A prototype camera is in the laboratory. Credit: Florian Willomitzer.
A new high-resolution camera has been invented by researchers at the university that can see the unseen, including around corners and through scattering media, such as skin, fog or the human skull.
The new method works by scattering light onto hidden objects and then returning it to a camera. The light signal is reconstructed to reveal hidden objects. The method has the potential to image fast moving objects, such as the beating heart through the chest or speeding cars, due to its high temporal resolution.
The study will be published in the journal Nature Communications.
Non-line-of-sight (NLoS) is a new field of research that uses cameras to look at objects behind obstructions. Compared to other NLoS technologies, the Northwestern method can quickly capture full-field images of large areas. The computational camera could potentially image through the skin to see the smallest capillaries.
The method has obvious potential for medical scans, early-warning navigation systems for automobiles, and industrial inspection in tightly confined spaces, but the researchers believe there are many more applications for it.
The first author of the study said that the technology will bring in a new wave of capabilities. Our current sensor prototypes use visible or IR 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. We have only scratched the surface, because it can be applied to many areas.
Willomitzer is an electrical and computer engineering professor at the school. Oliver Cossairt is an associate professor of computer science and electrical and computer engineering. The researchers at Southern Methodist University collaborated with the researchers at the other universities.
The light is scattered.
Seeing around a corner and seeing an organ inside the human body are both similar challenges, according to Willomitzer. Both deals with scattering media, in which light hits an object and scatters in a way that a direct image of the object can't be seen.
"If you have ever tried to shine a flashlight through your hand, you have experienced this phenomenon," Willomitzer said. You see a bright spot on the other side of your hand, but you don't know what's behind it. The light that passes the bones is scattered throughout the tissue in all directions.
The goal is to intercept the scattered light in order to reconstruct the time of travel to find the hidden object. That presents its own challenge.
"If you want to measure light's time of travel with high precision, you need extremely fast detectors," Willomitzer said. Such detectors can be very expensive.
Waves were tailored.
The synthetic light wave created by merging light waves from two lasers can be tailored to different scattering scenarios, eliminating the need for fast detectors.
"If you can capture the entire light field of an object in a hologram, you can reconstruct the object's three-dimensional shape in its entirety," Willomitzer said. "We use synthetic waves instead of normal light waves to do this hologram scanning."
There have been many attempts to recover images of hidden objects. These methods have problems. They either have low resolution or need a large probing area to measure the light signal.
The new technology is the first to combine high spatial resolution, high temporal resolution, a small probing area and a large field of view in a single method. The camera can image tiny features in tightly confined spaces and hidden objects in large areas with high resolution even when the objects are moving.
Walls are turning into mirrors.
Light only travels on straight paths, so an opaque barrier must be present in order for the new device to see around corners. The light from the sensor unit bounces off the barrier and hits the object around the corner. The light bounces back and forth between the barrier and the sensor unit.
Willomitzer said that it was like we could plant a virtual computational camera on every remote surface to see the world from the surface's perspective.
This method could be used to prevent accidents when driving through a mountain pass or through a rural forest. Willomitzer said that the technique turns walls into mirrors. It gets better as the technique can be used at night and in foggy weather.
The high-resolution technology could be used for medical and industrial purposes. Instead of using a flexible camera, synthetic wavelength holography could use light to see around the many folds inside the intestines.
Synthetic wavelength holography could image inside industrial equipment while it is still running, which is impossible for current endoscopes.
"If you want to inspect the inside of a turbine, you would use an ethnometer," Willomitzer said. When the device is moving, some defects show up. You can't look inside the turbine from the front while it's running. Our sensor can look inside a turbine to see structures that are small.
Willomitzer believes that the technology will eventually be used to help drivers avoid accidents. He said that it's still a long way to go before we see these kinds of imagers built in cars or approved for medical applications. "Maybe 10 years or even more, but it will come."
Fast non-line-of-sight and high-resolution and wide field of view using synthetic wavelength holography is more information. There is a DOI: 10.1038/s41467-021-26776-w.
Nature Communications is a journal.
The new Holographic camera sees the unseen with high precision, retrieved 17 November 2021, fromphys.org.
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