The Complete Handbook of Thermal Imaging Cameras

Chapter 1: Understanding Thermal Imaging Cameras

Thermal imaging cameras are a specific class of measuring tool, and the first thing to say about them is that they are not cameras. That is, they do not capture a picture of visible light reflecting off an object. A thermal imaging camera measures electromagnetic radiation emitted by the object itself, in the infrared portion of the spectrum, and converts that radiation into a visual display of temperature. This is a fundamental distinction, and making it matters because it defines how you use the instrument, what you can see with it, and, most importantly, the types of errors you are likely to make.

Put simply: everything with a temperature above absolute zero emits infrared radiation. The hotter an object is, the more radiation it emits, and the shorter the wavelength at which the peak of that radiation occurs. A thermal camera uses a lens made of a material that is transparent to infrared—typically germanium or a chalcogenide glass—to focus that radiation onto a sensor. The sensor, which is almost always an uncooled microbolometer in anything a professional or hobbyist is likely to buy today, is a grid of tiny thermal detectors. Each detector changes its electrical resistance when it heats up, and that change in resistance is measured, digitized, and assigned a color or a shade of gray on a screen. The result is a two-dimensional map of the radiation coming off the scene, which the camera interprets as a map of temperature.

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