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