Intriguing Solution for Around-the-Corner Imaging
The U.S. Defense Department’s Advanced Research Projects Agency (DARPA) and NASA funded new research that will enable non-line-of-sight imaging. Drawing on the lessons of classical optics, scientists from the University of Wisconsin-Madison and the Universidad de Zaragoza in Spain have shown that it is possible to image complex hidden scenes using a projected “virtual camera” to see around barriers. Once perfected, it could be used in a wide range of applications, from defense and disaster relief to manufacturing and medical imaging. NASA envisions the technology as a potential way to peer inside hidden caves on the moon and Mars.
The research published in Nature offers a solution enabling imaging complex hidden scenes, seeing around multiple corners and taking video. The researchers believe that the technology can be made to be both inexpensive and compact, meaning real-world applications are just a matter of time.
“This non-line-of sight imaging has been around for a while,” says Andreas Velten, a professor of biostatistics and medical informatics in the UW School of Medicine and Public Health and the senior author.
The basic idea of non-line of-sight imaging, Velten says, revolves around the use of indirect, reflected light, a light echo of sorts, 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 and 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.
“We send light pulses to a surface and see the light coming back, and from that we can see what’s in the hidden scene,” Velten explains.
According to phsy.org, the new research addresses the primary limitations to existing non-line-of-sight imaging technology, including varying material qualities of the walls and surfaces of the hidden objects, large variations in brightness of different hidden objects, complex inter-reflection of light between objects in a hidden scene, and the massive amounts of noisy data used to reconstruct larger scenes.
The team applied the same math used to interpret images taken with conventional line-of-sight imaging systems. The new method surmounts the use of a single reconstruction algorithm and describes a new class of imaging algorithms that share unique advantages.
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.
According to Velten, the technique can be applied to create virtual projected versions of any imaging system, even video cameras that capture the propagation of light through the hidden scene. Velten’s team, in fact, used the technique to create a video of light transport in the hidden scene, enabling visualization of light bouncing up to four or five times, which can be the basis for cameras to see around more than one corner.
The technology could be further and more dramatically improved if arrays of sensors can be devised to capture the light reflected from a hidden scene. The experiments described in the new Nature paper depended on just a single detector.
In medicine, the technology holds promise for things like robotic surgery. Now, the surgeon’s field of view is restricted when doing sensitive procedures on the eye, for example, and the technique developed by Velten’s team could provide a more complete picture of what’s going on around a procedure.
