X-RAYS ARE GREAT FOR detecting the outlines of a hidden object, such as a gun in a suitcase. But they don't reveal what makes up the object. Researchers at AT&T Bell Laboratories in Holmdel, N.J., have built an instrument that reveals the object's composition by hitting it with a different frequency of energy: a "T-ray."

Terahertz rays, or T-rays, are electromagnetic pulses of around 1 trillion cycles per second. That's in the frequency range above microwaves and below infrared light. Researchers led by Martin C. Nuss, a technical staff member, create short pulses consisting of a broad band of T-ray frequencies and hit materials with them. A detector examines how the various frequencies are absorbed or dispersed by the material. The signals are converted to audio waves and analyzed with Bell Labs' speech-recognition technology. The researchers have created a library of characteristic signals for various materials.

T-rays could be used to measure fiber and metal content in brake pads, metals in semiconductors, and water content in cookies or plants.

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