Holographic Camera Peers Through Fog, Around Corners

A high-resolution camera that can capture images around corners and through scattering media, such as skin, fog, and potentially the human skull, combines high temporal resolution, a small probing area, and a large angular field of view into a single device. A team of researchers at Northwestern University has introduced the device, which uses a holographic method and reconstructive algorithm to reveal hidden objects.

Called synthetic wavelength holography, the method indirectly scatters coherent light onto hidden objects, which then scatters again and travels back to a camera. From there, the algorithm reconstructs the scattered light signal, which reveals 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.

A setup of one of the camera prototypes in the laboratory. Courtesy of Florian Willomitzer, Northwestern University.

The researchers believe the method has potential to support applications beyond noninvasive medical imaging, early-warning navigation systems for automobiles, and industrial inspection in tightly confined spaces.

“Our current sensor prototypes use visible or infrared light, but the principle is universal and could be extended to other wavelengths,” said Florian Willomitzer, research assistant professor in the Department of Electrical and Computer Engineering and first author of the study. “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.”

Seeing around a corner and imaging an organ within the body may seem like very different challenges. According to Willomitzer, the two are 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, therefore, is to intercept the scattered light in order to reconstruct the inherent information about its time of travel to reveal the hidden object.

“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.”

To eliminate the need for fast detectors, Willomitzer and his colleagues merged lightwaves from two lasers to generate a synthetic lightwave 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 said. “We do this holographic imaging around a corner or through scatterers — with synthetic waves instead of normal lightwaves.”

Previous attempts at non-line-of-sight imaging suffered from low resolution, an extremely small angular field of regard, and/or the requirement of a time-consuming raster scan. Others required large probing areas to measure the scattered light signal.

Because light travels only on straight paths, an opaque barrier such as a wall, shrub, or automobile must be present 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, and 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.

The research was published in Nature Communications (www.doi.org/10.1038/s41467-021-26776-w).