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. ... 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.
—Herb Brody, "Taming the terahertz: t-rays could be more versatile than x-rays," Technology Review, June 2000

Binbin Hu and Martin Nuss, of the Advanced Photonics Research Department at Bell Labs, Holmdel, N.J., introduced their T-ray imaging system at the international Conference on Lasers and Electro-Optics (CLEO) here this week.

The researchers used laser pulses each lasting only 100 femtoseconds (one tenth of a trillionth of a second) to generate, detect, and measure electromagnetic pulses — T-rays — each lasting a picosecond (a trillionth of a second). ...

The digital signal processor was programmed to recognize the characteristic shapes of transmitted waveforms and identify the particular material at the spot illuminated by the T-ray beam. This information was obtained for every point or "pixel" on each object. Many compounds changed the T-rays in characteristic ways, due to absorption or reflection. Molecules and chemical compounds, particularly in the gas phase, showed strong absorption lines that can serve as "fingerprints" of the molecules. Metals and other materials with high electrical conductivity were completely opaque to terahertz radiation.

The T-ray imaging technique is notable in that it can distinguish between different chemical compositions inside a material even when the object looks uniform in visible light. Also, most plastics are transparent to T-rays, so it can "see" inside plastic packaging.
—"Just how lean is that ground beef?," PR Newswire, May 25, 1995