Researchers at Duke University have devised a way to see through walls using a narrow band of microwave frequencies without any advance knowledge of what the walls are made of. Other than having obvious application for security, this approach could also lead to inexpensive devices to help construction workers easily locate conduits, pipes and wires.
"Most technologies that can see through walls use a broad range of frequencies, which makes them expensive," said Daniel Marks, associate research professor of electrical and computer engineering at Duke. "They also don't have a very good resolution. So while they might be fine for seeing a person moving on the other side of a wall, they're terrible for finding thin conduits or wires."
Current approaches typically rely on knowing what material a wall is made of, before attempting to see through it. This allows the software to predict how the wall will affect the scanning waves so that it can separate the echoes and distortions from the solid objects that are being pursued.
Marks and his colleagues, David R. Smith, a James B. Duke Professor of Electrical and Computer Engineering, and Okan Yurduseven, a postdoctoral researcher in electrical and computer engineering at Duke, take advantage of a wall’s symmetry instead.
Because walls are usually flat and uniform in all directions, they distort waves in a symmetrical fashion. The newly described technology uses this symmetry to its advantage.
"We wrote an algorithm that separates the data into parts -- one that shows circular symmetry and another that doesn't," explained Yurduseven. "The data that doesn't have any symmetry is what we're trying to see."
This technique uses a single frequency to scan because it cuts down on the number of interference patterns that are created by the wall, and single-frequency emitters are much less expensive than broadband emitters. Sticking to a narrow range means that a future device would be easier to pass the tests of the Federal Communications Commission (FCC), because it would be easy to avoid interfering with microwave frequencies dedicated to other technologies, like Wi-Fi, cell phone service and Bluetooth.
The researchers built a prototype device to see how well it would work. In the laboratory, they constructed a couple of different kinds of wall and then placed objects behind them that workers might want to find, like studs, electrical conduits, wires and junction boxes.
Looking at raw data after scanning through gypsum plasterboard, it is difficult to make out anything beyond just a metal junction box. But after analyzing the data and removing the symmetrical patterns, the pictures are considerably clearer.
"We envision combining this technique with a machine vision system that someone could move over a wall to see what's inside," said Marks. "We think the technology has the price point and sensitivity to make an impact on the market."
The paper on this research was published in Optica.