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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 â€œTI era.â€
This monthâ€™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.
Pixels â€“ 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 â€“ 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 â€œself-preservation.â€ The end result in both situations was that the thermal imager would have an all-white display. This â€œwhite outâ€ 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 â€œwhite out.â€ 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 â€“ 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 itemâ€¦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 â€œwhite out.â€ 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 â€œhottestâ€ color, which is normally red.
Dynamic range â€“ 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 â€œsaturation pointâ€.