In mid-August the Los Angeles County Metropolitan Transportation Authority and the Transportation Security Administration announced Metro has paid $100,000 each for several TSA-approved portable terahertz millimeter-wave screening devices. Made by the U.K.-based company ThruVision, the devices will be deployed within the city's metro rail system to detect at a distance weapons capable of causing mass casualties.

The reality of mass screening in a public-transit system immediately stirred a sense of unease. Soon after the announcement, the American Civil Liberties Union (ACLU) of Southern California filed a public records request seeking more information about the devices. The Los Angeles rollout follows tests by the TSA there, in New York City's Port Authority Bus Terminal and at Union Station in Washington, D.C.

Essentially, these terahertz devices are fancy cameras that image heat shed from the human body. They are called “terahertz” because instead of visible light they “see” at .25 trillion hertz. Unlike airport scanners, which require passengers to stop individually and wait while the scanners actively bounce radiation off their bodies and measure the reflections, these devices are intended to scan passengers at a distance of 10 meters while up to 2,000 passengers an hour flow toward them—for example, down an escalator. The operator looks for solid, dark patches in the passengers’ images that indicate a large concealed object. “When there is an anomaly,” says Dave Sotero, a communications manager for LA Metro, “then there is a law enforcement component and a bomb-sniffing dog. They will be pulled to one side, and the dog will run by, and if it doesn't smell anything the person goes on their way.”

Cops-as-voyeur worries are apparently unfounded. The devices see through clothes, but “we see no anatomical detail,” says Colin Evans, ThruVision's managing director. “Just a hot or cold pixel, and that’s what we’re displaying.” Warmer objects show up as brighter; cooler ones as dark. Evans notes a concealed weapon heated to human bodily temperature would disappear from the image, but suggests such a temperature match would be hard to achieve, and even harder to maintain. The systems being deployed in LA are intended to scan more than 2,000 passengers per hour at a distance of 30 feet.

In 2002 Rutherford Appleton Labs in the U.K. developed the terahertz technology as part of the European Space Agency program to allow satellites to look for holes in the ozone layer. Over the last 15 years the U.S., U.K. and several waves of private owners have collectively invested $50 million in developing it further. ThruVision hand-builds all its systems at its Oxfordshire, England, headquarters, but is setting up an assembly facility in Cape Canaveral, Fla. In the three years since it reached commercial production, Evans says the company has manufactured approximately 300 units, which have been sold to 18 countries. The largest single deployment is in Asia, where customs authorities have bought about 40 units to look for smuggled cash and drugs.

The heart of the device is a block of electronics about the size of a 1990s tower personal computer. It comes housed in a musician’s black case, akin to the one Spinal Tap might use on tour. At the front: a large, square white plate, the terahertz camera and, just above it, an ordinary closed-circuit television (CCTV) camera. Mounted on a shelf inside the case is a laptop that displays the CCTV image and the blobby terahertz image side by side.

An operator compares the two images as people flow past, looking for unexplained dark areas that could represent firearms or suicide vests. Most images that might be mistaken for a weapon—backpacks or a big patch of sweat on the back of a person's shirt—are easily evaluated by observing the terahertz image alongside an unaltered video picture of the passenger.

It is up to the operator—in LA’s case, presumably a transport police officer—to query people when dark areas on the terahertz image suggest concealed large weapons or suicide vests. The device cannot see inside bodies, backpacks or shoes. “If you look at previous incidents on public transit systems, this technology would have detected those,” Sotero says, noting LA Metro worked “closely” with the TSA for over a year to test this and other technologies. “It definitely has the backing of TSA.”

How the technology works in practice depends heavily on the operator’s training. According to Evans, “A lot of tradecraft goes into understanding where the threat item is likely to be on the body.” He sees the crucial role played by the operator as giving back control to security guards and allowing them to use their common sense.

Because the detector operates passively at a distance—unlike airport scanners—and doesn’t display anatomical details, both the company and Sotero say the system respects personal privacy. For Jay Stanley, a senior policy analyst with the ACLU, however, “It's still a search.” Also, Stanley raises the possibility of racial profiling in interpreting ambiguities: Operators may perceive the dark blob around a person's middle as a money belt on a white person but explosives on a person of color.

In a demonstration of the system at ThruVision's offices, the operator immediately spotted the large bunch of keys in my back left pocket but didn’t notice the closed medium-size Swiss Army knife in my back right pocket until told it was there. He was unsurprised: the knife is about the smallest size they expect to be detectable at that distance and resolution, and it is not among the items LA Metro wants to detect.

This raises the question of future development. Security expert Bruce Schneier argues if the costs of such systems drop the way they have with other electronics devices, we could see them proliferate the way CCTV cameras have, driven by “cost and fear.” Even if resolution improves only slowly, it is easy to imagine the technology being gradually improved to detect all sorts of things that are currently too small, particularly in the U.K. where carrying knives is restricted.

Evans believes, however, we will not see that happen because the precision required to manufacture these systems makes them expensive to build. Inside their front-end modules a scanning mirror directs the incoming body heat through eight sensor channels. At the back of each channel resides more electronics that amplify the terahertz signal and feed it into the video-processing engine that paints the picture on the laptop screen. Increasing the resolution involves adding more channels, and, he notes, these channels account for the bulk of the machine's cost—and the cost goes up with the number of channels. He adds, though, fewer mainstream off-the-shelf components are available for this part of the spectrum compared with lower frequencies with shorter wavelengths such as military radar. Systems that operate at lower frequencies, he says, are “inferior because they’re more prone to interference from marble floors, glass walls and warm air.”

Ultimately will these devices make public places safer? Opinions vary drastically. Schneier, for one, is a skeptic. “It makes no sense, because all it does is force an attacker to make minor changes to their plans,” adding that he sees the technology as a step toward “militarization of the police.” Evans responds: The scanners offer an alternative to leaving mass transit unprotected or increasing the visible police presence as terrorists shift their focus away from airports. “It’s part of the solution,” he says. “We don’t claim it’s the whole solution, and anyone who does is over-claiming their technology.” But the enormity of the problem makes even that more modest goal a challenge. “A bomb can be set off anywhere in a free society, Stanley says. “When and where is the trade-off worth it? A lot of terrorism is not really very fussy about what’s attacked. Are we going to screen everybody every time people get together in one place?”