It works because of the detector and the laser the scientists chose. Instead of using a laser that uses visible light, Buller's team employed an infrared laser. These lasers travel farther because they aren't disrupted as much by sunlight. On top of that, infrared light is reflected by a wider range of objects - it won't get absorbed by clothing. It's also a lot safer for people's eyes.
Then there's the detector that picks up the reflected light. The Watt University group designed one that is more sensitive, and picks up even individual photons - it can see much fainter reflections than conventional designs.
Their imager would be useful for military personnel, for example, who want to know what a distance object is, but that is too far away to see. But 3-D images aren't just for surveillance. Getting a good 3-D picture of a mountainside or snowy peak could tell park rangers whether landslides or avalanches are likely, for instance.
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Despite it's ability to see so far, the imager can't see faces - one of the things that doesn't reflect infrared light well is human skin, though that changes if you sweat. And the algorithm that builds the pictures is still on the slow side - it takes several seconds to make a picture. The team also wants to improve the range, and it's possible, they think, to push it to 6 miles.