How Does Thermal Imaging Work?

If you’ve ever seen photos or videos where everything is a red and yellow mess, that’s called thermography—more colloquially known as thermal imaging. Here’s how it works.

Thermal imaging is used in all sorts of different scenarios—utility and energy companies use it to see where a house might be losing heat through door and window cracks. Police helicopters use it to locate suspects at night. Weather stations use it to track storms and hurricanes. It’s used in the medical field to diagnose different disorders and diseases. And some home security cameras, like the one on the Ring Doorbell, can use it too.

What Is Thermal Imaging?


In the most basic of terms, thermal imaging allows you too see an object’s heat radiating off itself. Thermal cameras more or less record the temperature of various objects in the frame, and then assign each temperature a shade of a color, which lets you see how much heat its radiating compared to objects around it.

Colder temperatures are often given a shade of blue, purple, or green, while warmer temperatures can be assigned a shade of red, orange, or yellow. For example, in the image at the top of this post, you’ll notice the person is covered in shades of red, orange, and yellow, while other areas are blue and purple. That’s because she’s radiating more heat than surrounding objects.

Some thermal cameras use a grayscale instead. Police helicopters, for instance, use a greyscale to make suspects stand out.

How Does Thermal Imaging Work?


An example of a professional thermal camera.

Thermal cameras detect temperature by recognizing and capturing different levels of infrared light. This light is invisible to the naked eye, but can be felt as heat if the intensity is high enough.

All objects emit some kind of infrared radiation, and it’s one of the ways that heat is transferred. If you hold your hand over some hot coals on the grill, those coals are emitting a ton of infrared radiation, and the heat is transferring to your hand. Furthermore, only about half of the sun’s energy is given off as visible light—the rest is a mix of ultravoilet and infrared light.

The hotter an object is, the more infrared radiation it produces. Thermal cameras can see this radiation and convert it to an image that we can then see with our eyes, much like how a night vision camera can capture invisible infrared light and convert it to an image that our eyes can see.

Inside of a thermal camera, there are a bunch of tiny measuring devices that capture infrared radiation, called microbolometers, and each pixel has one. From there, the microbolometer records the temperature and then assigns that pixel to an appropriate color. As you might have guessed, this is why most thermal cameras have an extremely low resolution compared to modern TVs and other displays—in fact, a very good resolution for a thermal camera is only around 640×480.

How Is It Different Than Night Vision?

Technically, thermal imaging can be a form of night vision, and it’s used as such. But if your goal is to simply see in the dark, it’s a bit overkill.

In police helicopters, for instance, thermal night vision is great to have, since it can easily differentiate a person from the rest of the environment. This not only makes it easier to spot suspects in the dark, but even in broad daylight it makes it much easier to find someone who may have blended in with their surroundings.

However, most thermal cameras rely on longer wavelengths of infrared, whereas your typical night vision security camera captures shorter wavelengths of infrared, and is much cheaper to manufacturer. Thermal cameras, on the other hand, have the ability to capture longer wavelengths of infrared, allowing it to detect heat.

Images by Heather Cowper/Flickr, NASA, NASA/Flickr, Kecko/Flickr

Craig Lloyd writes about smarthome for How-To Geek, and is an aspiring handyman who loves tinkering with anything and everything around the house. He's also a mediocre gamer, aviation geek, baseball fan, motorcyclist, and proud introvert.