IAFTER LOCKED room in a busy city, some terrorists are holding hostage. The curtains are usually drawn, leaving no direct line of sight to outsiders. In a building across the street, a team of engineers is assigned a task: they can get all the equipment they need, but they need to paint as clear a picture as possible of what’s going on in the room.
This was the challenge presented to Daniele Faccio, then at Heriot-Watt University, Edinburgh, in 2015 by Dstl, a UK government defense laboratory. He and his team eventually found a way to see around corners from a distance of 50 meters – which was considered impressive at the time, even though the system they came up with could only detect the movement and position of hidden objects, rather than take pictures of them. to make. . But now Xu Feihu and Pan Jianwei of China’s University of Science and Technology in Hefei have blown that record out of the water. As they describe in the Proceedings of the National Academy of Sciencesthey have managed to see around the corner from a distance of more than a kilometer.
This kind of visionless imaging is based on two principles. One is that objects are visible to an observer when light reflected from them makes its way to that observer’s eyes or instruments. The other is that at least some light bounces off all but the blackest, most absorbent surfaces. As a result, something that is hidden from an observer’s field of view can still be visible if it is close enough to a wall that can serve as a reflective surface. In this case, the observer can illuminate the wall with a tightly focused beam of light (probably a laser in practice), knowing that some of the light will bounce off the wall’s beam to illuminate the hidden object, and that some of the this lighting in turn is reflected from where it came through the wall. The fraction of the original ray returned by this trilogy of reflections can be miniscule and the information contained within it can seem hopelessly jumbled. But enough clever math can make it a picture of the thing it reflects.
Mirror, mirror, you are the wall
Dr. Xu and Dr. Pan ran their trial at night to minimize the amount of background light that could have interfered with the results. Their targets, a human dummy on an experimental run and a giant ‘H’ on another, were hidden behind a barrier in an apartment in a Shanghai apartment building. Their laser and receiver equipment was located in a second apartment building 1.43 km away. The receiving device, an instrument called a single-photon avalanche diode (SPAD), as its name suggests, was so sensitive that it could detect and count individual photons, the particles that make up beams of light. This was just as good, because of every seven million billion photons fired across the aperture by the laser, only one returned.
Because each firing of the laser yielded so little information, the researchers had to take many shots to build an image. To this end, they envisioned a grid on the target wall, 64 points wide and 64 deep. They fired the laser at each dot in turn and then fed the data from the SPAD in an algorithm capable of reconstructing, albeit vague, an image of the hidden object (see image).
The military applications of this technology are emerging. After all, they were the reason why Dstl was Dr. Faccio sponsored. But others are also interested. America’s space agency, NASA, has paid for such work in the past in hopes of placing a laser on a satellite orbiting a distant world. This would make it possible to photograph the otherwise invisible interiors of caves on the surfaces of moons and planets. And, in a more practical sense, engineers in the autonomous auto industry would be keen on a technology that allows their cars to see other drivers happily racing through blind corners.
For now, such applications remain well into the future. Capturing the experimental data in Shanghai took several hours, which is of little use, both on the road and in fast-moving situations such as hostage-taking. The amount of light lost between bounces also limits how far an object can be from the reflective wall before the technique is no longer useful. The doll used by Dr. Xu and Dr. Pan was three inches from the wall, which is probably close to that limit. These caveats aside, performing the trick over a distance of nearly 1½ km is staggering progress over previous efforts. It would not be surprising, says Dr. Faccio, now that it is known what is possible if that record were also broken.
This article appeared in the Science and Technology section of the print edition under the heading ‘Round the bend’