Add image compression to the list of nifty applications for metamaterials. Metamaterials guide light waves to create “invisibility cloaks” and bend sound waves to make theoretical noise reduction systems for urban areas. But these materials are tuned to particular wavelengths; some invisibility cloaks don’t work at all visible wavelengths because they leak those wavelengths of light. Now researchers have capitalized on that leakiness to build a new functional device: a microwave imaging system that compresses an image as it's being collected—not afterward as our digital cameras do.

Every pixel in a picture from our digital cameras corresponds to a pixel of information recorded on the detector inside the camera. Once a camera collects all the light intensity information from a scene, it promptly discards some of it and compresses the data into a JPEG file (unless you explicitly tell it to save raw data). You still end up with a decent picture, though, because most of the discarded data was redundant.

Compressive sensing aims to ease this process by reducing the amount of data collected in the first place. One way to do this is with a single pixel camera, developed in 2006. These devices capture information from random patterns of pixels around the image, essentially adding the light intensity values of several pixels together. If you know something about the structure of that image—say clusters of bright stars set against a dark sky—you’ll be able to capture that image with fewer measurements than a traditional camera.

Later, an algorithm takes the set of measurements you’ve collected and combines it with its prior knowledge about the scene. The math works out so that the computer produces the exact image of the scene from the huge set of possible reconstructed images based on the small set of measurements.

Now John Hunt and his colleagues at Duke University have built a new microwave compressive imaging system that uses a material, rather than lenses or mirrors, to create patterns of radiation that speckle a scene. With this new system, the researchers can gather 400 pixels of data with only 10 measurements (a 40:1 compression ratio).

Here’s how it works: microwaves travel through a thin strip of metamaterial containing precisely engineered coils of metal spaced throughout plastic. The radiation leaks out of the material at particular coils along the 40-cm path, and the waves interfere with each other. This resulting pattern of light travels to an object, bounces off its surface and returns to a detector near the original metamaterial aperture. The detector identifies something in the scene by combining the intensity of the scattered beams with the wave pattern leaving the aperture—even though this only captures a small fraction of the light that entered the metamaterial.

The researchers need to collect different images of that object by sending different frequencies of radiation, between 18-26 GHz, through the metamaterial. These frequencies leak out at different spots along the metamaterial waveguide, creating a new pattern of radiation that hits the sensor. Using a set of these measurements, an algorithm reconstructs an image of the scene, showing the viewing angle and the distance between the object and the camera—all within 100 milliseconds. That’s fast enough to record a video of a moving object. Current detectors for microwave or millimeter imaging are expensive, so systems often use fewer or smaller detectors. Developing a system that can compress images as they’re being collected is a way to reduce imaging cost without sacrificing image quality.

This new metamaterial compressive imaging system has no moving parts or lenses. It’s also thin. And the imaging system’s speed means that perhaps one day you’ll just walk through the millimeter wave scanners at the airport, rather than stopping for the detector to move around your body, Hunt says.

He also imagines building a metamaterial imaging system into the body panels of a car to create a collision avoidance system that can see through dust and fog.

Willie Padilla of Boston College thinks this work will usher in a new era of compressive sensing. Without lenses, this metamaterial device reduces much of the complexity in conventional imaging systems, so it could be a way to get state-of-the-art imaging at a drastically reduced cost, he says.

Science, 2013. DOI: 10.1126/science.1230054 (About DOIs).