How Thermal Imaging Works

Thermal image of a hand with and adhesive bandage on it.
©GIPhotoStock/Visuals Unlimited/Corbis

In the aftermath of the Boston Marathon bombings, the manhunt to end all manhunts was underway. There was just one problem -- in spite of their massive advantage in manpower and firepower, authorities couldn't seem to find the perpetrators.

Tipped off by a suspicious homeowner, they finally narrowed their search to a large, covered boat sitting in a driveway. Because the suspect was hidden from sight, they couldn't visually confirm his exact position in the boat, nor could they see whether he was armed. Officers were working in the dark, blind to danger. That's when a thermographic camera helped save the day.


That camera, mounted to a helicopter circling overhead, clearly showed the man lying prone on the floor of the boat. It also revealed that the person was alive and moving. Aided by the visual information from the helicopter, a SWAT team was finally able to approach the boat and apprehend the suspect.

A thermographic camera (or infrared camera) detects infrared light (or heat) invisible to the human eye. That characteristic makes these cameras incredibly useful for all sorts of applications, including security, surveillance and military uses, in which bad guys are tracked in dark, smoky, foggy or dusty environs ... or even when they're hidden behind a boat cover.

Archaeologists deploy infrared cameras on excavation sites. Engineers use them to find structural deficiencies. Doctors and medical technicians can pinpoint and diagnosis problems within the human body. Firefighters peer into the heart of fires. Utility workers detect potential problems on the power grid or find leaks in water or gas lines. Astronomers use infrared technology to explore the depths of space. Scientists use them for a broad range of experimental purposes.

There are different types of thermal imaging devices for all of these tasks, but each camera relies on the same set of principles in order to function. On the next page we'll pull off the blinders on exactly how thermal imaging works.


Light Enlightenment

An illustration of the infrared portion of the electromagnetic spectrum.
2011 HowStuffWorks

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.


Thermal Imaging Heats Up

Sir William Herschel, the astronomer who discovered infrared wavelengths. He’s also credited with discovering the planet Uranus.
©Stock Montage/Getty Images

Thermographic cameras are high-tech, modern-day devices. But the discovery of infrared light came a long, long time ago.

In 1800, a British astronomer named Sir William Herschel discovered infrared. He did so by using a prism to split a ray of sunlight into its different wavelengths and then holding a thermometer near each color of light. He realized that the thermometer detected heat even where there was no visible light -- in other words, in the wavelengths where infrared exists.


Throughout the 1800's, a series of intrepid thinkers experimented with materials that changed in conductivity when exposed to heat. This led to the development of extremely sensitive thermometers, called bolometers, which could detect minute differences in heat from a distance.

Yet it wasn't until after World War II that infrared research really started heating up. Rapid advances took place, in large part thanks to the discovery of transistors, which improved the construction of electronics in a multitude of ways.

These days, the evolution of infrared cameras has diverged into two categories, called direct detection and thermal detection.

Direct detection imagers are either photoconductive or photovoltaic. Photoconductive cameras employ components that change in electrical resistance when struck by photons of a specific wavelength. Photovoltaic materials, on the other hand, are also sensitive to photons, but instead of changing resistance, they change in voltage. Both photoconductive and photovoltaic cameras both require intense cooling systems in order to make them useful for photon detection.

By sealing the imager's case and cryogenically cooling its electronics, engineers reduce the chance of interference and greatly extend the detector's sensitivity and overall range. These kinds of cameras are pricey, more prone to failure and expensive to fix. Most imagers don't have integrated cooling systems. That makes them somewhat less precise than their cooled counterparts, but also much less costly.

Thermal detection technology, however, is often integrated into tools called microbolometers. They don't detect photons. Instead, they pick up on temperature differences by sensing thermal radiation from a distant object.

As microbolometers absorb thermal energy, their detector sensors rise in temperature, which in turn alters the electrical resistance of the sensor material. A processor can interpret these changes in resistance and use the data points to generate an image on a display. These arrays don't need any crazy cooling systems. That means they can be integrated into smaller devices, such as night vision goggles, weapons sights and handheld thermal imaging cameras.


Thermal Imaging Intricacies

Thermal images work a little like the human eye. Only instead of picking up on visible, reflected light, thermal imaging devices detect the heat released by an object.

