The New York Police Department, with assistance from the Pentagon, is testing a scanning device that can remotely detect concealed firearms, Police Commissioner Ray Kelly said Tuesday.

The device measures natural radiation emitted by people and can detect when this flow of energy is impeded by an object, such as a gun.

"This technology has shown a great deal of promise as a way of detecting weapons without a physical search," Kelly said in a speech before a police group in Manhattan.

Known as terahertz imaging detection, the technology functions similarly to night-vision goggles, which detect infrared radiation. But unlike much infrared radiation, the terahertz wavelength is not blocked by clothing.

"With terahertz, you will be able to identify a gun as a gun," said John Federici, a physics professor at the New Jersey Institute of Technology.

The device is being developed in conjunction with a Department of Defense counter-terrorism unit, which has expressed interest in the technology to thwart suicide bombers and similar threats. The device could be mounted in a squad car or placed in an area with a high number of shootings.

A prototype is being tested at a police shooting range in the Bronx and has proven effective at distances up to 16 feet, Paul J. Browne, an NYPD spokesman, told The New York Times. The department hopes to increase the detection's effective distance to around 80 feet, Browne said.

Kelly did not specify when it would be ready for the streets. "The development work is moving forward and we hope to utilize the sensor as soon as it meets our requirements," he said.

Civil libertarians took a cautious stance on the technology, which Kelly said would only be used under "reasonably suspicious circumstances." "We find this proposal both intriguing and worrisome," Donna Lieberman, executive director of the New York Civil Liberties Union, said in a statement.

If effective, Lieberman said, the device could essentially bring to an end the NYPD's controversial "stop-and-frisk" campaign, which subjects more than half-a-million New Yorkers per year to largely random search by the police. Police officials have defended the searches as necessary to remove illegal guns from the streets, but civil liberties groups have decried them as a racially biased and unconstitutional invasion of privacy.

Yet the scanners could themselves be considered a breach of privacy. "The ability to walk down the street free from a virtual police pat-down is a matter of privacy," Liberman said.

"If the NYPD is moving forward with this, the public needs more information about this technology, how it works and the dangers it presents," she said.

Some terahertz detection devices, particularly those designed to work at long distances, emit radiation when scanning an object or person. But that radiation would not be at a high enough level to cause harm, according to Federici, the physicist.

"This is a lot different than nuclear radiation or X-rays," he said. "It doesn't really cause any damage."