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Light polarisation promises significant boost in 3D cameras

Posted: 07 Dec 2015 ?? ?Print Version ?Bookmark and Share

Keywords:MIT? 3D camera? polarisation? cellphone?

A team of researchers at MIT has discovered that by manipulating the polarisation of light, the physical phenomenon behind polarised sunglasses and most 3D movie systems, they can enhance the resolution of conventional 3D imaging devices as much as 1,000 times. According to them, the novel method could open the door to high-quality 3D cameras built into cellphones.

The technique could also pave the way for the ability to snap a photo of an object and then use a 3D printer to produce a replica, and in the development of driverless cars.

"Today, they can miniaturise 3D cameras to fit on cellphones," said Achuta Kadambi, a PhD student in the MIT Media Lab and one of the system's developers. "But they make compromises to the 3D sensing, leading to very coarse recovery of geometry. That's a natural application for polarisation, because you can still use a low-quality sensor, and adding a polarising filter gives you something that's better than many machine-shop laser scanners."

The researchers describe the system, which they call Polarised 3D, in a paper they're presenting at the International Conference on Computer Vision in December. Kadambi is the first author, and he's joined by his thesis advisor, Ramesh Raskar, associate professor of media arts and sciences in the MIT Media Lab; Boxin Shi, who was a postdoc in Raskar's group and is now a research fellow at the Rapid-Rich Object Search Lab; and Vage Taamazyan, a master's student at the Skolkovo Institute of Science and Technology in Russia, which MIT helped found in 2011.

When polarised light gets the bounce

If an electromagnetic wave can be thought of as an undulating squiggle, polarisation refers to the squiggle's orientation. It could be undulating up and down, or side to side, or somewhere in-between.

Polarisation also affects the way in which light bounces off of physical objects. If light strikes an object squarely, much of it will be absorbed, but whatever reflects back will have the same mix of polarisations that the incoming light did. At wider angles of reflection, however, light within a certain range of polarisations is more likely to be reflected.

Polarised light

By combining the information from the Kinect depth frame in (a) with polarised photographs, MIT researchers reconstructed the 3D surface shown in (c). Polarisation cues can allow coarse depth sensors like Kinect to achieve laser scan quality (b). Courtesy of the researchers

This is why polarised sunglasses are good at cutting out glare: Light from the sun bouncing off asphalt or water at a low angle features an unusually heavy concentration of light with a particular polarisation. So the polarisation of reflected light carries information about the geometry of the objects it has struck.

This relationship has been known for centuries, but it's been hard to do anything with it, because of a fundamental ambiguity about polarised light. Light with a particular polarisation, reflecting off of a surface with a particular orientation and passing through a polarising lens is indistinguishable from light with the opposite polarisation, reflecting off of a surface with the opposite orientation.

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