New Views on Airport Screening

By Joseph Straw

Checked Baggage

Explosives detection systems (EDS) are large, belt-fed machines used to scan unopened checked luggage, in which explosives pose the only critical onboard risk. Drawing on technology adapted from medicine, EDSs are basically computed tomography (CT) x-ray machines, which, like the new AT/DV x-ray machines, use software to interpret data from multiple x-ray diodes.

These machines, most manufactured by either L-3 or GE Security, have a tube-shaped gantry consisting of a massive lead sheath enclosing a spiral array of 11 x-ray diodes. The tube rotates around the bags as they ride a conveyor belt through the machine. The machine’s software interprets transmitted x-rays to generate a detailed, three-dimensional image of the bag and its contents, including identification of potential explosives.

Some forms of EDS have been around for a decade or more, but the technology continues to advance. For example, the throughput of traditional EDS machines is roughly 300 bags per hour. Rapiscan, however, plans to field a technology that can work four times faster, called the Real Time Tomography (RTT) system, Kant says.

Rather than a rotating gantry with 11 diodes, the RTT systems rely on 400 stationary diodes performing the same function, providing 1,000 to 1,200 data points for software analysis in a single instant. The vastly increased amount of data will aid in detection, while the instantaneous scan would speed the process.

The added speed of the RTT system would require multiple screeners to review baggage images if the machines are to provide their maximum throughput. Kant, however, says that one RTT machine would replace several traditional EDS machines.


Passenger screening is also evolving to keep up with changing threats. Threats now include not only guns, knives, and plastic explosives but also liquid explosives, radioactive materials, and pathogens. These threats have increased the need to move beyond the traditional metal detector, but that creates special challenges because humans cannot be subjected to the same x-rays as baggage.

Full-body imaging. One way that this challenge is being met is with the full-body scan systems, which use backscatter x-ray or millimeter wave technology. These generate much lower levels of radiation (less than 10 microREM versus 100 milliREM allowed per year). While they rely on different bands of the electromagnetic spectrum, backscatter and millimeter wave machines operate on the same principle. Like radar or sonar, the machines project energy onto an object, and the software interprets what is reflected back.

Generally, the waves penetrate clothing unaffected, are absorbed by hard objects like guns or explosives, and are “scattered,” or reflected back to varying degrees by organic material, including flesh.

A backscatter machine is about the size and shape of a vending machine; the subject gets scanned twice—once while standing facing the machine, then again while facing away from it. The millimeter wave machines that TSA has purchased, manufactured by L-3 Communications, are hexagonal booths with dual sensors that simultaneously sweep across a subject’s front and posterior. They produce a photo negative-like image of a bare body with inorganic threats in black.

The technology works, but it has run into opposition based on privacy concerns. The American Civil Liberties Union has dubbed the machines a “virtual strip search” and “an assault on the essential dignity of passengers that citizens in a free nation should not have to tolerate.”

TSA defends the technology. The agency notes that it is far less invasive than the traditional physical search. In fact, passengers subjected to secondary screening at Phoenix Sky Harbor International Airport, when given a choice between a physical pat down and a full-body scan, choose the latter 90 percent of the time, according to TSA. Civil libertarians counter that most people don’t know exactly what the images entail.

“Determining how the public feels about this is going to affect the future of it,” Howe says.

To address the privacy concern, TSA has asked manufacturers to tweak the algorithm to blur the face in the image. Certain backscatter systems offer what may be a more desirable privacy feature: the subject’s body is presented not as a full image, but instead as a white outline reminiscent of the chalk outline at a crime scene. Threat objects are superimposed.

To boost privacy further, the TSO who views the full scans is sequestered from the checkpoint. If the TSO spots a potential threat, he or she radios the checkpoint to order a pat down.

In addition, the TSA’s policy requires that the scan images be deleted the moment review of the scan has been completed, Howe says. Asked about the value that such threat images might have in criminal investigations, prosecutions, or for intelligence, Howe says the TSA wrestled with the question, but opted to make a clear commitment for the sake of privacy.

“If [a suicide bomber] was going to get screened, they’d have blown themselves up already. And if you call over law enforcement, you’re not going to need the image,” because they will have the actual item, Howe says.

As of this summer, the TSA operated backscatter machines manufactured by American Science & Engineering Inc. (AS&E) at three airports—Los Angeles International, JFK, and Phoenix—for secondary screening versus a pat down. A total of 38 L-3 millimeter wave machines were in use at nine major airports for primary continuous screening, with three more airports planned by the end of the year, says Howe.

Howe notes that a millimeter wave scan takes 15 seconds compared to the backscatter’s 40 seconds—extra time that adds up at high-volume checkpoints.

Checkpoints also still use magnetometers. Given the prevalence of nonmetallic threats and the prevalence of harmless metal items both on—and in—passengers’ bodies, such as medical devices, Joe Reiss, vice president of marketing for AS&E, questions the ongoing value of magnetometers.

Howe, however, predicts that walk-through magnetometers will be present at TSA checkpoints for the foreseeable future. Each one costs about $5,000, while a new millimeter wave scanner costs roughly $200,000.

As for the privacy issue—it’s not going away. Cathleen A. Berrick, director of homeland security and justice issues for the independent Government Accountability Office, tells Security Management that the TSA has yet to fully allay privacy concerns surrounding the use of full-body scans.

Special needs. Some passengers, such as those with casts or prostheses, need special consideration when going through a screening checkpoint. TSA observes strict procedures to protect the rights and dignity of all passengers with disabilities. The question is how to meet security objectives without causing distress to these special-needs passengers.

With that in mind, TSA policy allows TSOs  to inspect and touch devices like prostheses, but agents cannot compel travelers to remove them for inspection or screening. That’s where the technology can help.

To mitigate the risk presented by these devices, TSA, working with the imaging company Spectrum San Diego, Inc., developed CastScope. Roughly the size of an office water cooler, CastScope features a moveable backscatter array approximately the length of a person’s forearm or lower leg. As with full-body backscatter scanners, the system’s x-rays penetrate casts or prostheses to reveal any threat objects within. Each scan takes only 2.5 seconds, according to the company.

TSA began pilot testing of the CastScope early in 2007. It has since purchased 37 of the machines, according to the agency.      



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