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T-ray scanners are developing rapidly, but so are the threats facing the personnel who would use them.

David L. Shenkenberg, FEATURES Editor,

On Sept. 21, 2007 – just six years and 10 days after two planes took off from Boston’s Logan International Airport and rammed into the World Trade Center in New York – MIT undergraduate Star Simpson entered the very same airport wearing some LED lights attached to a plastic board, with attached battery dangling, on the front of her hooded sweatshirt.

Unremarkably, she had gone to the airport to wait for someone – in this case, a man who identified himself as her boyfriend. She went to the information booth to ask about incoming flights, but little did she know that an airport staffer standing there mistook the LED lights for a bomb and decided to notify a state trooper. When she left the terminal, several troopers confronted her with MP5 submachine guns.

“Thankfully, because she followed instructions as was required, she ended up in a cell as opposed to the morgue,” state police Maj. Scott Pare later said.

Although in any other situation Star’s LED lights would have looked like something a child had made – especially with the Play-Doh used to hold it together – to the state troopers at the airport which just a few years earlier had been pivotal to a tragedy, the setup looked like a bomb. Star Simpson declined to comment for this story.

She told authorities that she had arranged the LED lights in the shape of a star so that she would stand out at a career fair. Ironically, just weeks after Star’s arrest, the city of Boston hosted the 11th International Symposium on Wearable Computers, a convention featuring electronics that people can wear on their bodies.

At a time when airport security will accost passengers if they bring too much hair gel in their carry-on luggage, perhaps most people would not dare to bring such a lighted display into an airport. However, the officials’ reaction to a homemade invention bodes ill for well-meaning scientists en route to a conference, for example, who might absentmindedly take their work onto a plane.

Suicide bombers

How can the police distinguish bombs from other electronic devices? Friend from foe? In the near future, a T-ray spectral imager might provide the answer. Functioning much like the infrared night-vision goggles used by the military, T-ray spectral imagers detect invisible heat waves in the infrared portion of the electromagnetic spectrum.

Jason C. Dickinson and others in Jerry Waldman’s lab at the University of Massachusetts Lowell have produced some of the most eye-catching images with T-rays. All of the T-ray images featured here are from their group and are reprinted with permission of SPIE.

Also resembling these military thermal imagers, some T-ray scanners produce colorful pictures – beautiful reds, yellows, purples, blues and greens. These are not real colors because infrared is invisible: They are assigned by a computer or computer chip based on the heat intensity. Without this added artistic measure, the images just look black and white.

Unlike the military thermal imagers in use today, however, a T-ray imager detects radiation in a region of the infrared that is absorbed by many explosives and illegal drugs, from 1 to 3 THz.

T-ray wavelengths range from microwaves to those at the very end of the infrared that are 1 mm long. Although we have microwave ovens in our kitchens, T-ray spectral imagers are just now becoming practical enough to emerge from research labs and enter the real world.

A “T-ray” – short for “terahertz radiation” – laser emits waves of infrared light in the terahertz spectrum, which have a frequency on the order of a trillion times per second. A T-ray spectral imager needs a source of T-rays, such as a laser, and these waves must hit the object under analysis. The device also requires a camera with a receiver that can detect the T-rays that bounce back from an object as well as those that are absorbed by that object. Also required is a computer chip or computer that can process the image.


Methinks this mannekin is hiding something…

Although researchers have produced and detected T-rays, they have experienced some trouble making the signal strong enough to detect distant objects, such as the roadside bombs in Iraq that have killed numerous soldiers. If the military were more effective in detecting these bombs remotely, more lives could be saved.

We also hear often about suicide bombers. In the June issue of Science, Michael Silevitch of Northeastern University in Boston commented that the military wants to detect a suicide bomber a football field away, so that soldiers can be dispatched to the terrorist before he kills civilians or the troops’ fellow servicemen.

Last year, a team of researchers led by Alan Lee in the laboratory of professor Qing Hu at MIT and their colleagues at Sandia National Laboratories in Livermore, Calif., reported at the Conference on Lasers and Electro-Optics (CLEO) in May that they had performed T-ray spectral imaging at a distance of 25 m, or half the length of an Olympic swimming pool. The Hu group has already recorded T-ray movies – a step closer to imaging moving people with bombs or suspected bombs on their bodies.

