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MIT researchers say they can increase the resolution of conventional 3D imaging devices by as much as 1000, by exploiting light’s polarisation properties.
The technique could lead to high-quality 3D cameras built into smartphones. With depth perception built in, future engineers and consumers may be able to quickly make 3D scans of objects and reproduce them via 3D printing.
The development of affordable but high resolution 3D cameras would also boost driverless car technology.
According to MIT Media Lab PhD student Achuta Kadambi, while 3D cameras are currently found on smart phones, the quality is less than ideal.
“They make compromises to the 3-D sensing, leading to very coarse recovery of geometry,” Kadambi explained.
With the MIT technology, it is possible to use a low-quality sensor, but add a polarising filter to yield results better than many machine-shop laser scanners.
Polarisation affects the way in which light bounces off of physical objects. If light strikes an object squarely, most 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.
In other words, the polarisation of reflected light carries information about the geometry of the objects it has struck.
However, due to the fundamental ambiguity about polarised light, it has been difficult and comptuationally expensive to “reverse engineer” polarised light to gather geometrical information. An algorithm would have to canvas all the possible combinations of of either of the two orientations of every surface, in order to identify the one that makes the most sense geometrically.
The MIT researchers found they could reduce the complexity of the task by adding coarse depth estimates provided by some other method, such as the time a light signal takes to reflect off of an object and return to its source.
However, even with this added information, calculating surface orientation from measurements of polarised light is complicated, but it can be done in real-time by a graphics processing unit, the type of special-purpose graphics chip found in most video game consoles.
The researchers modified a Microsoft Kinect (which gauges depth using reflection time) and placed an ordinary polarizing photographic lens in front of its camera.
In each experiment, the researchers took three photos of an object, rotating the polarizing filter each time, and their algorithms compared the light intensities of the resulting images.
On its own, at a distance of several meters, the Kinect can resolve physical features as small as a centimeter or so across. But with the addition of the polarization information, the researchers’ system could resolve features in the range of tens of micrometers, or one-thousandth the size.
For comparison, the researchers also imaged several of their test objects with a high-precision laser scanner, which requires that the object be inserted into the scanner bed. They found the Polarised 3D system still offered the higher resolution.
According to the researchers, the smart phone implementation would probably utilise grids of tiny polarisation filters that can overlay individual pixels in a light sensor. Such an implementation would reduce the camera’s resolution, since three pixels worth of light would be captured for each image pixel. However, this is no worse than current colour filters.
Polarised 3D could also find applications in driverless cars. Today’s experimental self-driving cars are highly reliable under normal illumination conditions, but rain, snow and fog can deceive their vision algorithms.
Polarised 3D can combat this by exploit information contained in interfering waves of light to handle scattering.