A SINGLE pulse of light is all it takes to generate high-resolution
three-dimensional images of a scene. The technique, developed by researchers in
Israel, is fast enough to create moving 3D pictures in real time, making it
ideal for robot vision or mapping landscapes.
Today鈥檚 3D imaging methods are either cumbersome or slow. One technique
involves a complicated set-up of cameras around a scene. Another uses lidar, the
laser equivalent to radar, to scan the scene, which takes time. The new
technique is similar to the scanning system, but can measure the time it takes
individual photons to reach the detector. This also gives it higher
resolution.
The researchers use a short, wide pulse of laser light, so all the photons
start their journey at the same time. As photons bounce off a distant object and
arrive at the detector, they strike a screen called a multichannel plate
detector (MCP), a device often used in night-vision cameras. Photons hitting the
MCP trigger the release of many more photons at that point. These are emitted in
a characteristic pattern鈥攍ots at first, decaying to zero over a few
nanoseconds. This decay time is what provides the vital depth information.
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It works like this. The detector itself consists of two cameras pointing at
the MCP. One has a gate in front of it that shuts off the light after one
nanosecond. The closer an object is to the detector, the more time the photons
have to reach the camera before the gate closes. So the parts of the image
corresponding to nearby objects appear brighter than the parts of the image
showing distant objects.
Meanwhile the other camera, having no gate, receives all the photons emitted
by the MCP. To get distance information, Zajfman explains, 鈥測ou need to divide
one picture by the other, pixel by pixel鈥�.
Electrical engineer Paul Evans of Nottingham Trent University likes the idea
for its simplicity. 鈥淵ou immediately have this depth map, instead of having to
interpret information from multiple views,鈥� he says. 鈥淐runching data takes
迟颈尘别.鈥�
But the technique isn鈥檛 suitable for imaging close objects. 鈥淏y the time you
have sent the light, it is already back, and you have no time to react,鈥� says
Zajfman. But he says the method would work for anything a few metres away or
more, and the only limit would be the power of the laser light source. This
would make it useful for scanning landscapes, for example, for robots or
vehicles trying to manoeuvre around trees or hills, or for underwater
exploration.
The team has just been granted a patent for the idea, and has built a
prototype detector.
