Researchers at Northwestern University have invented a new high-resolution camera that can see the invisible – even around corners and through propagating devices such as skin, fog or possibly even the human skull.
A new method called synthetic wavelength holography works indirectly by scattering coherent light onto hidden objects, which then re-diffuse and travel back to the camera. From there, the algorithm reconstructs the scattered light signal to reveal the hidden objects. Thanks to its high time resolution, the method can also be used to photograph fast-moving subjects, such as a heart pounding in the chest or cars climbing around the corner.
The study will be published in the journal on November 17 Nature communication.
A relatively new field of research involving the imaging of objects behind occlusions or scattering materials is called non-line-of-sight (NLoS) imaging. Compared to similar NLoS imaging techniques, the Northwestern method can quickly capture images of large areas over a large area with submillimeter accuracy. At this level of resolution, a computational camera could possibly shoot through the skin to see even the smallest capillaries in action.
While the method has obvious potential for non-invasive medical imaging, automotive early warning navigation systems, and industrial inspections in tightly enclosed spaces, the researchers believe the potential applications are endless.
“Our technology brings a new wave of imaging capabilities,” said Florian Willomitzer, Northwestern’s first author of the study. “Current sensor prototypes use visible or infrared light, but the principle is universal and could be extended to other wavelengths. The same method could be applied to radio waves in space exploration or underwater acoustic imaging, for example. It can be applied to many areas and we’re just scratching the surface.”
Willomitzer is an assistant professor of electrical and computer engineering at the McCormick School of Engineering in Northwestern. Northwestern co-authors include Oliver Cossairt, Assistant Professor of Computer Science and Electrical and Computer Engineering, and former Ph.D. student Fengqiang Li. Northwestern researchers worked closely with Prasanna Rangarajan, Muralidhar Balaji and Marc Christensen, all of whom are researchers at Southern Methodist University.
Captures stray light
Seeing around the corner compared to describing an organ inside the human body may seem like very different challenges, but Willomitzer said they are closely related. Both deal with scattering media, where light hits an object and scatters so that a live image of the object can no longer be seen.
“If you’ve ever tried to light a flashlight through your hand, you’ve experienced this phenomenon,” Willomitzer said. “You see a bright spot on the other side of your hand, but in theory there should be a shadow on the side of your bone that reveals the structure of the bones. Instead, the light passing through the bones dissipates in all directions in the tissue., Blurring the shadow completely.”
The goal is thus to capture scattered light in order to reconstruct the inherent information about its travel time to reveal a hidden object. But it brings its own challenge.
“Nothing is faster than the speed of light, so if you want to measure the time it takes for light to be very accurate, you need very fast detectors,” Willomitzer said. “Such detectors can be terribly expensive.”
To eliminate the need for high-speed detectors, Willomitzer and colleagues combined light waves from two lasers to create a synthetic light wave that can be specifically tailored for holographic imaging in different scattering scenarios.
“If you can capture the entire light field of an object in a hologram, you can reconstruct the three-dimensional shape of the object in its entirety,” Willomitzer explained. “We’re doing this holographic imaging around the corner or through scatterers – with synthetic waves instead of ordinary light waves.”
Over the years, NLoS imaging has attempted to retrieve images of hidden objects. But there are usually one or more problems with these methods. They either have a low resolution, a very low perspective, require a time-consuming raster scan, or require large measurement ranges to measure the stray light signal.
However, the new technology solves these problems and is the first method of imaging around corners and through scattering media that combines high spatial resolution, high temporal resolution, small measurement range, and large angular field of view. This means the camera can capture small features in tight spaces as well as hidden objects in large areas at high resolution – even when subjects are moving.
“turning walls into mirrors”
Because the light travels only along straight paths, an opaque obstacle (such as a wall, bush, or car) must be present for the new device to see around the corners. The light comes out of the sensor unit (which can be mounted on top of the car), bounces off an obstacle and then hits an object behind the corner. The light then bounces back into the obstacle and eventually back into the sensor unit sensor.
“It’s like we could plant a virtual computing camera on every remote surface to see the world from a surface perspective,” Willomitzer said.
For people who drive through winding roads through a mountain pass or meander through a rural forest, this method can prevent accidents by exposing other cars or deer just out of sight behind the bend. “This technology turns walls into mirrors,” Willomitzer said. “It gets better when the technology can work even at night and in foggy weather.”
In this way, high-resolution technology could also replace (or complement) endoscopes in medical and industrial imaging. Instead of needing a flexible camera that can turn corners and rotate in tight spaces – for colonoscopy, for example – synthetic wavelength holography could use light to see the many folds inside the gut.
Similarly, synthetic wavelength holography could be described inside industrial devices while they are still running – an achievement that is impossible for current endoscopes.
“If you have a running turbine and want to check for faults inside, you usually use an endoscope,” Willomitzer said. “But some faults only show up when the device is in motion. You can’t use the endoscope and look inside the turbine while it’s running. Our sensors can look inside the running turbine to detect structures less than a millimeter in size.”
Although the technology is currently a prototype, Willomitzer believes it will eventually be used to help drivers avoid accidents. “There’s still a long way to go before we see such imaging devices built into cars or approved for medical applications,” he said. “Maybe 10 years or even more, but it will come.”
Non-visual imaging with picosecond time resolution
High-speed non-visual imaging with high resolution and wide field of view using synthetic wavelength holography, Nature communication (2021). DOI: 10.1038 / s41467-021-26776-w
Provided by Northwestern University
Quotation: New holographic camera sees invisible with high resolution (2021, November 17), retrieved November 17, 2021 at https://phys.org/news/2021-11-holographic-camera-unseen-high-precision.html
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