Making 3-D imaging 1,000 times better

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

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

Algorithms exploiting light’s polarization boost resolution of commercial depth sensors 1,000-fold

MIT researchers have shown that by exploiting the polarization of light — the physical phenomenon behind polarized sunglasses and most 3-D movie systems — they can increase the resolution of conventional 3-D imaging devices as much as 1,000 times.

The technique could lead to high-quality 3-D cameras built into cellphones, and perhaps to the ability to snap a photo of an object and then use a 3-D printer to produce a replica.

Further out, the work could also abet the development of driverless cars.

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

The researchers describe the new system, which they call Polarized 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 polarized light gets the bounce

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

Polarization 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 polarizations that the incoming light did. At wider angles of reflection, however, light within a certain range of polarizations is more likely to be reflected.

This is why polarized 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 polarization. So the polarization 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 polarized light. Light with a particular polarization, reflecting off of a surface with a particular orientation and passing through a polarizing lens is indistinguishable from light with the opposite polarization, reflecting off of a surface with the opposite orientation.

This means that for any surface in a visual scene, measurements based on polarized light offer two equally plausible hypotheses about its orientation. Canvassing all the possible combinations of either of the two orientations of every surface, in order to identify the one that makes the most sense geometrically, is a prohibitively time-consuming computation.

Polarization plus depth sensing

To resolve this ambiguity, the Media Lab researchers use 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. Even with this added information, calculating surface orientation from measurements of polarized 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’ experimental setup consisted of a Microsoft Kinect — which gauges depth using reflection time — with an ordinary polarizing photographic lens placed 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.

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‘Groovy’ hologram creates strange state of light at visible and invisible wavelengths

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Nanostructured device controls the intensity, phase, and polarization of light for wide applications in optics

Applied physicists at the Harvard School of Engineering and Applied Sciences (SEAS) have demonstrated that they can change the intensity, phase, and polarization of light rays using a hologram-like design decorated with nanoscale structures.

As a proof of principle, the researchers have used it to create an unusual state of light called a radially polarized beam, which—because it can be focused very tightly—is important for applications like high-resolution lithography and for trapping and manipulating tiny particles like viruses.

This is the first time a single, simple device has been designed to control these three major properties of light at once. (Phase describes how two waves interfere to either strengthen or cancel each other, depending on how their crests and troughs overlap; polarization describes the direction of light vibrations; and the intensity is the brightness.)

“Our lab works on using nanotechnology to play with light,” says Patrice Genevet, a research associate at Harvard SEAS and co-lead author of a paper published this month in Nano Letters. “In this research, we’ve used holography in a novel way, incorporating cutting-edge nanotechnology in the form of subwavelength structures at a scale of just tens of nanometers.” One nanometer equals one billionth of a meter.

Genevet works in the laboratory of Federico Capasso, Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering at Harvard SEAS. Capasso’s research group in recent years has focused on nanophotonics—the manipulation of light at the nanometer scale—with the goal of creating new light beams and special effects that arise from the interaction of light with nanostructured materials.

Using these novel nanostructured holograms, the Harvard researchers have converted conventional, circularly polarized laser light into radially polarized beams at wavelengths spanning the technologically important visible and near-infrared light spectrum.

“When light is radially polarized, its electromagnetic vibrations oscillate inward and outward from the center of the beam like the spokes of a wheel,” explains Capasso. “This unusual beam manifests itself as a very intense ring of light with a dark spot in the center.”

“It is noteworthy,” Capasso points out, “that the same nanostructured holographic plate can be used to create radially polarized light at so many different wavelengths. Radially polarized light can be focused much more tightly than conventionally polarized light, thus enabling many potential applications in microscopy and nanoparticle manipulation.”

The new device resembles a normal hologram grating with an additional, nanostructured pattern carved into it. Visible light, which has a wavelength in the hundreds of nanometers, interacts differently with apertures textured on the ‘nano’ scale than with those on the scale of micrometers or larger. By exploiting these behaviors, the modular interface can bend incoming light to adjust its intensity, phase, and polarization.

Holograms, beyond being a staple of science-fiction universes, find many applications in security, like the holographic panels on credit cards and passports, and new digital hologram-based data-storage methods are currently being designed to potentially replace current systems. Achieving fine-tuned control of light is critical to advancing these technologies.

“Now, you can control everything you need with just a single interface,” says Genevet, pointing out that the polarization effect the new interface has on light could formerly only be achieved by a cascade of several different optical elements. “We’re gaining a big advantage in terms of saving space.”

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NEXT GENERATION 3-D THEATER: OPTICAL SCIENCE MAKES GLASSES A THING OF THE PAST

While their experimental results are promising, it may be several years until this technology can be effectively deployed in your local movie theater

From the early days of cinema, film producers have used various techniques to create the illusion of depth – with mixed results. But even with digital technology, the latest Hollywood blockbusters still rely on clunky glasses to achieve a convincing 3-D effect.

New optics research by a team of South Korean investigators offers the prospect of glasses-free, 3-D display technology for commercial theaters. Their new technique, described in a paper published today in the Optical Society’s (OSA) open-access journal Optics Express, can bring this added dimension while using space more efficiently and at a lower cost than current 3-D projection technology.

“There has been much progress in the last 10 years in improving the viewers’ experience with 3-D,” notes the team’s lead researcher Byoungho Lee, professor at the School of Electrical Engineering, Seoul National University in South Korea. “We want to take it to the next step with a method that, if validated by further research, might constitute a simple, compact, and cost-effective approach to producing widely available 3-D cinema, while also eliminating the need for wearing polarizing glasses.”

Polarization is one of the fundamental properties of light; it describes how light waves vibrate in a particular direction—up and down, side-to-side, or anywhere in between. Sunlight, for example, vibrates in many directions. To create modern 3-D effects, movie theaters use linearly or circularly polarized light.  In this technique, two projectors display  two similar images, which are slightly offset, simultaneously on a single screen. Each projector allows only one  state of  polarized light to pass through  its lens. By donning the familiar polarized glasses, each eye perceives only one of the offset images, creating the depth cues that the brain interprets as three dimensions.

The two-projector method, however, is cumbersome, so optical engineers have developed various single projector methods to achieve similar effects. The parallax barrier method, for example, succeeds in creating the illusion of 3-D, but it is cumbersome as well, as it requires a combination of rear projection video and physical barriers or optics between the screen and the viewer. Think of these obstructions as the slats in a venetian blind, which create a 3-D effect by limiting the image each eye sees. The South Korean team has developed a new way to achieve the same glasses-free experience while using a single front projector against a screen.

In their system, the Venetian blinds’ “slat” effect is achieved by using polarizers, which stop the passage of light after it reflects off the screen. To block the necessary portion of light, the researchers added a specialized coating to the screen known as a quarter-wave retarding film. This film  changes the polarization state of light so it can no longer pass through the polarizers.

As the light passes back either through or between the polarizing slates, the offset effect is created, producing the depth cues that give a convincing 3-D effect to the viewer, without the need for glasses.

The team’s experimental results reported today show the method can be used successfully in two types of 3-D displays. The first is the parallax barrier method, described above, which uses a device placed in front of a screen enabling each eye to see slightly different, offset images. The other projection method is integral imaging, which uses a two-dimensional array of many small lenses or holes to create 3-D effects.

“Our results confirm the feasibility of this approach, and we believe that this proposed method may be useful for developing the next generation of a glasses-free projection-type 3-D display for commercial theaters,” notes Lee.

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via The Optical Society
 

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