Just as x-ray technology came along in the 1890s-allowing doctors to peer beneath flesh to see bones and organs-another promising imaging technology is now emerging from an underused chunk of the electromagnetic spectrum: the terahertz frequencies. These so-called t-rays can, like x-rays, see through most materials. But t-rays are believed to be less harmful than x-rays. And different compounds respond to terahertz radiation differently, meaning a terahertz-based imaging system can discern a hidden object's chemical composition. Thanks to this power, "terahertz imaging is getting hotter and hotter," says Xi-Cheng Zhang, a terahertz pioneer at Rensselaer Polytechnic Institute. Potential applications range from detecting tumors to finding plastic explosives. And since t-rays penetrate paper and clothing, a terahertz camera could detect hidden weapons.

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