The group used an array of detectors, called a focal plane array because the array is in the focal plane of a lens. The detectors were electron bolometers, which, along with the quantum cascade laser, have proved extremely promising for T-ray spectral imaging.

The electron bolometer is already used as the detector in existing military night-vision goggles. These commercially available instruments can accept some light from the terahertz portion of the infrared spectrum and can operate at room temperature. Some have been modified to accept more T-rays.

As the T-ray source, the MIT-Sandia team used a quantum cascade laser, which is made of small and thin wafers of semiconductor material, often the size of a fingernail.

Stripped down

Federico Capasso and others at Bell Labs developed the quantum cascade laser in 1994. Now a professor at Harvard University in Cambridge, Mass., Capasso and his group at the university have generated radiation of about 5 THz at room temperature with the tiny laser, as reported by Kate Greene in an MIT Technology Review article dated May 23, 2008. In the past, the quantum cascade laser required some type of cooling system, usually big tanks of liquid nitrogen.

By making a miniature laser that can operate at room temperature, these researchers have come a long way toward rendering T-ray lasers more practical. Ideally, soldiers, policemen and security guards would have a handheld device that can work at room temperature and/or even in the desert heat of the Middle East. Trying to haul a giant laser with big cooling tanks through the streets of Baghdad would not be ideal.

At the Riken Advanced Science Institute in Saitama, Japan, Chiko Otani’s group has developed the technology to move a T-ray beam quickly in any direction while simultaneously changing the frequency, he said. That kind of versatility undoubtedly will prove useful for future applications.

In the future, soldiers may move portable T-ray spectrometers on the battlefield, but a different type of T-ray scanner is already in use in pilot programs at airports around the world, said Xi-Cheng Zhang of Rensselaer Polytechnic Institute in Troy, N.Y., who is an expert in T-ray imaging technology. But according to Alan Lee of the MIT group, the airport infrared scanners in limited use today have some differences from the ones used to detect explosives from a distance. They are designed primarily to detect concealed weapons, not explosives. Instead of radiation from a T-ray laser, the scanners detect T-rays that come naturally from our bodies. Because T-rays are a type of infrared radiation, which is essentially heat, what these scanners detect is our body heat.

Our body heat tends to range up to higher frequencies in the gigahertz range, or millimeter waves, so these scanners detect higher frequencies than do the spectral imagers used to detect explosives. It’s a trade-off, though, because lower frequencies penetrate clothing more.

T-rays can pass through almost anything but water. Because the body is mostly water, the radiation, similar to an x-ray, can pass through clothes yet show the outline of the person being scanned. X-rays can cause cancer – hardly what airline passengers want to take home from their vacation experience; T-rays, on the other hand, are safe.

The American Civil Liberties Union and other advocates have voiced concerns about the technology because it peeks under clothing. Though the images currently do not have the resolution to show more than the ghostly profile of a person, the outline does include intimate regions of the body. Giving security guards a show may be a small price to pay for ensuring the safety of airline passengers.

This dude with big hair thought we wouldn’t notice his handgun.

In addition to water, T-rays do not pass easily through metal, so they can show the outline of a metal gun or knife. And although metal detectors also can find metal weapons, mankind in its ingenuity has made weapons out of materials other than metal, such as plastic explosives and even ceramic guns. Because of the density of these nonmetallic objects, T-rays can detect them. This is the primary reason why T-ray scanners are superior to conventional metal detectors.

As Xi-Cheng Zhang notes, the inability of T-rays to pass through metal also could limit applications of the technology. Just as Superman’s enemies concealed contraband from the comic book hero’s x-ray vision by hiding it in lead-lined chambers, which block x-rays, bad guys in real life could conceal contraband from soldiers and policemen by hiding it in metal chambers, which block T-rays.

However, in some real-life scenarios, hiding contraband in metal chambers would not help. For example, if a terrorist hid a weapon in a metal container and walked through a full-body T-ray scanner in the airport, the scanner would detect the metal container, which would arouse suspicion and prompt airport security to search the terrorist. The technology is quite effective for security purposes, just as infrared imagers outside of the terahertz range already have proved quite useful.

Photonics Spectra
Oct 2008
That branch of science involved in the study and utilization of the motion, emissions and behaviors of currents of electrical energy flowing through gases, vacuums, semiconductors and conductors, not to be confused with electrics, which deals primarily with the conduction of large currents of electricity through metals.
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