Device can see around corners and through scattering media like fog and human tissue
From:
Northwestern University
November 17, 2021 -- Northwestern
University researchers have invented a new high-resolution camera that can see
the unseen -- including around corners and through scattering media, such as
skin, fog or potentially even the human skull.
Called synthetic wavelength holography,
the new method works by indirectly scattering coherent light onto hidden
objects, which then scatters again and travels back to a camera. From there, an
algorithm reconstructs the scattered light signal to reveal the hidden objects.
Due to its high temporal resolution, the method also has potential to image
fast-moving objects, such as the beating heart through the chest or speeding
cars around a street corner.
The study will be published on Nov. 17
in the journal Nature Communications.
The relatively new research field of
imaging objects behind occlusions or scattering media is called
non-line-of-sight (NLoS) imaging. Compared to related NLoS imaging
technologies, the Northwestern method can rapidly capture full-field images of
large areas with submillimeter precision. With this level of resolution, the
computational camera could potentially image through the skin to see even the
tiniest capillaries at work.
While the method has obvious potential
for noninvasive medical imaging, early-warning navigation systems for
automobiles and industrial inspection in tightly confined spaces, the
researchers believe potential applications are endless.
"Our technology will usher in a new
wave of imaging capabilities," said Northwestern's Florian Willomitzer,
first author of the study. "Our current sensor prototypes use visible or
infrared light, but the principle is universal and could be extended to other
wavelengths. For example, the same method could be applied to radio waves for
space exploration or underwater acoustic imaging. It can be applied to many areas,
and we have only scratched the surface."
Willomitzer is a research assistant
professor of electrical and computer engineering at Northwestern's McCormick
School of Engineering. Northwestern co-authors include Oliver Cossairt,
associate professor of computer science and electrical and computer
engineering, and former Ph.D. student Fengqiang Li. The Northwestern
researchers collaborated closely with Prasanna Rangarajan, Muralidhar Balaji
and Marc Christensen, all researchers at Southern Methodist University.
Intercepting scattered light
Seeing around a corner versus imaging an
organ inside the human body might seem like very different challenges, but
Willomitzer said they are actually closely related. Both deal with scattering
media, in which light hits an object and scatters in a manner that a direct
image of the object can no longer be seen.
"If you have ever tried to shine a
flashlight through your hand, then you have experienced this phenomenon,"
Willomitzer said. "You see a bright spot on the other side of your hand,
but, theoretically, there should be a shadow cast by your bones, revealing the
bones' structure. Instead, the light that passes the bones gets scattered
within the tissue in all directions, completely blurring out the shadow image."
The goal, then, is to intercept the
scattered light in order to reconstruct the inherent information about its time
of travel to reveal the hidden object. But that presents its own challenge.
"Nothing is faster than the speed
of light, so if you want to measure light's time of travel with high precision,
then you need extremely fast detectors," Willomitzer said. "Such
detectors can be terribly expensive."
Tailored waves
To eliminate the need for fast
detectors, Willomitzer and his colleagues merged light waves from two lasers in
order to generate a synthetic light wave that can be specifically tailored to
holographic imaging in different scattering scenarios.
"If you can capture the entire
light field of an object in a hologram, then you can reconstruct the object's
three-dimensional shape in its entirety," Willomitzer explained. "We
do this holographic imaging around a corner or through scatterers -- with
synthetic waves instead of normal light waves."
Over the years, there have been many
NLoS imaging attempts to recover images of hidden objects. But these methods
typically have one or more problems. They either have low resolution, an
extremely small angular field of regard, require a time-consuming raster scan
or need large probing areas to measure the scattered light signal.
The new technology, however, overcomes
these issues and is the first method for imaging around corners and through
scattering media that combines high spatial resolution, high temporal
resolution, a small probing area and a large angular field of view. This means
that the camera can image tiny features in tightly confined spaces as well as
hidden objects in large areas with high resolution -- even when the objects are
moving.
Turning 'walls into mirrors'
Because light only travels on straight
paths, an opaque barrier (such as a wall, shrub or automobile) must be present
in order for the new device to see around corners. The light is emitted from
the sensor unit (which could be mounted on top of a car), bounces off the
barrier, then hits the object around the corner. The light then bounces back to
the barrier and ultimately back into the detector of the sensor unit.
"It's like we can plant a virtual
computational camera on every remote surface to see the world from the
surface's perspective," Willomitzer said.
For people driving roads curving through
a mountain pass or snaking through a rural forest, this method could prevent
accidents by revealing other cars or deer just out of sight around the bend.
"This technique turns walls into mirrors," Willomitzer said. "It
gets better as the technique also can work at night and in foggy weather
conditions."
In this manner, the high-resolution
technology also could replace (or supplement) endoscopes for medical and
industrial imaging. Instead of needing a flexible camera, capable of turning
corners and twisting through tight spaces -- for a colonoscopy, for example --
synthetic wavelength holography could use light to see around the many folds
inside the intestines.
Similarly, synthetic wavelength
holography could image inside industrial equipment while it is still running --
a feat that is impossible for current endoscopes.
"If you have a running turbine and
want to inspect defects inside, you would typically use an endoscope,"
Willomitzer said. "But some defects only show up when the device is in
motion. You cannot use an endoscope and look inside the turbine from the front
while it is running. Our sensor can look inside a running turbine to detect
structures that are smaller than one millimeter."
Although the technology is currently a
prototype, Willomitzer believes it will eventually be used to help drivers
avoid accidents. "It's still a long way to go before we see these kinds of
imagers built in cars or approved for medical applications," he said.
"Maybe 10 years or even more, but it will come."
https://www.sciencedaily.com/releases/2021/11/211117100106.htm
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