Over the past two years, this column has concentrated on helping firefighters prepare themselves to use their thermal imagers (TIs) more effectively at emergency incidents. The articles have included suggestions about how and when to use TIs, as well as how to practice with them around the firehouse. To date, though, the column has not addressed those fire departments whose leaders are contemplating how to step into the “TI era.â€
This month’s column defines in plain English several key terms that firefighters will hear as they consider a new TI purchase. While a number of phrases and acronyms may appear on TI technical sheets, this mini-glossary should help firefighters understand the more important technical terms.
Operational Terms
Pixels – These are the independent squares of material on the infrared detector that sense and react to the infrared energy. Pixel size, as well as the number of pixels on the detector, helps determine the resolution and quality of the thermal image. However, the processing hardware and software play a greater role in determining picture quality. The detectors in the fire service are available as 160 x 120 pixels or 320 x 240 pixels. Most of the small-format TIs use a 160 x 120 detector. The 320 x 240 detectors have four times more pixels than the smaller detectors. With most TI displays being three to four inches in size, the average user will not see a substantial image quality difference between the two sizes.
White out – This term is a holdover from the early days of fire service TIs. The first handheld TIs introduced to the fire service could be easily overloaded by an intense heat source, such as a fire. These systems would either fail as a result of the thermal overload, or they would shut down as a means of “self-preservation.†The end result in both situations was that the thermal imager would have an all-white display. This “white out†could only be eliminated by removing the TI from the environment and giving it time to recover, or by replacing the damaged sensor.
All of the technologies available in thermal imagers today are immune to “white out.†The sensors can be overloaded by a fire, but they do not suffer irreparable damage in the process. In short, white out is no longer a concern for departments buying a modern thermal imager.
Saturation – This term reflects the fact that every TI sensor has a maximum amount of energy that it can receive and process. If the sensor is exposed to more heat (thermal energy) than it can measure, then it has reached its saturation point Therefore, if a sensor can receive up to 1,000 degrees Fahrenheit in energy, then it cannot detect or display a difference between a 1,000-degree and a 1,500-degree Fahrenheit item…the most it can sense is 1,000 degrees Fahrenheit. If a large number of pixels become saturated, then an image may be mostly white or clouded by white. This is not “white out.†The detector, and thus the TI, is performing properly. It has been exposed to a significant heat source and is generating a mostly white image as a result. This should be a danger sign to firefighters that environmental conditions are unsafe.
Simplistically, the saturation temperature is the temperature at which an object must be displayed as white on the TI. If the TI has a colorization system, then the object will be displayed as the “hottest†color, which is normally red.
Dynamic range – This has two meanings. The technical definition of dynamic range relates to how many temperatures can be displayed in any given scene. Each TI has a maximum range of temperatures between black (cold) and white (hot). The larger this range, the more gray scales are available to the system and the greater is the range of temperatures that can be shown in a given image. In a very dynamic scene, this larger range generally results in a higher quality image. Dynamic range, when used conversationally in the fire service, refers to the maximum temperature that the detector can receive before it becomes saturated. In this sense, the term is synonymous with “saturation pointâ€.
Microbolometer – 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 “fire†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 “BST,†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 user’s control. The challenges and restrictions of surface temperature measurement have been addressed in other articles.
Gain level – 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, “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 TI has switched from high-gain to low-gain mode. Two examples of symbols that indicate low-gain mode are “EI†and “Lâ€.
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 – 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.
Conclusion
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 protected]. Stay safe.
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 public safety official in Central Kentucky. If you have questions about thermal imaging, please send them to [email protected].
