The technology, called millimeter wave, is a new category of sensing so unobtrusive that it seems like something out of "Star Trek."
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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 2 centimeters to 1 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 companies--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.
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 handheld version. Its chief technology officer, John A. Lovberg, compared the technology to 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.
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."
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Millivision has a $60,000 device it has sold to government agencies for testing, 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.
During the last eight years, the department has given about $7 million in research grants to companies working on millimeter wave technologies. Sara V. Hart, director of the National Institute of Justice, said in an e-mail message: "We want law enforcement and corrections officers to be able to detect any weapon, such as a bomb, gun, knife or nonmetallic weapon, from a safe distance. This would enable officers to make immediate protection decisions to protect themselves and the public."
She said that the technology could be "practical for use within the next few years" and that her agency had bought several systems for evaluation.
The energy levels detected by millimeter wave sensors are extremely small--mere whispers of energy measured in femto-joules, or quadrillionths of a joule. (A joule is the amount of energy of one watt applied for one second.) Chips, which are used in the sensors to process the data and run at speeds similar to the frequencies involved, 80 to 100 gigahertz, are still quite costly to make. But the machines that manufacture the chips have gotten cheaper because they are closely related to the machines that make chips for cell phones.
Lovberg of Trex said that advances in electronics are making millimeter wave technology more affordable. "Ten years ago, you couldn't buy amplifiers in this spectrum," he said of the amplifiers used in the sensors. But semiconductors made of gallium arsenide or indium phosphide can run in the frequencies required, he said. Those chips have been commercialized for use in other devices, and are now available for use in millimeter wave technology.
Tom Byrne, a member of the executive board of the Center for Commercialization of Advanced Technology, a public-private consortium in San Diego that provided a $75,000 grant to Trex, said, "In the past there was a lot of expense in the receiver because the signal is a low-grade, weak signal, and that pushed the price up." But now, the receivers are getting more sensitive, Byrne said, and that is reducing the price and making the sensors potentially viable.
Some companies are working on "active" millimeter wave systems, which are more like radar in bouncing energy off the person being studied, Byrne said.
In fact, low-power radar is already in use at some airports to search arriving passengers. But radar systems raise potential health concerns from exposure to the energy, and also privacy issues because body parts are clearly visible. Active millimeter systems could raise the same worries.
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