Light does not "bend", so we cannot see objects hidden around corners or behind walls. But scientists at the Massachusetts Institute of Technology in Cambridge have found a way to "see" hidden objects by exploiting the "echoes of light", in a similar way to sound echoes. They fired a pulse of laser light at a wall, and record the time at which the scattered light reaches a camera. Photons bounce off the wall onto the hidden object and back to the wall, scattering each time, and small fractions eventually reaches the camera, each at a slightly different time. Then they captured ultra-fast time-of-flight information (how long each photon takes to reach the camera). The camera is truly fast! It can record images every 2 picoseconds, the time it takes light to travel just 0.6 mm. This time resolution provides the key to revealing the hidden geometry. The technology is completed by a computer that compares images generated from different laser positions, and calculates the likely positions of the hidden object, in a similar way to the computational tomography that is used in CAT-scans. The technology is still in its infancy, because it takes several minutes to reveal the object, but in the future, it will be possible to reduce the time to <10 seconds.
Ultra-fast camera can create images of hidden objects using scattered laser light.
The ability to see objects hidden behind walls could be invaluable in dangerous or inaccessible locations, such as inside machinery with moving parts, or in highly contaminated areas. Now scientists at the Massachusetts Institute of Technology in Cambridge have found a way to do just that.
They fire a pulse of laser light at a wall on the far side of the hidden scene, and record the time at which the scattered light reaches a 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.
"We are all familiar with sound echoes, but we can also exploit echoes of light," says Ramesh Raskar, head of the Camera Culture Research Group at the MIT Media Lab which carried out the study.
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, described today in Nature Communications1, 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.
Read the resthere.
This is fantastic!
Less than a picosecond is a very very short time. It is amazing what an increase of resolution can bring into the picture. And this is basically true of all sciences.
In this case, research opens up all kinds of applications and I'd expect the Pentagon to be very interested in this.