Today’s virtual reality headsets are universally large – closer to glasses than glasses, and even bigger and heavier than modern ski goggles. While Facebook has shown prototypes of larger and more complex Oculus VR headsets, it does not hide its interest in creating lightweight models that can be worn all day. It is now revealing a holographic optical architecture designed for thinner and lighter VR headsets, which are expected to appear in future “high-performance AR / VR” devices.
Discussed in a Siggraph 2020 research paper titled “Thin and Light Holographic Optics for Virtual Reality,” the system uses flat films to create a VR screen just 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 backlight, offering flat images or volumetric holograms depending on design sophistication. Depending on how many color, lighting, and alignment enhancement components a prototype contains, the thickness of the optical system can range from 11mm to just under 9mm.
In portable prototype form, each screen features a resolution of approximately 1,200 by 1,600 pixels, comparable to today’s virtual reality glasses, with a field of view that is a 93-degree circle or a 92-by-69-degree rectangle. That’s roughly comparable to the screen specs of a 571-gram Oculus Quest, but in a form factor similar to a lens that weighs less than 10 grams total, albeit with just an eye-screen in the prototypes. The researchers note that they could cut parts and change materials to achieve a weight of 6.6 grams equivalent to plastic aviator sunglasses, but would compromise performance by doing so.
There is an important caveat: The technology is not ready for short-term deployment. As of now, Facebook researchers have three prototypes, including a full-color display that is mounted on a test bench instead of a laptop, and two portable prototypes that offer green-on-black images in static or moving form. All prototypes use LCD laser lighting systems, and the full-color three laser prototype exhibits bright spots, requiring a defogger that is fine for still images but leaves color noise when displaying dynamic content. In addition to developing a more efficient custom defogger, the researchers say they could deliver a full-color portable prototype by adding a holographic backlight or lens, but they need more engineering work to get to that point.
Above: The external captures of the bench-mounted portable and full-color single-color displays do not do their color ranges justice, but provide an idea of their resolution.
Image credit: Facebook
Since most VR headsets today use large, deep lens systems, replacing them with a near-eye holographic system could reduce their volume by 75% or more, although other components like processors and storage would still have to be inside or downloaded wirelessly to a separate Home. Facebook’s stylish glasses prototypes have most of their components mounted off-screen, meaning their real-world deployment would be closer to Nreal’s Light AR glasses than Oculus Quest, depending on a smartphone or another device to create your images. Scheduled for launch this year, Light weighs 88 grams and has a 52-degree field of view.
At this stage, the researchers believe that the system is closer to offering a viable lightweight virtual reality solution than Microsoft’s HoloLens 2 and Magic Leap 1, which are limited by fields of view of approximately 40 degrees, much less than necessary to fill the eye with digital content. Over time, Facebook’s goal is to offer resolution that matches normal human vision and a large vision box so that the eyes can rotate to view the content instead of looking forward in a small window. Headphones can be used for everything from office productivity displaying a virtual computer and other tools to social interactions and Facebook-based games.
Facebook is not the only company working on holographic optical systems for mixed reality applications. Apple-acquired company Akonia was working on solutions years ago, and has tried to patent a retina hologram projector system among other XR display technologies. British startup WaveOptics is another player, backed by China’s Goertek, while Samsung and Sony have backed a DigiLens thin-film holographic waveguide display. As an alternative, Fraunhofer from Germany has developed micro-screens that are much smaller than traditional virtual reality displays and can get closer to the user’s eyes, reducing the volume of the optical system by 75% and the weight by 50%.
