A small, millimeter and microwave-detecting camera that can see through solid materials in real time has been developed. Soon, the device could be adapted and used in law enforcement and security where, among other uses, its inventors envision airport scanners that screen passengers for weapons or explosives as they walk by.
The camera features a one-dimensional aperture made from a copper-based metamaterial. Fashioned from plastics or metals, metamaterials behave in ways that ordinary materials naturally do not. Some can cloak objects. Others can reveal them. Here, scientists used the copper-based metamaterial as an aperture for microwaves, the telecommunications workhorses that populate the longer end of the electromagnetic spectrum. By connecting the aperture to an image-reconstructing computer, the researchers can capture information from a scene in real time, with no moving parts.
This new microwave- and millimeter-wave-imaging technology could cut the cost, size, and speed scanners and open them up to other applications. The technologyrelies on metamaterials and computational-imaging techniques. The researchers send microwaves at different frequencies, ranging from 18 to 26 gigahertz, one at a time into one end of the waveguide. As light of a certain frequency travels down the structure, it encounters resonator elements, some of which are designed to resonate at that frequency. So the light radiates out of the metal strip at those resonator spots. The emerging light waves interfere constructively and destructively and create beams that point at a variety of angles. “What propagates away from the structure is a set of beams pointing in different directions,” Hunt says.
“You can see through certain materials that you can’t see through with optical light – such as clothing or wood. But at the same time, you can still see plastics, metal, skin,” said graduate student John Hunt of Duke University, co-author of a description of the device published today in Science. “Dust and fog and rain, things that might be in the air are essentially invisible at these frequencies.”
The metamaterial aperture shuttles microwaves reflected from a scene to a computer, which then reconstructs the scene using mathematical algorithms the team developed. The whole process takes just 100 milliseconds and requires no moving parts and no image compression – meaning that the camera could capture moving scenes in near real time, and without losing details.
Traditional cameras rely on lenses that guide light to detectors comprising millions of pixels. Human eyes use a similarly organized system: a light-focusing lens, plus light- and color-detecting rods and cones arranged on the retina. Because optical wavelengths are short, a detector array can fit in the back of an eye or a tiny camera.
The camera Hunt helped design is different. The metamaterial aperature is only 40 centimeters long and it doesn’t move. It’s a circuit-board-like structure consisting of two copper plates separated by a piece of plastic. One of the plates is etched with repeating boxy structures, units about 2 millimeters long that permit different lengths of microwaves to pass through. Scanning the scene at various microwave frequencies allows the computer to capture all the information necessary to reproduce a scene.
To recreate a scene in three dimensions the team will need to build a two-dimensional aperture. That’s not far off, Hunt said. Then, the applications for such technology will be broad, he said, especially because the device is inexpensive to build, light, and portable.
“As a replacement to an airport scanner — you can just walk right past it,” Hunt said. “No more long lines.” And that’s just one idea. Adapting the system differently could yield a quicker baggage scanner. Embedding a microwave-detecting camera in the front of self-driving cars could help vehicles navigate scene-obscuring snow, rain, and fog. Embedding one in the wing of an airplane with one would eliminate the need for space-consuming radar imagers. Designing a hand-held, metal-detecting device could produce the ultimate stud-finder.
Lining the front of a police officer’s vest could help the officer detect concealed weapons — guns and knives — and distinguish them from cellphones.
“It will be exceptionally cheap,” Padilla said. “You can indeed simply adhere it to a wall and it can perform imaging.”
ABSTRACT – By leveraging metamaterials and compressive imaging, a low-profile aperture capable of microwave imaging without lenses, moving parts, or phase shifters is demonstrated. This designer aperture allows image compression to be performed on the physical hardware layer rather than in the postprocessing stage, thus averting the detector, storage, and transmission costs associated with full diffraction-limited sampling of a scene. A guided-wave metamaterial aperture is used to perform compressive image reconstruction at 10 frames per second of two-dimensional (range and angle) sparse still and video scenes at K-band (18 to 26 gigahertz) frequencies, using frequency diversity to avoid mechanical scanning. Image acquisition is accomplished with a 40:1 compression ratio.