New holographic camera sees the unseen with high precision
Device can see around corners and through scattering media like fog and
human tissue

Date:
November 17, 2021
Source:
Northwestern University
Summary:
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.



FULL STORY ========================================================================== 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 journalNature
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." ========================================================================== Story Source: Materials provided by Northwestern_University. Original
written by Amanda Morris. Note: Content may be edited for style and
length.


========================================================================== Journal Reference:
1. Florian Willomitzer, Prasanna V. Rangarajan, Fengqiang Li,
Muralidhar M.

Balaji, Marc P. Christensen, Oliver Cossairt. Fast non-line-of-sight
imaging with high-resolution and wide field of view using synthetic
wavelength holography. Nature Communications, 2021; 12 (1) DOI:
10.1038/ s41467-021-26776-w ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/11/211117100106.htm

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