As you already know, objects both hot and cold emit heat. As that heat moves outward from the object, a thermal imaging device can see it. Like a camera, these devices have an optical lens, which focuses the energy onto an infrared detector. This detector has thousands of data points so that it can detect subtle changes in temperature, from about minus 4 degrees Fahrenheit (minus 20 degrees Celsius) to 3,600 degrees Fahrenheit (2,000 degrees Celsius).


Then, the detector constructs a thermogram, which is basically a temperature pattern. The data from the thermogram is transformed into electrical signals and zipped to a processing chip in the camera. That chip converts the thermogram's raw data into visual signals that appear on a display screen. The whole process works very quickly, updating about 30 times per second.

Many imagers show objects as monochrome pictures, with hotter areas shown as black and cooler areas as gray or white. On a color imager, hot objects jump off the screen as white, yellow, red and orange, while cool areas are blue or violet. These are called false color images, because the device artificially assigns colors to each area of the image -- unlike a regular camera, which creates true color images that show objects as they appear in real life.

Depending on the relative warmth of each object in view, the resulting image may offer striking visual detail, such as a full picture of a man holding a gun. In instances where temperature gradations are less distinct, the image may be fuzzier and less definitive.

Picture quality changes depending on whether the imager is active or passive. Active systems actually warm the surface of a target object using a laser or other energy source in order to make it more visible to its detector (and also anyone standing near the target area). For example, some car manufacturers warm vehicle parts as they pass through the factory, making any flaws in construction more visible to thermal cameras. Passive systems just detect the heat that the object emits naturally. Both systems have their pros and cons, but the simplicity of passive systems makes them far more common.


Night Vision ... Nope

Don’t be confused. Night vision imaging (pictured here) is not the same as thermal imaging.

Early versions of infrared detectors were big, unwieldy and noisy. Contemporary cooled systems are much improved, but even now they are still heavy, bulky and expensive, and often attached to large vehicles or planes so that they can be moved to a location and then put to use.

One popular cooled system, for example, is the FLIR SAFIRE III, which was used to narrow the search for the Boston bombing suspect [source: Peluso]. This unit is tough enough for military use and stabilized with an onboard gyroscope, and it works on land vehicles or on aircraft. It also weighs 100 pounds and costs around $500,000 as of 2013. "Cheaper" detecting units often run into tens of thousands of dollars, making them too expensive for the general public.


Uncooled products are much less expensive, and they are a lot smaller, too. Take the Extech i5 -- it costs around $1,600 and it weighs the same as a can of soda. It has a rechargeable lithium-ion battery, a 2.8-inch (7.1-centimeter) color LCD screen and, like a typical digital camera, it stores its pictures to a removable flash card.

Or consider the FLIR Scout PS24 monocular, which retails for roughly $2,000. It's only 6.7 inches (17 centimeters) long, so hikers, hunters and security professionals can take it wherever they roam. In spite of its small size, it has a color display and is waterproof, too.

Some of these imagers offer nifty features such as picture-in-picture displays, interchangeable lenses, laser pointers (so you can see exactly where you're pointing the camera), integrated GPS, WiFi connectivity and even microphones so that you can add voice comments to each image.

The Extech and FLIR products are both based on microbolometer technology. They're much different than most of the night-vision or infrared illuminated cameras common at the consumer level. You know these gadgets -- they produce that sickly green glow in movies and TV shows.

That kind of night vision doesn't detect heat. Instead, those products greatly amplify wisps of ambient light in order to reveal objects in the dark. In other words, they still need visible light being reflected off of those objects or they won't work very well.

The same goes for infrared illuminated cameras. These cameras project an infrared beam (think of your TV's remote control), which bounces off target objects and reflects light back towards the camera sensor.


Super-hot Tech

In May 2009, the Budapest Airport used a a thermographic camera at a security gate to monitor passenger temperatures to screen for possible carriers of influenza A(H1N1).

Thermal imagers are continually improving in sensitivity and features. But they are not a perfect technology.

Sure, these cameras can see heat signatures within vehicles, homes and other dense materials. But any physical material (such as glass windows) that blocks heat will reduce the device's effectiveness. You can even buy clothing that will counter some heat seeking sensors [source: Maly].


There's also the matter of interpreting the images that appear on a camera's display. The often fuzzy, changeable pictures are simply representations of temperature and not actual pictures, so making sense of them depends on the user's expertise. Inexperienced people may misinterpret those images, especially in scenarios with extenuating circumstances such as inclement weather or interference.

Expense will continue to be an issue for anyone without deep pockets. Even the most affordable imagers cost many hundreds of dollars, and they have only a fraction of the capability of those deployed by government and military agencies.

