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 questions, answer questions and really get a feel for what is on the minds of firefighters when it comes to thermal imaging. On the drive home, I was reflecting on the questions and interactions over the previous five days and felt like I had spent quite a lot of time at the show explaining technical terminology to firefighters. Some were confused by sales pitches that they had heard and others were confused by sales literature they had read. While a thermal imager (TI) is still a highly technical piece of equipment (resulting in the need for technical literature and sales presentations), you do not need to be an engineer to understand the terminology.

This month, we will define 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.

• 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 80x60, 120x120, 160x120 or 320x240 pixels. Most TIs use a 160x120 detector. The 320x240 detectors have four times more pixels than the smaller detectors. Generally speaking, the higher the resolution will yield a better potential for image quality. Price will rise or fall with resolution more than with any other feature of a thermal imager. The key is to determine what resolution is sufficient for your needs and budget.

• 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 shut down as a means of "self-preservation." The end result in both situations was that the TI 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 TI.

• 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°F item and a 1,500°F item — the most it can sense is 1,000°F. 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.

Simply put, 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 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 [email protected].

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 [email protected].