Introduction to Infrared (Part 1): The Physics Behind Thermal Imaging

Have you ever wondered why images taken with a thermal camera are a mesh of red, blue, and yellow, or why they are black and white in night vision?

Our new blog series will provide those answers—and much more. We will look at the underlying physics and technology behind Forward-Looking Infrared technology, more colloquially known as thermal imaging; helping you to choose the right camera.

Today, thermal is used in many different scenarios—utility and energy companies use it to see where a house might be losing heat through cracks. The police use it to locate suspects from helicopters. Thermal cameras are used in advanced vehicles to see and classify items that are difficult to scrutinize with typical cameras on an autonomous car. Weather stations use it to track storms and hurricanes. It’s used in the medical field to diagnose different disorders and diseases. Thermal imaging cameras are mounted on ships to help the crew spot icebergs and passengers overboard.

There are many other exciting applications for this technology. If you are interested to learning more, make sure to follow our blog series Introduction to Infrared (IR).

How does a thermal camera work?

Human eyes can see objects illuminated by the sun or another form of light at specific wavelengths in the visual spectrum. In contrast, thermal cameras “see” heat or electromagnetic radiation emitted by objects within the infrared spectrum.

Infrared light is electromagnetic radiation of small particles named photons. All objects at temperatures above absolute zero (-273°C or -459.69°F) emit infrared radiation; this is how heat is transferred and detected by infrared cameras and can operate even in complete darkness.

Though it’s not visible to the human eye, it’s possible to feel infrared radiation. If you hold your hand close to the side of a steaming cup of coffee – you feel the heat emitting from the cup. Thermal cameras can see this radiation and convert it to an image that we can see with our eyes.

How is thermal different from traditional cameras?

A thermal camera produces an image similar to that of a visible camera. But unlike a visible camera, infrared sensors detect electromagnetic waves of a different wavelength from those of light. This gives thermal cameras the ability to “see” heat, or more technically, infrared radiation. The hotter an object is, the more infrared radiation it produces.

In other words, infrared imaging allows us to see an object’s heat radiating off its surface; therefore measuring the temperature of various objects in the frame and assigning each temperature a shade of a color. This also enables cameras to be used as a thermographic camera for accurate temperature measurement.

Colder temperatures are often represented as some shade of blue, purple, or green, while warmer temperatures are red, orange, or yellow.

Some cameras use a grayscale instead. Night time footage from security cameras is always in black and white. There is a good reason behind that; human eyes can differentiate between black and white better than they can differentiate other shades of colors, such as red or blue. Because of that, most night vision cameras use a monochrome filter to make it easier for us to understand what’s on the image. That is also why police helicopters use grayscale to make suspects stand out.

What’s the difference between uncooled and cooled cameras?

A thermal imaging camera uses either uncooled or cooled sensor to detect electromagnetic radiation.

In the more common uncooled thermal camera, the infrared-detecting elements are contained in a unit that operates at room temperature. In contrast, cooled cameras use a detector cryogenically cooled to around 77 degrees K, -321 degrees F (-196 degrees C). As a result of their elements being cooled, these cooled systems offer much better sensitivity compared to the uncooled systems, and therefore capable of distinguishing between smaller temperature changes.

Now, let’s get into physics of a IR infrared thermal imaging camera for a moment.

Electromagnetic Spectrum

The infrared spectrum constitutes only a part of the whole electromagnetic spectrum (as shown on the visual) and in its turn has three effective ranges depending on the wavelength:

• Long-Wave IR (LWIR) (7.5-14µm) – typically used by uncooled IR cameras;
• Medium-Wave IR (MWIR) (3-5µm) – typically used by cooled IR cameras;
• Short-Wave IR (SWIR) (1-3µm) – typically used in active illumination night vision technology.

A longer wave enables a photon travel through environment with larger particles (e.g. dust or fog). Hence uncooled devices designed for 7.5-14µ wavelengths are better suited for dusty or foggy environments. Uncooled cameras also provide more affordable infrared solutions than their cooled counterparts.

Examples of such cameras include PTZ infrared camera systems Accuracii Mini HD, Accuracii XRU HD, and Accuracii ML HD; security cameras Sii OP and Accuracii TO; driver vision enhancement military cameras Tavor BS, and pilot’s enhanced vision system EVS AP 640.

Photons have more energy when wavelength is shorter. Hence, cooled devices supporting 3-5µ are suitable for longer range surveillance tasks.

Examples include the Accuracii XR HD PTZ System and pilot’s enhanced vision system EVS AP.

Hopefully, this has answered some questions. In the next post in this series we will get “under the hood” of infrared cameras and take a look at finer, more technical details like detector, pixel, and pitch.

Read Part 2 of the series: Cooled vs. uncooled cameras, sensitivity, resolution, frame rate