Three companies are racing to market a new form of technology for detecting concealed weapons, using physics borrowed from radio astronomy and manufacturing techniques from cellular phone makers.

The technology, called millimeter wave, is a new category of sensing so unobtrusive that it seems like something out of “Star Trek.”

Unlike conventional systems such as metal detectors, which sense magnetic fields created by certain materials or objects, or X-ray machines, which pass rays through objects, millimeter wave sensors are passive and rely on detecting energy emitted by objects.

The energy the sensors look for is in an unfamiliar part of the electromagnetic spectrum, different from the usual visible light or infrared. At wavelengths of two centimeters to one millimeter, the energy is much longer than light or infrared waves, and thus able to pass through clothing and similar material. Human bodies radiate the energy at a rate higher than metal, plastic or composite materials, so those objects can be spotted under clothing, in silhouette.

The sensors have been successfully demonstrated in laboratories and have been sold mostly to government agencies for evaluation.

With research grants from the National Institute of Justice, the technology arm of the Justice Department, and from the Defense Department and other federal agencies, three small firms – Brijot Imaging Systems, Millivision Technologies and Trex Enterprises – are working to manufacture portable sensor units.

“Millimeter wave imagery is remarkably well understood, but no one’s been able to build anything cheap enough and small enough to be practical,” said Brian Andrews, president and chief executive of Brijot Imaging, which is a partner with Lockheed Martin, the giant contractor.

Mr. Andrews says his company has done just that, with a $60,000 box that is supposed to be able to see from 5 to 45 feet, depending on the lens attached. A computer scans the images and looks for anomalies that could be weapons.

A second company, Trex Enterprises of San Diego, has a unit that sells for $50,000 (so far only to government customers), and is working on a hand-held version. Its chief technology officer, John A. Lovberg, compared the technology to infrared technology, which is also “passive,” meaning that the sensor measures natural emissions rather than bouncing energy off the object being observed, as radar does. Infrared is used in a variety of settings, including military aviation.

Mr. Lovberg said that the technology mounted on cars or planes could also help drivers or pilots see through fog or smoke. Millimeter wave sensors, he said, can show “the difference between a road and a tree and a metal street sign.”

Millivision, of South Deerfield, Mass., is marketing a detector about the size and shape of telescopes used by serious amateurs. The company is partly owned by L-3 Communications, a major manufacturer of scanning equipment. The potential market, said William J. Caragol, vice president for business development, is “any entrance that you pass through where there’s a need for security.”

“It expands,” he said, “to every office building, stadiums – fill in the blank.”

Millivision has a $60,000 device it has sold to government agencies for testing, Mr. Cargol said. With L-3, it is developing a portal with controlled-temperature conditions, for more accurate scanning. The Millivision sensor, he said, can spot a ceramic gun that a metal detector would miss.

The Justice Department expects to use the sensors as security tools for courthouses and other buildings, but says they could also have commercial applications.

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