A technique was developed that enables reconstruction of images in great detail. Researchers computed millimeter- and micrometer-scale shapes of curved objects, providing an important component to a larger suite of non-line-of-sight (NLOS) imaging techniques.
Most of what people see — and what cameras detect — comes from light that reflects off an object and bounces directly to the eye or the lens. But light also reflects off the objects in other directions, bouncing off walls and objects. A faint bit of this scattered light ultimately might reach the eye or the lens but is washed out by more direct, powerful light sources. NLOS techniques try to extract information from scattered light — naturally occurring or otherwise — and produce images of scenes, objects, or parts of objects not otherwise visible.
Other NLOS imaging systems can understand room-size scenes or even extract information using only naturally occurring light. The new technique, which is complementary to those approaches, enables NLOS systems to capture fine detail over a small area. In this case, an ultrafast laser was used to bounce light off a wall to illuminate a hidden object. By knowing when the laser fired pulses of light, the researchers could calculate the time the light took to reflect off the object, bounce off the wall on its return trip, and reach a sensor. The time-of-flight technique is similar to LiDAR used by self-driving cars to build a 3D map of the car’s surroundings.
Previous attempts to use these time-of-flight calculations to reconstruct an image of the object have depended on the brightness of the reflections off it. But in this study, a new method was developed based purely on the geometry of the object, which in turn enabled creation of an algorithm for measuring its curvature.
An imaging system was used that is effectively a LiDAR capable of sensing single particles of light to test the technique on objects such as a plastic jug, a glass bowl, a plastic bowl, and a ball bearing. The technique was combined with an imaging method called optical coherence tomography to reconstruct the images of U.S. quarters. In addition to seeing around corners, the technique proved effective in seeing through diffusing filters such as thick paper.
The technique has been demonstrated only at short distances — a meter at most. But the technique, based on geometric measurements of objects, might be combined with other, complementary approaches to improve NLOS imaging. It could also be employed in other applications such as seismic imaging and acoustic and ultrasound imaging.
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