Firehouse Magazine, June 2006
The Meaning Behind the Mumbo-Jumbo
By Jonathan Bastian
Over the past two years, this column has concentrated on helping firefighters prepare themselves to use their thermal imagers more effectively at emergency incidents. Suggestions have been included about how and when to use TIs, as well has how to practice with them around the firehouse. To date, though, the column has not dedicated significant attention to those fire departments still contemplating how to step into the “TI era.”
This month, this column will define, in laymen’s terms (or firemen’s terms) 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 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 far greater role in determining picture quality. The detectors in the fire service are available as 160x120 pixels or 320x240 pixels. Most of the small-format TIs use a 160x120 detector. The 320x240 detectors have four times, or 300%, more pixels than the smaller detectors.
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 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 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 is said to be “saturated.” Therefore, if a sensor can receive up to 1000°F in energy, then it will not be able to display a difference between a 1000°F item and a 1500°F item ... the most it can sense is 1000°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.
If the TI has a colorization system, then the saturation will be indicated by the “hottest” color (normally red).
Dynamic Range: This has two meanings. The technological 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. The second meaning of “dynamic range” is probably more common in the fire service. It refers to the maximum temperature that the detector can receive before it is saturated. This usage of the term is synonymous with “saturation point.”
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 then 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 appears to freeze. The picture freeze is normal on all fire service microbolometers.
The other type of sensor is a ferroelectric detector. These are commonly referred to as BST detectors, 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. 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.
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 “L”.
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°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 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.
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 jonathan_bastian@bullard.com. Stay safe.