New way to see through walls of unknown materials

Scientists have devised a way to see through walls without any advance knowledge of the material they are made of.
Besides having applications in the realm of security, the approach could 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 at Duke University in the US.
"They also do not have very good resolution. So while they might be fine for seeing a person moving on the other side of a wall, they are terrible for finding thin conduits or wires," said Marks.

Current approaches also typically rely on knowing what material the wall is made out of before trying 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 being sought.

Since walls are generally flat and uniform in all directions, they distort waves in a symmetrical fashion.
The new technology described in the journal Optica uses this symmetry to its advantage.
"We wrote an algorithm that separates the data into parts - one that shows circular symmetry and another that does not," said Okan Yurduseven, a postdoctoral researcher at Duke.

"The data that does not have any symmetry is what we are trying to see," said Yurduseven.
The technique uses only a single frequency to scan because it cuts down on the number of interference patterns created by the wall and single-frequency emitters are much less expensive than broadband emitters.
Sticking to a narrow range also means that a future device would be easy to avoid interfering with microwave frequencies dedicated to other technologies, such as Wifi, cellular phone service and Bluetooth, researchers said.
They built a prototype device to see how well it would work.
In their laboratory, the researchers constructed a couple of different kinds of walls and then placed objects behind them that a worker might want to find, like studs, electrical conduits, wires and junction boxes.

Looking at the raw data after scanning through gypsum plasterboard, it is difficult to make out anything other than a metal junction box, which is four inches wide and two inches thick.
However, after analysing the data and removing the symmetrical patterns, the pictures clear considerably, and each individual component is easily recognised.
"We envision combining this technique with a machine vision system that someone could move over a wall to see what is inside," said Marks.
"We think the technology has the price point and sensitivity to make an impact on the market," he said.