The images on the screen looked like the infrared bodies you see on ghost hunting shows, but the green figures that flashed at the Pentagon City Metro station Tuesday were the heat signatures of passengers walking past a camera-like scanning device.

Metro Transit Police anti-terrorism officers were looking for blank — or cold — spots on the bodies, which could indicate something as benign as a laptop or as deadly as a gun or suicide vest. It was part of a test the police were conducting with a system from the Transportation Security Administration. that detects explosives.

Such detection devices can help law enforcement spot explosives and other weapons without funneling people through metal detectors or disrupting passenger flow. “Passive” detection systems that scan crowds have been around for more than 15 years, according to Capt. Jim Baumberger, branch chief of public area security infrastructure protection for the Department of Homeland Security. But transit systems are turning more and more to this type of system as terrorist attacks of public spaces become more common.

The system tested by Metro was created by Thruvision, a publicly traded company that said it has sold more than 250 screening devices across the world, including to the TSA and the Los Angeles County Metropolitan Transportation Authority. The $90,000 systems have also been tested by New Jersey Transit and the New York Metropolitan Transportation Authority.

In the days before Thanksgiving, the TSA said, Amtrak used it to screen passengers at Washington’s Union Station.

The systems come mounted to a black trunklike box or mounted on a rolling tripod. As a person walks by, the system captures the “energy” of a person and flashes a green human shape from an Ethernet connection onto a laptop that a police officer is watching.

“They measure the naturally occurring energy coming off your body to see if there’s something blocking your energy,” Baumberger said.

The device does not display anatomical body parts and can detect — from up to 32 feet away — metal, plastic, ceramic, gel, liquid, powder and paper hidden in people’s clothing, according to Thruvision. The devices can focus on screening fields of nearly 34 inches by nearly 15 inches to as large as 11 feet by 8 feet. A Thruvision fact sheet says the system can be used only inside at an “ambient temperature less than” 82 degrees.

The equipment does not emit radiation of any kind and is portable.

“The fact that it’s mobile and portable speaks for itself,” Metropolitan Transit Police Chief Ronald A. Pavlik Jr. said Tuesday morning as four officers assigned to an anti-terrorism team watched on laptop screens hundreds of green figures go by at Pentagon City. Signs warned passengers that officers were testing the equipment. “We want to see how it reacts in this environment.”

Pavlik said he was impressed by the devices and would like to purchase at least one. Even if that’s not possible, he said, he plans to borrow the systems from TSA for major events such as the inauguration. TSA officials said the devices can be borrowed by governmental agencies, and local, state and regional police.