Heat signatures can be masked

Firefighters need to understand how a thermal imager interprets heat signatures to successfully use this life-saving tool. (Photo courtesy of Bullard.)

By Manfred Kihn

When looking through the lens of a thermal imager (TI), what is it you are looking at? We know that TIs are designed to process emitted infrared heat signatures and convert that information to an LCD screen to be interpreted by the firefighter. We also know that the basic shades are white (identifying objects as hot or warm), black (identifying objects as cold or cooler), and gray (for all the temperatures in between).

It’s important to note that the TI doesn’t really care how warm or cold an object is, simply how much warmer or colder it is than the object next to it, which brings us back to the question: What is it you are looking at? Keep reading to find the answers. 

Remember, a TI is detecting Infrared Radiation (IR), also known as heat, which comes from anything with molecular activity, whether it be passive, active, or direct emitters. Emissivity is defined as a measure of a material’s ability to radiate absorbed energy. Picture a brick and a shirt lying on a sidewalk in Phoenix, Arizona, in July. Both warm up, but the brick absorbs more heat than the shirt and, once the sun goes down, the shirt gives its heat up faster than the brick. This means that the emissivity of the shirt is lower than the brick because it radiates heat faster. Defining passive, active, and direct emitters below will help us better understand what our TI is telling us:  


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