For a few weeks in July, a commuter rail station in New Jersey enjoyed the same screening protection as that surrounding the soldiers and civilians in Baghdad's Green Zone. Using millimeter waves--wavelengths of light shorter than microwaves but longer than infrared--a walk-through portal produced images of passengers before they boarded the trains. Like an x-ray, the technology creates a revealing picture that can highlight items, such as plastic guns, that typical transit security sensors fail to detect. And, essential for transit hubs such as New Jersey's Exchange Place Station, which serves at least 200,000 passengers a day, it does so quickly.

Be they trains, planes or automobiles, the world's transportation network needs better protection. In the U.S. alone, Government Accountability Office investigators snuck bomb components through more than 20 airports and the recent roll-up of a terror plot in the U.K. highlighted the inability of current technologies to detect items such as liquid explosives or their precursors. But a slew of new devices could help fill this security gap, including millimeter-wave cameras.

Developed by companies ranging from industry heavyweights General Electric (GE) and L-3 Communications to smaller firms such as QinetiQ, millimeter wave detectors work in one of two ways: active or passive. Active devices bombard people with millimeter waves to reveal what may be hidden inside clothing, whereas passive devices rely on collecting the ambient waves in the environment. "Millimeter wave covers a broad range of frequencies--from 30 to 300 GHz. At some, the sky illuminates the object, the image being collected in much the same way as an optical camera," explains John Salkeld, QinetiQ's director of optronics. "At others, you can pick up emission from the human body."

Whether active or passive, millimeter waves' real attraction lies in what it is not: overly revealing. But revealing is exactly what security experts desire for passenger and luggage screening. And for that, x-ray remains the best probing wave. Already, x-ray machines form the core of checked baggage security, peering inside suitcases much as doctors peer inside bodies using CAT scans. Carry-on baggage screening also enhances x-ray imagery by overlaying color-specific highlights that identify the type of material.

Carry-on luggage is a cluttered affair, however, and adding so-called backscatter x-ray machines--those that pick up the x-rays scattered by materials, rather than just those that pass through or are absorbed--can help clarify images. These devices can detect items otherwise obscured in baggage (such as the water bottle glowing to the right of the normal colored x-ray image pictured on the next page). Such x-rays have been offered as a solution for passenger screening as well, though radiation and privacy concerns have limited their application in the U.S.

Besides seeing what passengers bring on board, security officials also want to sniff them. The most common "smelling" devices are trace detection portals, known as puffers, which work by loosening particles on a passenger's clothing with blasts of air and then analyzing them for traces of explosives or other suspicious chemicals. But the machines have proved susceptible to malfunction, prompting a halt in their installation at airports, and some experts question their effectiveness. "Airplane security stops the sloppy and the stupid," argues Bruce Schneier, chief technology officer of Counterpane Internet Security. Puffer makers, though, are quick to defend their technology. "It doesn't take much to be sloppy," counters Jay Hill, chief technology officer for GE Security. "We're talking about picogram concentrations."

Regardless of whether terrorists are sloppy or stupid, they do have a wide array of explosive tools at their disposal: from highly volatile chemical bombs manufactured from relatively common ingredients, such as the hexamethylene triperoxide diamine (HMTD) suspected in the foiled London terror plot, to the military grade plastic explosive Semtex. Because of the problems with existing puffers, some security experts are looking at alternatives, such as quadrupole resonance, terahertz detectors and neutron bombardment machines.