The key advances are devices and circuitry that emit and sense radiation in the terahertz band of the electromagnetic spectrum, which extends from the upper edge of microwaves to the near infrared. The rays are reflected by metal but go through most other materials. Water soaks up the radiation, so human tissue, which is mostly water, absorbs it. But unlike X-rays, terahertz rays are thought to be harmless. Terahertz radiation ("T-rays") can't penetrate much past your skin, and it lacks the energy to ionize molecules in human tissue the way X-rays do, so it cannot cause cancers by smashing up your DNA. T-ray technology will probably find its first big uses in security-related applications, now an enormously fast-growing business because of recent high-profile terrorist attacks, write John F. Federici, Dale Gary, Robert Barat, and Zoi-Heleni Michalopoulou in the July issue of IEEE Spectrum. In a terahertz image, a gun or a knife shines through whatever clothing it's concealed in. Even a plastic knife shows up, because of the way its sharp edges scatter the radiation. But some terahertz images have another ability, one not even claimed by the comic-book specs: not only can they see hidden objects, but they can tell what those objects are made of, write Federici and his colleagues, who are all professors at New Jersey Institute of Technology, in Newark. Many explosives, including all the plastic explosives popular with terrorist groups, reflect and transmit a characteristic combination of terahertz waves that make them distinguishable from other materials, even those that might seem identical to the eye and hand. That same chemical-discriminating capability also applies to pharmaceuticals and drugs. In essence, different materials appear as different colors to the terahertz imaging system. So future screening devices should be able to tell whether that's plastique in your pocket or just Play-doh, a package of sugar or an envelope of methamphetamines.
Some short-range imagers available now can also do spectroscopy, although the imaging rate is currently too slow for use in a walk-through scanner. But as the literally hundreds of engineers and scientists working on new terahertz sources and devices push the technology's limits, the article's authors expect to see a machine within the next five years that can do both imaging and spectroscopy at 50 meters or more.
RSS