Holographic optics for thin and lightweight virtual reality

Facebook Reality Labs (FRL) is always exploring new optical architectures to improve form factor, comfort, and optical performance. Last fall, at Oculus Connect 6, FRL Chief Scientist Michael Abrash introduced new miniaturization progress in VR with Half Dome 2 and 3, two prototypes that examine how varifocal displays can improve visual and physical comfort. This year, at the virtual SIGGRAPH conference, we’re presenting another research milestone on this path: a new optical architecture that is significantly more compact and offers the potential for better visual performance.

In this work, “Holographic Optics for Thin and Lightweight Virtual Reality,” researchers Andrew Maimone and Junren Wang propose a new class of near-eye displays, which combine the power of holographic optics and polarization-based optical folding — an approach that could be used to develop future sunglasses-like VR hardware. These two methods help keep the optics as thin as possible while making the most efficient use of space. We anticipate that such lightweight and comfortable form factors may enable extended VR sessions and new use cases, including productivity.

The design is demonstrated in a proof-of-concept research device that uses only thin, flat films as optics to achieve a display thickness of less than 9 mm while supporting a field of view comparable to today’s consumer VR products. The work demonstrates the promise of better visual performance, as well: Laser illumination is used to deliver a much wider gamut of colors to VR displays, and progress is made toward scaling resolution to the limit of human vision.

This video demonstrates video game animation, as shown on our proof-of-concept research. 

The approach

This image shows our research device display modules mounted into a frame. This research device was used to capture the green image shown below (some components are mounted externally).

Today’s VR displays have three primary components: a source of light (e.g., LEDs), a display panel that brightens or dims the light to form an image (e.g., an LCD panel), and a viewing optic that focuses the image far enough away so that the viewer’s eyes can see it (e.g., a plastic lens). As the first two components can readily be formed into thin and flat modules, most of the weight and volume go into the viewing optics. To significantly reduce the overall size and weight of VR displays, we combine two techniques: holographic optics and polarization-based optical folding.

Most VR displays share a common viewing optic: a simple refractive lens composed of a thick, curved piece or glass or plastic. We propose replacing this bulky element with holographic optics. You may be familiar with holographic images seen at a science museum or on your credit card, which appear to be three-dimensional with realistic depth in or out of the page. Like these holographic images, our holographic optics are a recording of the interaction of laser light with objects, but in this case the object is a lens rather than a 3D scene. The result is a dramatic reduction in thickness and weight: The holographic optic bends light like a lens but looks like a thin, transparent sticker.

However, even if the lens itself is made thin, the viewing optics as a whole may still be large — a considerable amount of empty space must be placed between the display panel and the lens to properly focus the image. Ordinarily, light from the display panel propagates forward to the lens and then continues toward the eye. However, when we apply polarization-based optical folding, light can be controlled to move both forward and backward within the lens so that this empty space can be traversed multiple times, collapsing it to a fraction of the original volume.

Wider color gamut

Shown on the left, a photograph captured with the proof-of-concept research device shown above. On the right, a photograph taken through a larger full-color benchtop prototype. We are currently working on achieving full color on the smaller research prototype.

When we apply holographic optics to a VR display, we must reevaluate all other optical components. Notably, holographic optics compel the use of laser light sources, which are more difficult to integrate but provide a much richer set of colors than the LEDs common in nearly all of today’s VR headsets, phones, computers, and televisions.

To illustrate the difference, the figure below shows the gamut of human-visible colors. A common set of colors reproducible on many displays today is the sRGB color space (illustrated by the smaller triangle). Note that it can capture only a small fraction of the colors that we can actually see. In contrast, the outer triangle represents the much larger set of colors that can be reproduced using the lasers on one of our research prototype displays. This allows the reproduction of vivid and saturated colors. Think of a brightly lit neon sign or the iridescent sheen of a butterfly wing.

This figure illustrates the gamut of human-visible colors. The sRGB space represents a common set of colors reproducible on many displays today. The outer triangle represents the larger set of colors reproducible on our research prototype.

What’s next

While it points toward the future development of lightweight, comfortable, and high-performance AR/VR technology, at present our work is purely research. In our technical paper, we identify the current limitations of our proposed display architecture and discuss future areas of research that will make the approach more practical. To our knowledge, our work demonstrates the thinnest VR display demonstrated to date, and we’re excited to see what the future holds.

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