Secret weapon? How thermal imaging helped catch bomb suspect

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Thermal-imaging devices have been used to seek out pot-growing operations, map Martian geology — and now, to watch the prime suspect in this week's Boston Marathon bombings as he was holed up in his last hiding place.

Authorities said helicopters equipped with thermal imagers spotted the heat signature of a person inside a tarp-covered boat, sitting in a backyard in Watertown, Mass. Police used the sensors after a woman in the area reported seeing a trail of blood leading to the boat. The thermal readings confirmed that there was someone under the tarp, and that the person was still alive.

"Probably been there all day," a senior police official told NBC News. "He wasn't moving at first. He is now."

After monitoring the body in the boat for more than an hour, police moved in and seized the wounded bombing suspect, 19-year-old Dzhokhar Tsarnaev.

Thermal imagers can spot the signature of a body or other heat source even if it's inside a house, a vehicle, or in this case, a vessel. Walls may stop visible-light wavelengths, but infrared wavelengths can still pass through. The variations in heat emissions can be picked up by camera chips designed to be sensitive to the infrared part of the spectrum. The signature would be particularly noticeable when there's a significant difference between the background temperature and the temperature of the heat source.

Police have long used such devices to find out whether marijuana is being grown inside a house, using heat lamps. In 2001, the Supreme Court ruled that the use of thermal scans to monitor heat sources inside a person's home should be considered a "search" under the Fourth Amendment, and thus would require a warrant. The court said such scans could reveal private details about the homeowner, including the time of night when "the lady of the house takes her daily sauna and bath."

Thermal imagers have been taken to other worlds — for instance, aboard NASA's Mars Odyssey orbiter, which analyzes variations in the composition of the Red Planet's surface using the Thermal Emission Imaging System, or THEMIS.

Immigration authorities have used thermal scanners to look for the signs of fever among arriving passengers, and researchers have been experimenting with them as a lie-detector technique.

In 2009, FBI investigators used thermal imagers to search for bodies in the neighborhood where Cleveland serial killer Anthony Sowell lived. That may well have been the most notorious case where the technology was brought to bear. Until now.