Human eyes are wonderfully complicated and intricate organs. They're made for seeing visible light. This light reflects off of objects, making them visible to us.
Light, which is a type of radiation, comes in more flavors than just the visible kind. The range of light spans an entire electromagnetic spectrum, comprised of visible and invisible light, as well as X-rays, gamma rays, radio waves, microwaves and ultraviolet light.
Wavelength (also called frequency) is what makes each of these types of light different from one another. At one end of the spectrum, for example, we have gamma rays, which have very short wavelengths. On the flip side of the spectrum, we have radio waves, which have much longer wavelengths. In between those two extremes, there's a narrow band of visible light, and near that band is where infrared wavelengths exist, in frequencies from 430 THz (tetrahertz) to 300 GHz (gigahertz).
By understanding infrared, we can use thermal imaging devices to detect the heat signature of just about any object. Nearly all matter emits at least a little bit of heat, even very cold objects like ice. That's because unless that object is at absolute zero (minus 459.67 degrees Fahrenheit or minus 273.15 degrees Celsius), its atoms are still wiggling and jiving, bumping around and generating heat.
Sometimes, objects are so hot that they put off visible light -- think about the red, blazing-hot coils on an electric stove or the coals in a campfire. At a lower temperature those objects won't glow red, but if you can definitely put your hand near them you can feel the heat, or infrared rays, as they flow outward towards your skin.
However, quite often our skin isn't very useful for detecting infrared. If you filled one cup with warm water and one with cool and set them on a table across a room, you'd have no idea which was which. A thermal imaging camera, however, knows instantly.
In a situation like this, humans rely on electronic tools for assistance. In essence, thermal imaging devices are a like a sidekick for our eyesight, extending our visual range so that we can see infrared in addition to visible light. Empowered with this expanded visual information, we become the superheroes of the electromagnetic spectrum.
But how can a digital device possibly pick up on invisible heat signals and create an image that makes sense to our eyes? On the next page you'll see how advances in digital processing make it possible.