HUMANS SEE WHAT WE SEE because the "film" in the backs of our eyes responds to certain frequencies of light. But what's visible to us is only a tiny slice of the light suffusing the world. The rest would be unseen without the likes of X-ray film, which responds to higher frequencies than human eyes, and infrared film, which captures frequencies too low for the eye.

At still lower frequencies, there's an almost untapped region of light that might prove even more revealing than X-rays, says Xi-Cheng Zhang, an associate professor of physics at Rensselaer Polytechnic Institute. He has devoted years to harnessing this slice of the spectrum, which is sandwiched between infrared and microwave radiation. This "terahertz" radiation passes through many objects, much like X-rays, and can theoretically produce internal views that are difficult or impossible to see with X-rays or other radiation. Examples include pictures of diseased tissue, or plastic bombs in suitcases.

The missing piece of the puzzle was "film" to register terahertz images. That's what Zhang has devised. It's a zinc-telluride crystal from which images can be extracted with a laser pickup similar to laser readers in CD-ROM players. Now, "the last unexplored region of the spectrum [can be] declared open for business," says Kevin R. Stewart, chairman of Molecular OptoElectronics Corp. in Watervliet, N.Y. Founded in 1993 after Stewart left General Electric Co.'s research lab in Schenectady, N.Y., MOEC has an exclusive license on Zhang's technology.

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