MIT, USA. Ultra-fast camera can create images of hidden objects using scattered laser light.

A new imaging system developed at MIT’s Media Lab could use opaque walls, doors or floors as ‘mirrors’ to gather information about scenes outside its line of sight. In doing so, the camera is able to see objects hidden behind walls. Such a camera could be invaluable in dangerous or inaccessible locations, such as inside machinery with moving parts, or in highly contaminated areas.

The camera fires a pulse of laser light at a wall on the far side of the hidden scene, and records the time at which the scattered light reaches back to the camera. Photons bounce off the wall onto the hidden object and back to the wall, scattering each time, before a small fraction eventually reaches the camera, each at a slightly different time. It’s this time resolution that provides the key to revealing the hidden geometry. The position of the 50-femtosecond (that’s 50 quadrillionths of a second) laser pulse is also changed 60 times, to gain multiple perspectives on the hidden scene.

A normal camera can only see objects that are right in front of it. Light that reaches the sensor from beyond the direct line of sight is too diffuse to convey useful information about the hidden scene, having been scattered by multiple reflections. The new set-up overcomes this problem by capturing ultra-fast time-of-flight information — that is, how long each photon has taken to reach the camera. This information is then decoded by a reconstruction algorithm conceived by team member Andreas Velten.

Ramesh Raskar, an associate professor at the MIT Media Lab, led the new research. Raskar says, his group has discovered that the problem of peering around a corner has a great deal in common with that of using multiple antennas to determine the direction of incoming radio signals. Going forward, Raskar hopes to use that insight to improve the quality of the images the system produces and to enable it to handle visual scenes with a lot more clutter.