Science Gazette

With incredible precision, a new holographic camera perceives the unseen

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The device can see around corners and through mediums that scatter light, such as fog and human tissue.

Researchers at Northwestern University have developed a new high-resolution camera that can see around corners and through scattering substances such as skin, fog, or even the human skull.

The new technology, known as synthetic wavelength holography, works by scattering coherent light onto concealed objects, which then scatters again and returns to a camera. The dispersed light signal is then reconstructed using an algorithm to reveal the concealed items. The technology may also picture fast-moving objects, such as the beating heart through the chest or rapid autos at a street corner, due to its high temporal resolution.

The work will be published in the journal Nature Communications on November 17th.

Non-line-of-sight (NLoS) imaging is a relatively recent study subject that involves imaging objects behind occlusions or scattering medium. The Northwestern approach, as compared to other NLoS imaging methods, can acquire full-field pictures of enormous regions quickly and with submillimeter accuracy. The computational camera may theoretically picture through the skin at this resolution to observe even the finest capillaries at action.

While noninvasive medical imaging, early-warning navigation systems for autos, and industrial inspection in restricted places are obvious applications, the researchers think the method’s uses are limitless.

“Our technique will bring in a new generation of imaging capabilities,” said Florian Willomitzer of Northwestern University, the study’s lead author. “Although our present sensor prototypes employ visible or infrared light, the idea is general and may be applied to other wavelengths as well. The same technology might be used to image radio waves for space exploration or underwater acoustic imaging, for example. It can be used in a variety of situations, and we’ve just scratched the surface.”

Willomitzer is an electrical and computer engineering research assistant professor at Northwestern University’s McCormick School of Engineering. Oliver Cossairt, an associate professor of computer science and electrical and computer engineering at Northwestern, and Fengqiang Li, a former Ph.D. student, are among the co-authors. The Northwestern researchers worked along with Southern Methodist University scholars Prasanna Rangarajan, Muralidhar Balaji, and Marc Christensen.

Detection of dispersed light

Although seeing around a corner and visualizing an organ within the human body may seem to be completely distinct tasks, Willomitzer said that they are really extremely similar. Both are concerned with scattering media, which occur when light strikes an item and scatters in such a way that a direct picture of the object is no longer visible.

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“You’ve experienced this phenomena if you’ve ever attempted to beam a flashlight through your hand,” Willomitzer said. “You see a brilliant light on the opposite side of your hand, but your bones should throw a shadow, showing the structure of your bones. Instead, the light that travels through the bones is dispersed in all directions inside the tissue, totally obscuring the shadow picture.”

The aim is to catch the dispersed light and reconstruct the intrinsic information about its transit time in order to disclose the concealed item. However, this poses its own set of problems.

“Nothing moves faster than light,” Willomitzer said, “therefore if you want to measure light’s duration of passage with great accuracy, you’ll need extraordinarily fast detectors.” “Such detectors may be quite costly.”

Waves that are made to order

Willomitzer and his colleagues combined light waves from two lasers to create a synthetic light wave that can be adjusted to holographic imaging in various scattering circumstances, eliminating the requirement for fast detectors.

“You may rebuild the item’s three-dimensional form in its entirety if you can capture the complete light field of an object in a hologram,” Willomitzer added. “We use synthetic waves instead of typical light waves to achieve this holographic imaging around a corner or via scatterers.”

Many NLoS imaging efforts to recover pictures of concealed objects have been made throughout the years. However, these approaches often have one or more flaws. They have limited resolution, a narrow angular field of view, a time-consuming raster scan, or huge probing regions to measure the dispersed light signal.

The new technique, on the other hand, overcomes these limitations and is the first approach for imaging around corners and through scattering substances to combine high spatial resolution, high temporal resolution, a tiny probing area, and a broad angular field of view. This implies the camera can capture little features in small regions as well as concealed items in broad areas with high resolution, even if the objects are moving.

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Making’mirrors out of walls’

Because light can only travel in straight lines, the new gadget will need an opaque obstacle (such as a wall, bush, or car) to view around curves. The light is produced by the sensor unit (which may be positioned on the roof of a vehicle), bounces off the barrier, and then strikes the item around the corner. The light then bounces back to the barrier, eventually landing in the sensor unit’s detector.

“It’s as if we could place a virtual computational camera on any distant surface and experience the world through the eyes of the surface,” Willomitzer said.

This technology might avert accidents by exposing other automobiles or animals just out of sight around the curve for persons driving routes bending around a mountain pass or winding through a rural woodland. Willomitzer said, “This technology transforms walls become mirrors.” “It becomes better since the approach may be used at night and in hazy weather.”

In this way, high-resolution imaging technologies might potentially replace (or enhance) endoscopes in medical and industrial imaging. Instead of requiring a flexible camera capable of turning corners and twisting through small areas, synthetic wavelength holography might utilize light to see through the numerous folds within the intestines, such as during a colonoscopy.

Similarly, synthetic wavelength holography could scan within industrial equipment while it was still operating, something that conventional endoscopes cannot achieve.

“You would normally use an endoscope if you had a functioning turbine and want to investigate faults within,” Willomitzer explained. “However, several flaws are only visible while the gadget is in motion. You can’t peek into the turbine with an endoscope from the front while it’s operating. Our sensor can identify objects that are less than one millimeter within a moving turbine.”

Despite the fact that the device is only a prototype, Willomitzer hopes it will one day be utilized to assist drivers in avoiding accidents. “It will be a long time before these kind of imagers are incorporated into automobiles or certified for medical uses,” he added. “It might take ten years or perhaps longer, but it will happen.”

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