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Errant information related to thermal imaging seems to permeate the fire service. I am not sure what causes this, but it seems like several topics simply will not die and keep getting reintroduced. What bothers me most is that, often times, these inaccuracies are reintroduced to the fire service under the guise of training. In the past several months, there have been no fewer than 10 articles published or promoted to the fire service – by media and manufacturers alike – that cover the topic of thermal imaging. All of them contained inaccuracies. Some are opinions stated as fact, while others are factually incorrect.
I would like to set the record straight regarding the five most common myths surrounding thermal imaging. I have covered each of these topics in more depth in previous columns, so I will only scratch the surface of each topic in this column.
Myth 1: Temperature sensing is accurate
This is nonsense, and you should confront it at every opportunity. The right way to phrase this is, “Temperature sensing is sometimes accurate enough to be relevant to decision making.” What’s the difference, you ask? Well, even under controlled laboratory conditions, radiometry (temperature sensing) is only marginally accurate, often to a plus or minus 10 percent. But when temperature sensing is inaccurate, it is often wildly inaccurate. This is not a statement about the quality of the thermal imager.
The primary causes of inaccuracies are well beyond the control of the manufacturer, but you could roll them all under one umbrella called “real life.” Real life is not a lab. Real life is full of smoke, extreme heat, shiny surfaces, super-heated gases, condensation and the like – all of which contribute to inaccuracies. Temperature sensing is not valueless; rather, it must be taken in context. Please remember that it can be wildly inaccurate, even under rather mundane, real-life circumstances.
Myth 2: Image freezing
Nearly every thermal imager sold today is of a microbolometer technology, but there are still many of the barium strontium titanate (BST) technology imagers in use. Don’t confuse yourself with what each means (see Myth 4 below), but suffice it to say that these two technologies make up 99% of all thermal imagers in the fire service today, and they behave very differently.
Both technologies must periodically calibrate their respective thermal sensors; however, microbolometers use an internal shutter to do so and a BST does not. Microbolometers offer a multitude of advantages over BST in terms of size, power consumption, durability, dynamic range, electronics integration and others, but one drawback is the utilization of a shutter for calibration. When the shutter closes – known as non-uniformity correction (NUC) – all incoming heat is blocked and the imager is blind for a split second. While the imager is blind, the image seen immediately prior to the NUC is frozen on the display. There is nothing you as a firefighter can do to make this stop or make it faster. NUC occurs at two times:
1. Whenever the thermal imager changes gain states from high sense to low sense or from low sense to high sense. When the imager is exposed to very high heat sources, it must turn its internal gain state down to what is known as low sense; however, when viewing less-intense heat sources, the thermal imager must turn its internal gain up to what is known as high sense. This allows for consistency of picture quality, but requires a calibration and an NUC.
2. Whenever the imager itself thinks it needs to. Left alone in a room, staring at a mundane, ambient scene, the thermal imager will NUC at fairly regular intervals and differ by manufacturer, but routinely in the range of every two to four minutes. When exposed to more dynamic scenes, however, the imager will NUC more often.