Here's The Science Behind The Technology That Ended The Boston Manhunt

Only four days after two blasts killed three and wounded nearly 200 in downtown Boston, the culmination of a massive law enforcement manhunt came down to an incredible technology: infrared imaging.

The helicopter that spotted bombing suspect Dzhokhar Tsarnaev was equipped with a forward looking infrared camera, commonly known as FLIR (pronounced, 'fleer'). Unlike night vision systems, which amplify visible light to illuminate things in low light, infrared doesn't depend on visible light at all.

Visible light — which is what we see — is just part of a larger rainbow of radiation on the electromagnetic spectrum. The spectrum ranges through short wavelength radiation, like X-rays and gamma rays, through visible light and all the way up to longer-wavelength microwaves and radio waves.

Infrared cameras pick up the electromagnetic waves in the infrared area of the spectrum, instead of the waves in the visible light section of the spectrum. Infrared radiation is just a tad longer wavelength than the color red.

These wavelengths carry more energy, so they are emitted by warmer objects, which is why your oven or toaster coils glow red (and large amounts of infrared) when they heat up.

On an infrared camera, heat from humans, animals, vehicles, etc., shows up as vivid and easily detectable signatures of white or black. For the search through Boston and the surrounding area, it proved invaluable.

Whatever the circumstance: completely blacked out, zero moonlight, through smoke, or underneath a boat tarp — if your body is still emitting heat, it's very difficult to hide from an infrared camera.

As the helicopters searched the neighborhood, a man living in a house in Watertown, Mass. walked outside and saw blood around his boat. After opening the tarp and seeing the suspect, he alerted police.

"We set up a perimeter around that boat and over the course of the next hour or so, we exchanged gunfire with the suspect who was inside the boat," Boston Police Chief Ed Davis said. "And ultimately, the hostage rescue team or FBI made an entry into the boat and removed the suspect who was still alive in the boat."

Over the boat, the helicopter pilots monitored Tsarnaev and relayed information to the SWAT team. After detecting him raising his arm on the camera, they were able to warn officers that he was still alive and to proceed cautiously.

The military and law enforcement have been using infrared cameras with great success for years, for everything from military reconnaissance to border security — but the cameras have been too expensive for civilians.

That's not the case anymore. While they won't be advanced as the one that nabbed Tsarnaev, you can get your own handheld infrared camera for under two grand.

Here's what the infrared video stream of the boat looked like: