Using walls as mirrors, cameras can look around the corners
IAFTER CLOSING in a busy city, some terrorists are holding a hostage. The curtains are mostly drawn and cut any direct line of sight for those outside. In a building across the street, a team of engineers is given the task: they may have the necessary equipment, but they must paint as clear a picture as possible of what is happening in the room.
This was the challenge given to Daniele Faccio in 2015, then at Heriot-Watt University in Edinburgh, by Dstl, a British defense laboratory. Eventually, he and his team found a way to look around corners from 50 yards out – which at the time was considered impressive, although the system they devised could only detect the movement and position of hidden objects, rather than taking pictures of them. to take . Now, however, Xu Feihu and Pan Jianwei of the University of Science and Technology in China, in Hefei, have blown the record out of the water. As they are described in the Proceedings of the National Academy of Sciences, they managed to look a distance of more than a kilometer around the corners.
Imaging without a line of sight depends on two principles. One is that objects are visible to an observer when light reflected from them moves to the observer’s eyes or instruments. The other is that at least some light reflects off all the blackest, most absorbent surfaces. As a result, something hidden from an observer’s line of sight may still be visible when it’s near a wall that may serve as a reflective surface. In this case, the observer can illuminate the wall with a tightly focused beam of light (in practice probably a laser), knowing that some of the light beam will bounce off the wall to illuminate the hidden object, and that some of this illumination will be bounced back again from where it came via the wall. The fraction of the original bar returned by this trilogy of reflections may be small, and the information it contains seems hopelessly confusing. But sufficiently clever math can turn it into an image of the thing that reflects it.
Mirror, mirror, you are the wall
Dr. Xu and Dr. Pan conducted their trial at night to reduce the amount of backlight that could affect the results. Their targets, a human dummy on one experimental flight and a giant ‘H’ on another, were hidden behind an apartment block in an apartment block in Shanghai. Their laser and receiving device was in a second apartment block that was 1.43 km away. The receiving device, an instrument called a single-photon avalanche diode (SPAD), was, as its name suggests, so sensitive that it could detect and count individual photons, of which the particles consist. It was just as good, because of every seven million billion photons shot through the gap by the laser, only one returned.
Because each shot of the laser yielded so little information, the researchers had to take many shots to build an image. For the purpose, they proposed a grid on the target wall, 64 points wide and 64 deep. They fired the laser in turn at each point and then the data from the SPAD in an algorithm that can reconstruct an image of the hidden object, even if it is fuzzly (see photo).
The military applications of this technology point to themselves. After all, that was why Dstl dr. Faccio sponsored in the first place. But others are also interested. America’s space agency, NASA, has in the past paid for such work in the hope of placing a laser on a satellite orbiting a distant world. This can allow the photography of the otherwise invisible inside of caves on the surfaces of moons and planets. And in a more practical way, engineers in the autonomous vehicle industry would like to have a technology that shows their cars how other motorists are driving fast around the blind spots.
For now, such applications remain far into the future. Capturing the experimental data in Shanghai took several hours, which is of little use on the road or in fast-moving situations such as hostage-taking. The amount of light lost between bounces also places a limit on how far an object can be from the reflective wall before the technique is no longer usable. The dummy used by Dr. Xu and Dr. Pan was 75 cm from the wall, which is probably near the limit. Apart from this reservation, performing the trick over a distance of almost 1½ km is a staggering progress on previous attempts. It would not be surprising, says Dr Faccio, now that it is known what is possible, if this record were to be broken as well.■
This article appears in the Science and Technology section of the print edition under the heading “Round the bend”