For all the advancements in virtual reality technology in recent years, one major factor still holding the space back is the size and relative discomfort of current headset design. Even the most compact and comfortable VR headsets today still resemble something like a cross between ski goggles and a motorcycle helmet, requiring massive headstraps to secure a heavy display that protrudes multiple inches away from the face. Reference designs for “eyeglasses” style VR displays help a bit, but they still look like coke-bottle spectacles from a steampunk cosplay event (and provide a limited field of view, to boot).
Now, researchers at Facebook Reality Labs are using holographic film to create a prototype VR display that looks less like ski goggles and more like lightweight sunglasses. With a total thickness less than 9mm—and without significant compromises on field of view or resolution—these displays could one day make today’s bulky VR headset designs completely obsolete.
In the newly published ACM Siggraph paper Holographic Optics for Thin and Lightweight Virtual Reality, researchers Andrew Maimone and Junren Wang detail the optics behind their lightweight prototype. The key to the thinness is a series of flat, polarized films that use a “pancake optics” light-folding technique to reflect the displayed image multiple times in a small space.
That design effectively extends the apparent focal length of the image (which is key to user eye comfort) without the need for a large physical space for the light to travel through. Holographic films used to focus the image onto the eye also eliminate the need for the kinds of bulky refractive lensing systems found in current headsets.
How does it look?
Despite the thinness, the prototype display is able to provide a roughly 90-degree horizontal field of view in testing. That’s a big improvement over thin-film AR displays like Microsoft’s Hololens or Magic Leap, and it’s comparable to VR headsets like the Oculus Rift and Quest (but less than higher-end headsets like the HTC Vive and Valve Index).
Since Facebook’s prototype display uses lasers instead of LCDs to create an image, it’s hard to make a traditional pixel-based resolution measurement. The resolution of the display varies as you get farther from the center as well, with what the researchers describe as “significant ghosting” once you get outside a central 60-degree field of view.
That said, the researchers say the images at the center of their prototype display can clearly define details only slightly larger than one arcminute in apparent size. That’s approaching the limits of 20/20 human vision, a level of detail that would require ridiculous pixel densities on a traditional LCD headset.
A coming revolution?
While there’s a lot of potential here, there are plenty of limitations to the current holographic display prototypes as well. For one, the current model only displays content in multiple shades of green, creating an effect somewhat similar to the red-tinted 3D images of Nintendo’s failed Virtual Boy (you can see a video of this green-tinted display in action in this Facebook Research blog post).
Getting full-color images in that form factor will require integrating and overlaying red and blue lasers as well, which will require some careful design and engineering work. On the plus side, though, those lasers should theoretically be able to provide a wider color gamut than the usual sRGB LCD displays in current VR headsets.
The current holographic prototypes also offload everything except a single monocular display away from the “sunglasses” frame to prevent additional bulk. As the researchers note, “A truly portable and practical display would integrate a pair of display modules, a computing platform, batteries, positional trackers, and all external components into a sunglasses-like frame.”
All that said, the prototype discussed here sounds like a revolutionary and altogether feasible new direction for future VR displays—one that could break virtual reality out of the significant limits of its current uncomfortable form factor. While we’re a long way off from this kind of technology ever making its way to any consumer-level products, it’s nice to see Oculus-owner Facebook pushing the state of the art for VR display.
Listing image by Facebook Research