Holographic camera sees through people and around corners with high precision

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 relatively new research field of imaging objects behind occlusions or scattering media is called non-line-of-sight imaging. Compared to related imaging technologies, the Northwestern method can rapidly capture full-field images of large areas with submillimetre precision.

With this level of resolution, the computational camera could potentially image through the skin to see even the tiniest capillaries at work, the researchers said.

While the method has obvious potential for non-invasive 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."

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," he 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. This 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 added. "Such detectors can be terribly expensive."

To eliminate the need for fast detectors, the researchers 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."

While non-line-of-sight technology does already exist, the team believes their new technology 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.