Facebook released new research today that the company says shows the “thinnest VR display yet proven,” in a proof-of-concept headset based on folded holographic optics.
Facebook Reality Labs, the company’s AR / VR R&D division, today released new research demonstrating an approach that combines two key features: polarization-based optical ‘folding’ and holographic lenses. In the paper, researchers Andrew Maimone and Junren Wang say they have used the technique to create a functional virtual reality display and lens that together are only 9mm thick. The result is a proof-of-concept VR headset that could really be called ‘VR glasses’.
The approach has other benefits beyond its incredibly compact size; The researchers say that it can also support a significantly wider color gamut than today’s virtual reality displays, and that its display is moving “toward resolution scaling to the limit of human vision.”
Let’s talk about how everything works.
Why are today’s headphones so great?
It’s natural to wonder why even the latest VR headsets are essentially as bulky as the first generation of headsets that launched in 2016. The answer is simple: optics. Unfortunately, the solution is not that simple.
All consumer virtual reality headsets on the market effectively use the same optical pipeline: a macro display behind a single lens. The lens is there to focus the light from the screen on your eye. But for that to happen, the lens must be within a few inches of the screen, otherwise it doesn’t have enough focusing power to focus the light on your eye.
That necessary distance between the screen and the lens is the reason why all headphones look like a box on the face. The focus is still used today because lenses and displays are known quantities; they are cheap and simple, and although bulky, they achieve a wide field of view and high resolution.
Many solutions have been proposed to make VR headsets smaller, and almost all of them include the use of new screens and lenses.
The new Facebook research proposes the use of folded optics and holographic optics.
Folded Optic
What are folded optics? It is not quite what it sounds like, but once you get it, it would be hard to find a better name.
While the simple lenses in today’s VR headsets need to be some distance from the screen to focus light on your eye, the concept of folded optics proposes to ‘bend’ that distance on itself, so that the light still traverse the same distance needed to focus, but it is road folds into a more compact area.
You can think of it as a piece of paper with an arbitrary width. When you fold the paper in half, the paper itself is still as wide as it was when you started, but its width takes up less space because you folded it on itself.
But how the hell do you do that with light? Polarization is the key.
It turns out that the light beams have an “orientation”. Normally, the orientation of the light beams is random, but you can use a polarizer to let only light in a specific orientation pass. You can think of a polarizer like the coin slot in a vending machine – it will only accept coins in one orientation.
Using polarization, it is possible to bounce light back and forth several times along an optical path before eventually letting it out and into the user’s eye. This approach (also known as ‘pancake optics’ allows the lens and screen to get much closer together, resulting in more compact headphones.
But to slim down further, to reduce the size of the lenses, Facebook researchers have turned to holographic optics.
Holographic optics
Instead of using a series of typical lenses (such as those found in a pair of glasses) in folded optics, researchers have formed the lenses into … holograms.
If that hurts your head, everything is fine. Holograms are crazy, but I will do my best to explain it.
Unlike a photograph, which is a recording of light on a airplane of space at any given time, a hologram is a recording of light in a volume of space at any given time.
When you look at a photograph, you can only see the light information contained in the plane that was captured. When you look at a hologram, you can look around the hologram, because the light information is captured in the entire volume (also known as the light field).
Now I’m going to blow your head off. What if when you captured a hologram, the scene you captured had a lens? It turns out that the lens you see in the hologram will behave just like the lens in the scene. Don’t Believe Me Watch this video at 0:19 and watch the magnifying glass on the scene and watch it magnify the rest of the hologram, even though it is part of the hologram.
This is the fundamental idea behind Facebook’s holographic lens approach. The researchers effectively ‘captured’ a hologram of a real lens, condensing the optical properties of a real lens into a paper-thin holographic film.
So the optics Facebook is employing in this design is literally a hologram of a lens.
