and search and rescue operations. He is currently a police officer in Lexington, Kentucky. If you have questions about thermal imaging, please send them to