Today’s virtual reality headsets are universally big — closer to goggles than glasses, and even then, larger and heavier than modern ski goggles. While Facebook has shown prototypes of bigger and more complex Oculus VR headsets, it’s made no secret of its interest in creating lightweight models that can be worn all day. Now it’s revealing a holographic optical architecture designed for thinner, lighter VR headsets, which it expects will appear in future “high performance AR/VR” devices.
Discussed in a Siggraph 2020 research paper titled “Holographic Optics for Thin and Lightweight Virtual Reality,” the system uses flat films to create a VR display only slightly thicker than today’s typical smartphones. Facebook’s “pancake optics” design combines several thin layers of holographic film with a laser projection system and directional backlights, delivering either flat imagery or volumetric holograms depending on the sophistication of the design. Depending on how many color, lighting, and alignment-enhancing components a prototype contains, the thickness of the optical system can range from 11mm to just under 9mm.
In wearable prototype form, each eye display features a resolution of roughly 1,200 by 1,600 pixels — comparable to current VR goggles — with a field of view that’s either a 93-degree circle or a 92-by-69-degree rectangle. That’s roughly comparable to the display specs of a 571-gram Oculus Quest, but in a glasses-like form factor that weighs less than 10 grams in total, albeit with only a single eye display in the prototypes. The researchers note they could cut parts and change materials to achieve a 6.6 gram weight equivalent to plastic aviator sunglasses, but would compromise performance by doing so.
There’s one major caveat: The technology isn’t ready for near-term deployment. As of now, Facebook’s researchers have three prototypes, including a full-color display that’s mounted on a test bench rather than wearable, and two wearable prototypes that deliver green-on-black imagery in either moving or static form. All of the prototypes use laser LCD illumination systems, and the three-laser full color prototype exhibits shimmering speckles, requiring a despeckler that’s fine for static images but leaves color noise when displaying dynamic content. In addition to developing a more performant custom despeckler, the researchers say that they could deliver a wearable full-color prototype by adding either a holographic backlight or lens, but need more engineering work to get to that point.
As most of today’s VR headsets use large, deep lens systems, replacement with a near-eye holographic system could cut their volume by 75% or more, though other components such as processors and storage would still need to be inside or wirelessly offloaded to a separate housing. Facebook’s glasses-styled prototypes have most of their components mounted outside of the displays, which means their real-world implementation would be closer to Nreal’s Light AR glasses than Oculus Quest, depending on a smartphone or other device to create their visuals. Scheduled for release this year, Light weighs 88 grams and has a 52-degree field of view.
At this stage, the researchers believe that the system comes closer to delivering a viable lightweight VR solution than Microsoft’s HoloLens 2 and Magic Leap 1, which are limited by roughly 40-degree fields of view, far less than is necessary to fill the eye with digital content. Over time, Facebook’s goal is to deliver both resolution matching normal human vision and a large viewing eye box so that eyes can rotate to see content rather than just staring forward at a small window. The headset could be used for everything from office productivity — displaying a virtual computer and other tools — to Facebook-powered social interactions and games.
Facebook isn’t the only company working on holographic optical systems for mixed reality applications. Apple-acquired company Akonia was working on solutions years ago, and has sought to patent a retinal hologram projector system among other XR display technologies. U.K. startup WaveOptics is another player, backed by China’s Goertek, while Samsung and Sony have backed a thin film holographic waveguide display from DigiLens. As an alternative, Germany’s Fraunhofer has developed microdisplays that are much smaller than traditional VR screens and can be brought closer to the user’s eyes, reducing the optical system’s volume by 75% and weight by 50%.
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