A new laser camera that can see around corners has been invented by MIT researchers. Objects can be seen in 3D using the
camera, which fires ultrafast laser pulses at walls ‘behind’ the area that can’t be seen, to capture a ghostly 3D reflection.
The camera ‘bounces’ ultrafast laser pulses off a wall behind a hidden subject – and computers inside ‘rebuild’ the information into a 3D image of what’s hidden. The technique is similar to using a mirror to see round a corner – but instead of a mirror, the ‘reflection’ is reconstructed from laser light that scatters back off a wall.
The camera ‘times’ the beams of light as they bounce back to its sensors, and builds an image, which is slightly wobbly, but precise to ranges of just one centimetre. MIT researchers Ramesh Raskar and Andreas Velten got around this issue using a laser, a beam-splitter and a sophisticated algorithm – the incredibly rapid laser bursts allow them to time the light as it ‘returns’ to the camera, building a 3D model.
They fired a laser through the beam-splitter and at a wall, with pulses occurring every 50 femtoseconds. (A femtosecond is a millionth of a billionth of a second, or the time it takes light to travel about 300 nanometers).
When the laser light hits the splitter, half of it travels to the wall, and then bounces to the object around the corner. The light reflects off the object, hitting the wall again, and then returns to a camera.
The other half of the beam just goes directly to the camera. This half-beam serves as a reference, to help measure the time it takes for the other photons (particles of light) to return to the camera.
The system performs the procedure several times, bouncing light off several different spots on the wall, so that it enters the room at several different angles.
The detector, too, measures the returning light at different angles. By comparing the times at which returning light strikes different parts of the detector, the system can piece together a picture of the room.
Using a special algorithm to analyze when the returning photons arrive and checking them against the reference beam, the scientists were able to reconstruct an image of the object they were trying to see.
Velten noted that when analyzing the photons, the ones that hit an object in a room will return sooner than the ones that bounce off a rear wall, and the algorithm accounts for that. They could even see three-dimensional objects, such as a mannequin of a running man used in the experiment.
The resolution isn’t, of course, anywhere near as good as a human eye. It can pick up centimeter-size details at a distance of a few meters, so it can only resolve relatively large objects. Raskar noted that a shorter exposure time could boost resolution.
The camera is currently using exposures measured in picoseconds. But even so, it is a useful method for detecting things that for whatever reason are not directly in the line of sight.
Raskar and Velten are no strangers to playing with photons. In December 2011, they demonstrated a camera that could capture frames a trillion times every second.
Raska hopes that a future version of the system could be used by emergency responders – firefighters looking for people in burning buildings or police determining whether rooms are safe to enter – or by vehicle navigation systems, which could bounce light off the ground to look around blind corners.