Getting a clearer view around the corner

Researchers from the US and Spain say they have drawn on the lessons of classical optics to show that it is possible to see effectively around barriers using a “virtual camera”.

Writing in the journal Nature, they describe technology and an approach that allow hidden scenes to be imaged despite the challenges that have limited many previous attempts at non-line-of-sight imaging to “fuzzy pictures of simple scenes”.

The work, by a team from the University of Wisconsin-Madison (UW), US, and Universidad de Zaragoza, Spain, has been funded largely by the US Defence Department and NASA.

The idea of non-line-of-sight imaging, says senior author Andreas Velten, from UW, uses indirect, reflected light to capture images of a hidden scene. 

Photons from thousands of pulses of laser light are reflected off a wall or another surface to an obscured scene, then the reflected, diffused light bounces back to sensors connected to a camera. The recaptured light particles or photons are then used to digitally reconstruct the hidden scene in three dimensions.

A great deal of recent research, Velten says, has focused on improving the quality of scene regeneration under controlled conditions using small scenes with single objects. 

His team has looked at it through a more conventional prism by applying the same math used to interpret images taken with conventional line-of-sight imaging systems. The new method, they say, surmounts the use of a single reconstruction algorithm and describes a new class of imaging algorithms that share unique advantages.

Conventional systems interpret diffracted light as waves, which can be shaped into images by applying well known mathematical transformations to the light waves propagating through the imaging system.

In the case of non-line-of-sight imaging, the challenge of imaging a hidden scene, says Velten, is resolved by reformulating the non-line-of-sight imaging problem as a wave diffraction problem and then using well-known mathematical transforms from other imaging systems to interpret the waves and reconstruct an image of a hidden scene. 

By doing this, the new method turns any diffuse wall into a virtual camera.

“What we did was express the problem using waves,” he says. “The systems have the same underlying math, but we found that our reconstruction is surprisingly robust, even using really bad data. You can do it with fewer photons.”

Velten’s team showed that hidden scenes could be imaged despite the challenges of scene complexity, differences in reflector materials, scattered ambient light and varying depths of field for the objects that make up a scene.

And they believe the ability to essentially project a camera from one surface to another suggests the technology could be developed to a point where it is possible to see around multiple corners.