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Comment by swagonomixxx | Hacker News Reader

I am desperately waiting for semi-high refresh rate (20Hz-30Hz) e-ink displays so that I can replace my traditional HD display with them.

They're significantly easier on the eyes and they're a joy to use. I don't particularly care about color. One thing I'm wondering though, is what are the technical challenges behind building such a display?

Ideally, I'd like to be able to use an Android device that renders onto e-ink. I don't particularly care for watching videos and such - most of my time on my phone is spent sending and reading IM's (Signal, text, e-mail) rather than viewing media, so lower refresh rates is not something I really care about.

Where are the shortcomings causing e-ink displays to have very low refresh rate? Does it have to do with modern rendering technology being so well optimized for color displays that they simply are not performant for e-ink?

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Here's how these work:

1) Take a fluid, color it mostly white(ish).

2) Suspend a bunch of tiny black "balls" inside this fluid. The balls need to be charged so they can be attracted to an electric field.

3) Squish this fluid between some glass.

4) Put a grid of electrodes on the back that you can address.

5) Now, using some clever AC patterns, you can address any "pixel" on your grid, and apply a charge there, that will attract or repel the little balls.

6) When you attract the balls, that spot turns black, because you just see the balls above the fluid.

7) When you repel the balls, that spot turns "white" because you just see the fluid, not the balls.

The trick is that moving the balls back and forth (6/7) is a physical process, and requires time. They _have_ to move a physical distance, and if that fluid lets them move too easily then they won't keep their color. The balls will move and they'll just drift back to some middle ground.

Also, if you're not careful with your approach, you can mess with the charge of the balls, and they'll no longer respond to the field the way you want. This is burn-in. (And it's why displays do full-black-to-full-white refreshes from time to time)

So until we can figure out how to move those balls faster in the fluid, we really aren't going to dramatically improve refresh rates with this technology.

Note: "Balls" is a simplification.

Thanks for this explanation, really interesting

Thank you for the simple explanation of this tech!

I wonder if it will be possible in the future to apply a similar concept for for a 3d e ink desplay where the grid pulls cells through a 3d medium.

3D doesn't really work that way.

If you want a volumetric display, maybe it will work, but the display itself will need to be the same size as the total volume of depth you want to create, e.g. digital objects can not appear outside the screen.

Due to the fact that eInk is a non-backlit technology, I can't imagine creating a glasses-free stereoscopic effect using eInk.

Take a look at the Onyx Boox Max series:

https://onyxboox.com/boox_max3

They're 13.3" displays, work with Android and you can use them as an external HDMI display. However, the refresh rate is still poor, not nearly 20-30Hz. The Technology Connections Youtube channel has made two very interesting videos, showing the many caveats of the device (it's an older model, but I'd guess most of it still applies). The main issues are poor software and slow CPU:

https://www.youtube.com/watch?v=7NfX0vlCa4k

I recommend going to https://mobileread.com/forums to read about user experiences with Onyx and other makers. They are interesting devices but the support level is not at the same level as a Kobo/Nook/Kindle. I used to have a Boyue Likebook Mimas, which was a great ereader hobbled by a slow processor and itchy software.

As far as I understand the technology, it's slow because you are physically pushing/pulling pigments around in little cells. It's simply not as fast a process as passing voltage across a crystal or shooting an electron beam.

> Ideally, I'd like to be able to use an Android device that renders onto e-ink.

You can buy one now. The money will (probably) be used to improve this technology.

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