Scientists have developed a detection system that could vastly improve the accuracy of human facial and activity recognition at long distances and through obstructions like fog, smoke, or camouflage. The researchers said their sensitive lidar system can generate high-resolution 3D images with double the efficiency of similar lidar systems being developed by other research groups, and at least 10 times better image resolution. At 325 meters, researchers were able to 3D image the face of one of their co-authors in millimeter-scale detail. The same system could be used to accurately detect faces and human activity at distances of up to a kilometer, the researchers said. The work is a collaboration between the Single-Photon Group at Heriot-Watt University, using equipment developed by NASA's Jet Propulsion Laboratory (JPL) at Caltech, MIT, and the James Watt School of Engineering at the University of Glasgow. “The results of our research show the enormous potential of such a system to construct detailed high-resolution 3D images of scenes from long distances in daylight or darkness conditions,” said lead author Aongus McCarthy, a research fellow at Heriot-Watt’s Institute of Photonics and Quantum Sciences. “For example, if someone is standing behind camouflage netting, this system has the potential to determine whether they are on their mobile phone, holding something, or just standing there idle. So there are a number of potential applications from a security and defense perspective.” The system uses pulses of laser light to measure the distances to objects in a scene. The team’s breakthrough involved being able to measure the time it took for a laser pulse to travel from the system to the object and back with an accuracy of approximately 13 picoseconds. This timing is around 10 times better than the researchers had been able to do previously. A depth scan of research co-author Gregor Taylor taken from 45 meters away using the new detection system. Courtesy of Heriot-Watt University “The timing is really phenomenal,” McCarthy said. “It allows us to measure variations in depth very, very accurately — on a millimeter scale — which means we can distinguish between closely separated surfaces at very long distances.” The system could lead to “step change improvements” in applications such as facial and human activity recognition, and the imaging of scenes through “clutter and atmospheric obscurants,” the researchers said. A key advantage of the system is being able to accurately measure distances in broad daylight — when scattered light from the sun typically has a negative impact on the measurement process. By using a laser wavelength greater than can be seen by the naked eye — at 1550 nanometers — the daylight background is significantly reduced. This wavelength is also ideal for very high transmission in the atmosphere and in optical fibers. Another advantage is that the laser output of the system is low power and “eye safe” — meaning the laser beams from the system are safe to look at from any distance. The researchers tested their system at three distances visible from their rooftop laboratory: a neighboring rooftop 45 meters away, a location on the ground 325 meters away, and a distant radio mast exactly one kilometer away. It was at the 45 meter and 325 meter locations that research co-author Gregor Taylor posed while his colleagues scanned his head. The researchers are interested in testing the system over much longer distances. “Could we recognize a vehicle type at 10 kilometers, whether it's a car or a van or a tank?” McCarthy asked. “These kind of distances would be of real interest.” McCarthy said the system could also be used to monitor the movement of buildings or rock faces to assess subsidence or other potential hazards. The team built the system using a highly advanced detector developed by NASA’s JPL and MIT called a superconducting nanowire single-photon detector. The detector has to be cooled to a very low temperature of approximately -272 ºC by using a cryocooler fridge designed and developed by the Quantum Sensors group of Robert Hadfield, professor of photonics at University of Glasgow’s James Watt School of Engineering. The research was published in Optica (www.doi.org/10.1364/OPTICA.544877).