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Microbolometer â€“ This is a type of infrared detector. The term refers to the way that 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 primary advantage of a microbolometer is that it can be designed to calculate surface temperatures based on the readings its pixels receive. All microbolometers have a shutter, which will â€œfireâ€ at different intervals to refresh the image. When this happens, the image on the display screen appears to freeze. The picture freeze is normal on all fire service microbolometers.
The other type of sensor is a ferroelectric detector, commonly referred to as â€œBST,â€ since the material on the sensor is barium strontium titanate. Ferroelectric detectors are not inherently better or worse than microbolometers; they merely operate on a different electrical principle. Ferroelectric detectors do not have a shutter, so there is no image freeze during operation. However, these detectors cannot calculate surface temperatures from their pixels.
Remember that any surface temperature measurement is subject to inaccuracy based on a number of factors outside the userâ€™s control. The challenges and restrictions of surface temperature measurement have been addressed in other articles.
Gain level â€“ Just as with a radio, an infrared detector must adjust its gain level to filter out background noise. Current fire service TIs have automatic gain adjustment systems, thus the firefighter does not have to concern himself with adjustments. The gain adjusts based on the amount of thermal energy in any scene. Microbolometers commonly have two gain levels, â€œnormalâ€ or high gain and â€œEI modeâ€ 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 TI has switched from high-gain to low-gain mode. Two examples of symbols that indicate low-gain mode are â€œEIâ€ and â€œLâ€.
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 and 300 degrees Fahrenheit.
Operational range â€“ 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 0 to 185 degrees Fahrenheit. 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 an hour or more 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 make more informed purchasing decisions. For additional terms and definitions, visit the Technology section of Firehouse.com.
If there are other terms you want to understand, or if you have questions in general about your TI, do not hesitate to contact me at email@example.com. Stay safe.
Jonathan Bastian is a thermal imaging specialist for Bullard. He is certified as a thermal imaging instructor by the Law Enforcement Thermographersâ€™ Association (LETA). He is also the author of the FD Training Network â€œFireNotesâ€ 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 public safety official in Central Kentucky. If you have questions about thermal imaging, please send them to firstname.lastname@example.org.