Those that have the dough, though, can perform some amazing feats. The security and surveillance aspects are almost a given -- bad guys have a lot fewer places to hide when cops and soldiers can track suspects even without visual line of sight, whether it's in an urban area, on national borders or inside buildings.

Using thermal cameras, fire fighters can locate people trapped inside structures, home in on hot spots and pinpoint structural problems before someone gets hurt. Scientists can find Arctic polar bear dens deep within snow banks. Ancient ruins often exhibit different heat signatures than the soil and rocks surrounding them, meaning archaeologists can use imagers to find their next excavation site.

Building inspectors carry thermal cameras to find leaks or deficiencies in roofs and insulation. Similarly, remediation workers can find water and subsequent mold growth behind walls, even in cases where a property owner had no idea there was a problem.

Power grid components that are overheating may lead to failure and then blackouts. To ward off outages, workers leverage imagers to spot deteriorating areas in a grid. Gas leaks are another major challenge for utilities, and thermal cameras can see leaks before they become bigger issues.

Worried about an epidemic? Install thermal cameras at high-traffic public areas like rail stations and airports and you can spot feverish folks in a crowd.

The list of uses goes on and on. And as companies invest more in research and development, thermal cameras will only get better and cheaper, and thus find a place in many more situations, from recreation to research. What's now a hot technology is only getting hotter, and we humans are seeing our world in a whole new way.


Lots More Information

Author's Note: How Thermal Imaging Works

We call them thermal cameras, but they aren't really cameras. Instead, thermal imagers are sensors. And for the moment, they are really, really pricey. I was fortunate enough to play with a handheld imager a few years ago when we were searching for the source of a mysterious water intrusion in a suburban home. Camera in hand, we found that one corner of the house was much cooler than other walls. We removed the drywall and found a hole just big enough to create a water problem during heavy downpours. We may have used the device for only a couple of hours, but it definitely proved its worth.

Related Articles

  • Atherton, Kelsey D. "How it Works: The Thermal Camera that Found the Boston Bomber." Popular Science. April 25, 2013. (May 3, 2013)
  • Beckhusen, Robert. "DARPA Finally Shrinks Massive Thermal Cameras into Handheld Devices." Wired. April 17, 2013. (May 3, 2013)
  • Boyle, Alan. "Secret Weapon? How Thermal Imaging Helped Catch Bomb Suspect." NBC News. April 20, 2013. (May 3, 2013)
  • Brown, Emily and Leinwand, Donna. "Helicopter, Infrared Cameras Help Confirm Suspect in Boat." USA Today. April 20, 2013. (May 3, 2013)
  • Electronic Design. "Infrared Sensors – The All-Purpose Detection Devices." May 7, 2009. (May 3, 2013)
  • European Space Agency. "Caroline and William Herschel: Revealing the Invisible." (May 3, 2013)
  • Dalesio, Emery P. "Thermal Imaging a Hot New Archaeology Tool." Los Angeles Times. May, 14, 2000. (May 3, 2013)
  • Davis, Joshua. "The Fire Rebels." Wired. June 2005. (May 3, 2013)
  • Drank, Nadia. "Infrared Images Reveal Frigid, Purple Penguins." Wired. March 5, 2013. (May 3, 2013)
  • FLIR product page. "Star SAFIRE III." 2011. (May 3, 2013)
  • FLIR Corporate Page. "Why Use Thermal Imaging?" (May 3, 2013)
  • FLIR Corporate Page. "What's the Difference Between Thermal Imaging and Night Vision?" (May 3, 2013)
  • Homeland Security News Wire. "FLIR Shows New Thermal Imaging Camera." May 3, 2007. (May 3, 2013)
  • IEC Infrared Systems. "Infrared Cameras: How They Work." (May 3, 2013).
  • Infrared Security Solutions product page. "LongView." (May 3, 2013)
  • Kondas, David A. "Introduction to Lead Salt Infrared Detectors." U.S. Army Armament Research, Development and Engineering Center. Feb. 1993. (May 3, 2013)
  • Parks, Bob. "Tool: Hot Shot Thermal Imager for Law Enforcement." Wired. Oct. 6, 2009. (May 3, 2013)
  • Rogalski, Antoni. "History of Infrared Detectors." 2012. (May 3, 2013)
  • Texas Instruments history page. "Defense." (May 3, 2013)
  • White, Jack R. "Herschel and the Puzzle of Infrared." American Scientist. May-June 2012. (May 3, 2013)