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."
Reference:
Fast non-line-of-sight imaging with high-resolution and wide field of view
using synthetic wavelength holography, Nature Communications (2021).
DOI: 10.1038/s41467-021-26776-w
Tags:
Physics