A group of Sweden-based researchers proposed a novel e-ink display solution that could make way for super compact, retina-level VR headsets and AR glasses in the future.

The News

Traditional emissive displays are shrinking, but they face physical limits; smaller pixels tend to emit less uniformly and provide less intense light, which is especially noticeable in near-eye applications like virtual and augmented reality headsets.

In a recent research paper published in Nature, a team of researchers presents what a “retinal e-ink display” which hopes to offer a new solution quite unlike displays seen in modern VR headsets today, which are increasingly adopting micro-OLEDs to reduce size and weight.

The paper was authored by researchers affiliated with Uppsala University, Umeå University, University of Gothenburg, and Chalmers University of Technology in Gothenburg: Ade Satria Saloka Santosa, Yu-Wei Chang, Andreas B. Dahlin, Lars Österlund, Giovanni Volpe, and Kunli Xiong.

While conventional e-paper has struggled to reach the resolution necessary for realistic, high-fidelity images, the team proposes a new form of e-paper featuring electrically tunable “metapixels” only about 560 nanometres wide.

This promises a pixel density of over 25,000 pixels per inch (PPI)—an order of magnitude denser than displays currently used in headsets like Samsung Galaxy XR or Apple Vision Pro. Those headsets have a PPI of around 4,000.

Image courtesy Nature

As the paper describes it, each metapixel is made from tungsten trioxide (WO₃) nanodisks that undergo a reversible insulator-to-metal transition when electrically reduced. This process dynamically changes the material’s refractive index and optical absorption, allowing nanoscale control of brightness and color contrast.

In effect, when lit by ambient light, the display can create bright, saturated colors far thinner than a human hair, as well as deep blacks with reported optical contrast ratios around 50%—a reflective equivalent of high-dynamic range (HDR).

And the team says it could be useful in both AR and VR displays. The figure below shows a conceptual optical stack for both applications, with Figure A representing a VR display, and Figure B showing an AR display.

Image courtesy Nature

Still, there are some noted drawbacks. Beyond sheer resolution, the display delivers full-color video at “more than 25 Hz,” which is significantly lower than what VR users need for comfortable viewing. In addition to a relatively low refresh rate, researchers note the retina e-paper requires further optimization in color gamut, operational stability and lifetime.

“Lowering the operating voltage and exploring alternative electrolytes represent promising engineering routes to extend device durability and reduce energy consumption,” the paper explains. “Moreover, its ultra-high resolution also necessitates the development of ultra-high-resolution TFT arrays for independent pixel control, which will enable fully addressable, large-area displays and is therefore a critical direction for future research and technological development.”

And while the e-paper display itself is remarkably low-powered, packing in the graphical compute to put those metapixels to work will also be a challenge. It’s a good problem to have, but a problem none the less.

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My Take

At least as the paper describes it, the underlying tech could produce XR displays approaching the size and pixel density that we’ve never seen before. And reaching the limits of human visual perception is one of those holy grail moments I’ve been waiting for.

Getting that refresh rate up well beyond 25 Hz is going to be extremely important though. As the paper describes it, 25 Hz is good for video playback, but driving an immersive VR environment requires at least 60 Hz refresh to be minimally comfortable. 72 Hz is better, and 90 Hz is the standard nowadays.

I’m also curious to see the e-paper display stacked up against lower resolution micro-OLED contemporaries, if only to see how that proposed ambient lighting can achieve HDR. I have a hard time wrapping my head around it. Essentially, the display’s metapixels absorb and scatter ambient light, much like Vantablack does—probably something that needs to be truly seen in person to be believed.

Healthy skepticism aside, I find it truly amazing we’ve even arrived at the conversation in the first place: we’re at the point where XR displays could recreate reality, at least as far as your eyes are concerned.

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Scott Hayden
Well before the first modern XR products hit the market, Scott recognized the potential of the technology and set out to understand and document its growth. He has been professionally reporting on the space for nearly a decade as Editor at Road to VR, authoring more than 4,000 articles on the topic. Scott brings that seasoned insight to his reporting from major industry events across the globe.
  • Christian Schildwaechter

    TL;DR: the low e-ink refresh rate was, is, and will be a huge problem; reflective displays need a minimum distance that make them more suitable for smartglasses than XR HMDs; this looks a bit like a new technology searching for an ultra-high PPI application, which led to XR displays despite not being that usable there.

    I haven't read the paper, but e-ink has fundamental speed problems. It is based on tiny, electrostatically charged pigments suspended in a liquid, usually oil in a microcapsule. To change the color, you apply a positive or negative field, and the differently charged black or white pigments move either up or down. This state is stable even without applying power, which makes e-ink displays great for low power e-readers or supermarket price signs.

    But having to physically move (or rotate) small colored balls in a liquid initially caused a screen refresh to take more than a second, often requiring a complete wipe first. Over time this improved, ways to display in color were found, and there are more technical approaches. The requirements in the article hint at one with a permanently applied field, but they still never got fast. So now running at 25Hz, usable for watching movies, is already a huge step forward, nothing they could easily double or triple.

    The second issue is this being a reflective display, so compared to self-emissive display like OLEDs, or transmissive display like LCD with backlights, you have to place the light source at some distance. Again because e-ink is based on pigments, with both white and black ones present at the same time. You are supposed to only see those floating on top. If you'd shine light through it, you'd always get gray.

    This causes problems with building slim headsets. OLEDs panels are basically thin plastic film, microOLEDs silicon wavers, LCD very thin glass/plastic sandwiches of color filters and LED backlights, all a few millimeters thick at the very most, while a projector reflecting an e-ink display would probably require more like 10-50mm or more in total distance, depending on lens and configuration. This might be much more useful for smartglasses that already use LCOS as another reflective display type to project an image onto the inside of the glasses. Smartglasses would also work better with lower refresh rates.

    Again, I haven't read the paper, so maybe they solved all this. My gut feeling is more that they found a way to create microscopic pigments for use in e-ink displays, but there is no point to go beyond a few hundred PPI on regular displays. Retina displays are called that for a reason, our eyes cannot perceive single pixels at normal viewing distance, which is why nobody (besides Sony since 2015) bothers offering 4K smartphones. Lacking a need for ultra-high resolution e-book readers, they may have come up with XR displays as an application still in need of much more PPI.

    By the time they solved all the issues with using e-ink in HMDs, we will probably have replaced microOLEDs with superior microLED displays that are still nowhere near ready today. But maybe by then everybody will be wearing smartglasses with displays too, and this will turn out to be perfect for those.

  • psuedonymous

    This sounds like a higher density but monumentally slower DMD (which has actuation times in the kHz range). Like with DMD (and other reflective SLM)-based HMDs, reflective optical architectures don't tend to lend themselves well to high etendue systems, so you end up with either small FoV and small eyebox (or more often, both).

    Basically, it's LCOS but worse.

  • cacarr

    Sounds like the perfect tech for display wallpaper. Minority Report-style e-paper. Changeable camouflage surface material for military hardware. Or with some waterproof coating on top of it, change your car's paint job at will.