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Trick of light makes microwave imaging simple

Metamaterials plus math equals quick, cheap system

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6:38pm, January 17, 2013
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The days of standing still, arms raised, in an airport security scanner may soon be a thing of the past. A new microwave imaging system offers a fast, inexpensive way to see through clothing and other objects that gathers data without involving complicated moving parts.

The new system, reported in the Jan. 18 Science, employs a thin copper strip as an aperture that collects a range of microwave-frequency light. Elegant math then converts those data into an image in less than a second.

“This definitely represents a less expensive and potentially faster alternative to current imaging methods,” says technologist Kevin Kelly of Rice University in Houston, who was not involved in the research. “You can imagine an MRI or PET scanner where instead of sitting in a machine for 50 minutes you sit in it for five minutes.”

In a digital camera, the lens focuses light onto an array of pixels on a detector. If you want a million-pixel image, you essentially need a million detectors, says John Hunt, a Duke University physicist who led the new work. That many-pixel, many-detector approach doesn’t work with microwaves, which are longer than waves in the visible part of the electromagnetic spectrum. So microwave imagers — used in car collision avoidance systems for their ability to see through fog and rain — have a single detector that must be slowly moved across a plane with the help of complicated, expensive gears.

The new imaging system also uses a single detector. But its fancy, specialized aperture means there’s no need for moving parts. The researchers etched tiny patterns — spirals and loops — into a super thin piece of copper, turning it into a metamaterial. Metamaterials often are exotic mixtures of matter that allow light to be manipulated, which makes them ideal for their invisibility-cloak possibilities (SN: 2/26/11, p. 12). While for cloaking purposes the material controls the direction of light, in the new study the metamaterial aperture allowed fine control over the frequency of light hitting the detector. Only light of a certain frequency can get through a particular etched section of the metamaterial screen.

“They get exquisite control of microwaves with just a sheet of copper with squiggles in it,” says metamaterials pioneer John Pendry of Imperial College London.

By sending out microwaves of different frequencies, recording some of the waves that bounce back and hit the detector and then transforming these data with elaborate algorithms, it’s possible to reconstruct a scene with far less data than is usually required.

“Instead of taking an organized set of measurements, if you mix them up randomly you can do really cool stuff,” says Kelly, who helped develop the first single-pixel imager.

The new system created images with about one-fortieth of the data usually needed for a similar image. Such compression is usually done after the fact, resulting in a JPEG or GIF file, but the new approach collects less data to begin with.

“This is a very clever technique of very economical sampling of an environment,” Pendry says. “It’s an extremely exciting development.”

So far, the researchers have taken images only of very controlled scenes of bright reflective objects in rooms with antireflective material on the walls. But the technology should work anywhere that you want to see shiny metallic objects, says Hunt. The approach might lead to cheap, handheld devices that look through walls at wires and pipes. Or police might have a vest with an imaging surface built into it that can see if a person has a knife concealed in a pocket. And the approach is also amenable to video: Soon you might walk through an airport without even knowing that you’re being scanned.

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