The New York Police Department is testing a new device it says can detect firearms concealed beneath layers of clothing, according to The Wall Street Journal.

The “T-Ray” machine detects terahertz radiation, a high-frequency electromagnetic natural energy that is emitted by people and can penetrated many materials, including clothing, the article says.

“If something is obstructing the flow of that radiation, for example a weapon, the device will highlight that object,” says Commissioner Raymond Kelly, who described the device in a speech Wednesday.

Privacy advocates are concerned by the devices, the article says, they also saw a potential benefit – it might render unnecessary the legally disputed police policy of stopping and frisking people who haven’t first been identified as suspects in crimes, WSJ reports.

The scanner highlights the person’s body in neon green, with firearm shapes standing out as black shapes. Additional images from the press event show the machine, tripod-mounted and about the size of an old-style projection television housed in blue plastic, WSJ reports. Officials say that in this form, the machine could be mounted on a truck and deployed to sites identified as prone to gun violence.

An NYPD spokesperson says the police aim to get the T-Ray technology in a device small enough to carry on an officer’s gun belt, but they provided no timetable for the deployment of such a device, the article says. However, last month, the California Institute of Technology developed a terahertz microchip, which raises the possibility that the technology could one day be packed into handheld devices, WSJ reports.