Camera Sees People in a New Light

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A prototype video camera that can see through walls, bodies and bags uses harmless radio waves instead of x-ray radiation. The technology, developed by graduate research associate Georgios Trichopoulos, with his advisor, professor Kubilay Sertel, at Ohio State University, could be a big help to security screeners at airports, since it can see items that x-ray machines miss and it can work in a video format — which means no need for a person to pause and stand still in the airport body scanner.

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There are some video cameras that can see through walls or even people, but they use infrared or x-ray wavelengths of light. Infrared works best when there’s heat or if it’s dark and x rays use radiation that can harm living cells.

Trichopoulos’ video camera uses a different wavelength of light called terahertz, which is a wavelength that’s longer than infrared but shorter than radio. There is radiation, but it’s nowhere near as powerful as that from x-rays, and won’t increase cancer risk in people that come into contact with it. Terahertz radiation is useful because not only can it reveal dense materials such as metals, it can also reveal less dense materials, such as plastics or even liquids.

X-ray imagers, on the other hand, can only pick out objects made from dense materials. They can detect the shape of a knife or gun, but the radiation sails right through softer substances such as drugs or plastic explosives.

If “terahertz imager” sounds familiar, it may be because some airport scanners rely on this technology, instead of x rays, to see through clothing. Anyone who has been through the airport security line, and has been asked to step into the chamber and put her arms up over her head for three seconds has probably been inside a terahertz body scanner. (There are other ones that use low doses of x-ray radiation, but they’re slowly being phased out.)

Unfortunately, terahertz scanners aren’t able to produce video. That’s why people have to stand still for three seconds.

Trichopoulos’ camera can create video. His technology uses a sensor that contains thousands of miniscule antennas, each less than a tenth of a millimeter across. Those antennae pick up the terahertz radiation. Terahertz radiation is all around us, reflecting off objects, but it’s not often “bright” enough for the sensor to pick up quickly. So the terahertz video camera has a transmitter that beams terahertz waves at the object it wants to scan, just as a TV camera might use spotlights to illuminate the subject.

Each of the tiny antennas picks up the radiation and sends an electrical signal to a computer that makes it into a pixel. Digital cameras work in exactly the same way.

Because Trichopoulos’ camera uses thousands of antennas, it can pick up the radiation fast enough for video. Other imagers that rely on terahertz technology use much fewer antennas and can only produce still images.

One might again draw an analogy with digital cameras: early models had fewer light-sensitive elements crammed onto the sensor, and so it took took longer to create an image, especially in low light, and the picture wasn’t sharp. It wasn’t until that technology improved that it was possible to make digital video on a typical point-and-shoot.

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Another innovation is that Trichopoulos’ camera is sensitive to several wavelengths in the terahertz band — an advance that can be likened to a video camera’s ability to record all of the rainbow-colored wavelengths of visible light to produce a color image.

There are other applications besides airport security for Trichopoulos video camera. Terahertz radiation can show how well hydrated human tissue is, for example, because water reflects some of waves. That capability could be used to get a better look at burns, for example, to see whether tissue can be saved.

Trichopoulos’s work was published in IEEE Transactions on Antennas and Propagation, and the camera fabrication was a collaboration between Ohio State and Traycer Systems Inc.

Top Photo: The terahertz camera prototype developed at Ohio State University

Credit: Georgios Trichopoulos