The device can see through corners and scattering media such as fog or human tissue.
Northwestern University researchers invented the high-resolution camera capable of seeing the unheard in corners and through scattering media like skin, fog, or even the human skull.
Synthetic wavelength holography is a new method that uses the indirect scattering of coherent light onto hidden objects. This scatters again and returns to a camera. An algorithm reconstructs the scattered light signal and reveals hidden objects. The method has the potential to image fast-moving objects, such as the beat of the heart or cars speeding around a corner.
The study will be published in Nature Communications today, November 17, 2021.
Non-line-of-sight (NLoS imaging) is a relatively new field in imaging objects behind scattering media or occlusions. The Northwestern method is faster than other NLoS imaging techniques and can capture full-field images over large areas with submillimeter precision. With this resolution, the computational camera can image through the skin to view even the smallest capillaries.
The method is helpful for noninvasive medical imaging and early-warning navigation systems in automobiles, and the researchers think it has many other potential uses.
Florian Willomitzer from Northwestern, the first author of this study, said, “our technology will usher in a new wave in imaging capabilities.” Our current prototypes of sensor technology use infrared or visible light. However, the universal principle could also be applied to other wavelengths. The same code could be used to detect radio waves in space or for underwater acoustic imaging. This method can be used in many areas, and we are only scratching the surface.
Willomitzer is an assistant professor of electrical engineering at Northwestern’s McCormick School of Engineering. Oliver Cossairt (associate professor of computer science, electrical and computer engineering) and Fengqiang Li (ex-Ph.D. student) are co-authors from Northwestern. Northwestern researchers worked closely with Muralidhar Balaji, Prasanna Rangarajan, and Marc Christensen, all Southern Methodist University researchers.
Recognizing scattered light
Although it might seem that seeing around corners and imaging an organ within the human body may be two different tasks, Willomitzer stated they are very similar. Both are concerned with scattering media. This is when light strikes an object and spreads it so that a direct image cannot be taken of the thing.
Willomitzer stated that if you’ve ever tried shining a flashlight through your hands, you’ve experienced this phenomenon. “You can see a bright spot on your other hand. However, there should be a shadow cast by your bones. This would reveal the bones’ structure. Instead, the light passing through the bones is scattered in the tissue, blurring the shadow image.
Therefore, the goal is to intercept scattered light and reconstruct its inherent information about the time of travel to reveal the hidden object. This presents its challenges.
Willomitzer stated, “Nothing can travel faster than light speed, so you will need high-speed detectors if you want light’s time to travel with high precision.” These detectors can be costly.
Tailored waves
Willomitzer and colleagues combined light waves from two lasers to create a synthetic lightwave that could be tailored for holographic imaging under different scattering conditions.
Willomitzer said, “If you capture the entire object’s light field in a Hologram, you can then fully reconstruct the object’s three-dimensional shape.” This holographic imaging is done around corners or through scatterers, using synthetic light waves instead of regular ones.
Many NLoS imaging efforts have been made to find hidden objects over the years. These methods often have one or two problems. These methods have limited resolution and require large probing areas to measure scattered light signals.
This new technology overcomes these problems and is the first to image around corners and through scattering media. It combines high spatial and temporal resolution with a small probing area and a large angle field of view. The camera can image small features in tight spaces, hidden objects in large areas of high resolution, and even objects that are moving.
Transforming ‘walls into Mirrors.’
Light only moves along straight lines, so an opaque barrier, such as a wall, shrub, or automobile, must be present to allow the new device to see around corners. The sensor unit, which could be mounted on top of a car, emits light that bounces off the barrier and hits the object at the corner. The light bounces off the wall, then hits the thing around the corner—finally, the light returns to the sensor’s detector.
Willomitzer stated, “It’s almost like we can place a virtual computational camera at every remote surface to view the world from its perspective.”
This method can prevent accidents when driving through mountain passes or rural forests. Willomitzer stated that this technique transforms walls into mirrors. It’s possible to use the process at night or in fog.
High-resolution technology could also supplement or replace endoscopes for industrial and medical imaging. Synthetic wavelength holography uses light to view the many folds in the intestines and replaces a flexible camera that can turn corners and navigate through tight spaces.
Synthetic wavelength holography can also image inside industrial equipment, which is currently impossible with endoscopes.
Willomitzer stated that an endoscope is used to inspect the inside of a turbine. Some defects are only visible when the device is moving. While the turbine is running, an endoscope cannot be used to look inside it from the front. The sensor can detect structures smaller than 1 millimeter inside a turbine.
Willomitzer is optimistic that the technology will eventually be used to prevent accidents. He said there is still much to be done before imagers can be built into cars and approved for medical purposes. It will happen in 10 years, maybe even longer.