Editor’s Note: This article was originally published in 2012, highlighting early experimental research from MIT that allowed cameras to reconstruct 3D images around corners using scattered laser light.
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 cannot be seen, to capture a ghostly 3D reflection.
Bouncing Light to See the Invisible
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 is hidden. The technique is similar to using a mirror to see around a corner, but instead of a mirror, the reflection is reconstructed from laser light that scatters back off a solid 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.
The Power of Femtoseconds
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 full 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, which is the time it takes light to travel about 300 nanometres.
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, hits the wall again, and then returns to the camera.
Timing the Photons
The other half of the beam goes directly to the camera. This half-beam serves as a reference to help measure the exact 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 also 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 an accurate picture of the hidden room.
Using a special algorithm to analyse 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.
Reconstructing 3D Objects
Velten noted that when analysing 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 easily accounts for that. They could even see three-dimensional objects, such as a mannequin of a running man used in the experiment.
The resolution is not anywhere near as good as a human eye. It can pick up centimetre-sized details at a distance of a few metres, so it can only resolve relatively large objects. Raskar noted that a shorter exposure time could drastically boost resolution.
The camera is currently using exposures measured in picoseconds. But even so, it is a highly useful method for detecting things that, for whatever reason, are not directly in the line of sight.
Future Applications
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.
Raskar hopes that a future version of the system could be used by emergency responders, such as firefighters looking for people in burning buildings or police determining whether rooms are safe to enter. It could also be utilised by vehicle navigation systems, which could bounce light off the ground to look around blind corners.
