Radar and its laser counterpart lidar can detect out-of-sight objects. But fog, rain, smoke and dust throw these tools off by scattering light and radio waves. Now Milad Akhlaghi and Aristide Dogariu, optics researchers at the University of Central Florida, have taken advantage of this property to track a moving object hidden by a simulated haze. By analyzing subtle changes an object creates in a pattern of scattered light, the researchers can instantly obtain the object's direction and speed. The technique, reported earlier this year in Optica, could advance collision-avoidance systems and help the military monitor shrouded targets.

“Tracking objects outside line-of-sight is a hot topic right now,” says Stanford University electrical engineer Gordon Wetzstein, who was not involved in the new work. Researchers have previously developed ways to image unseen objects—even around corners and behind walls—by bouncing microwave or laser pulses at them and carefully timing the signals' return. But these methods require fancy, expensive equipment.

The Florida team came up with a simpler approach using a cheap, low-power laser and a common light detector called a photomultiplier tube, which records the total intensity of photons falling on it. To demonstrate their method, the scientists placed a moving target—a symbol printed on a transparent sheet—inside a toaster-size box of frosted plexiglass. They shined laser light on the box, whose opaque wall created a random pattern of speckles detected by a photomultiplier tube on the other side of the box. As the object moved inside the box, the light pattern changed slightly. Using clever statistical analysis and modeling, the researchers can use these flickers to reconstruct the object's motion in 3-D.

Like radar, the new technique can spot an object but cannot tell its size or shape, Dogariu says. It can detect objects only up to a meter away right now, but “estimates indicate that kilometer ranges might be attainable,” he adds. One limitation is that a stationary target would go unnoticed because it would not change the speckle pattern, and the method relies on tracking these changes.

Daniele Faccio of Heriot-Watt University in Scotland, who was not part of the study, calls the technique “a very elegant, robust method that requires minimal resources.” To prove its real-world efficacy, the researchers will need to show that it works when the target is in a space filled with mist and not just behind a foggy two-dimensional wall.