Thermal Imaging Terminology

The Fire Department Instructors Conference (FDIC) is one of my favorite activities of the year, as I get an opportunity to interact with firefighters from all over the world who come by the booth to talk about thermal imaging. The show allows me to ask...


The Fire Department Instructors Conference (FDIC) is one of my favorite activities of the year, as I get an opportunity to interact with firefighters from all over the world who come by the booth to talk about thermal imaging. The show allows me to ask questions, answer questions and really...


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Dynamic range — This has two meanings. The technical definition of dynamic range relates to the ratio of signal to noise. When the noise overrides the signal, the object has exceeded its dynamic range. Each TI has a maximum range of temperatures between black (cold) and white (hot). In extreme cold (usually less than -40°F) the imager cannot discern a strong enough signal due to lack of heat and cannot discern anything colder. In a very hot scene, the signal will be overridden by noise (saturation). The difference between these two temperatures (low and high) is what is referred to conversationally in the fire service as the dynamic range.

Microbolometer — This is a type of infrared detector. The term refers to the way in which the individual pixels on the detector receive thermal energy and translate it into an electrical current for the software to analyze. Most new thermal imagers are microbolometers, based on detectors made of vanadium oxide or of amorphous silicon. The advantage of a microbolometer over older technology is improved durability, reduced size and reduced power consumption. All microbolometers have a shutter, which will "fire" at different intervals to refresh the image. When this happens, the image on the display appears to freeze. The picture freeze is normal on all fire service microbolometers.

Gain level — Current fire service TIs have automatic gain adjustment systems, so firefighters do not have to be concerned with adjustments. The gain adjusts based on the amount of thermal energy in any scene. Microbolometers commonly have two gain levels — "normal," or high gain, and "EI mode," or low gain. When these TIs switch modes, the shutter will fire and there will be a momentary freeze of the image. Some TIs display a symbol to indicate that the unit has switched from high-gain to low-gain mode. Two examples of symbols that indicate low-gain mode are "EI" and "L." The gain switch occurs when a certain number of pixels (set by the manufacturer) become saturated in high gain. Modern microbolometers usually switch gain levels between 200°F and 300°F.

Operational range — Many TI specification sheets will indicate an operational temperature range. This refers to the temperature of the detector, not the scene being scanned or the environmental temperature. If the detector itself has a temperature outside of the range, it loses electrical conductivity and will not produce a proper image. The newest TIs have operational ranges of -40°F to 185°F. Insulation and heat management devices inside the TI help keep the detector in this range during normal operations. Depending on the TI, it could take hours of exposure at an extreme temperature to actually make the detector temperature move outside its operational range.

This list of terms is not exhaustive, but it does address the more common terms used in TI marketing and sales. Firefighters evaluating TIs for purchase should find these definitions helpful in understanding the basic operating principles of thermal imagers, as well as making more informed purchasing decisions.

If there are other terms that you would like to understand, or if you have questions in general about your thermal imager, do not hesitate to contact me at brad_harvey@bullard.com.

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 e-mail him at brad_harvey@bullard.com.