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Comment by makomk | Hacker News Reader

This is a neat trick: "The richness of the colors is achieved by having all the colored pigments in every picture element (pixel) rather than the side-by-side pixel colors achieved with a CFA. This eliminates the light attenuation, which can be quite significant." I believe this is why non-backlit colour LCDs don't work so well - too much light attenuation from the fact that the colours are side-by-side and so only let a third of the light through the colour filters.

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Any idea how they manage to control which pigments are shown?

In traditional B&W E-Ink, black pigments are negatively charged, while white pigments are positively charged, so it's fairly straightforward to control which ones to show. I wonder how they do it for color, since they would need presumably 3 or 4 primary colors

It's easy, in our digital world, to forget: electric charge is not binary. A gamut of pigments that responded to a range of charges would serve.

That doesn't answer his question though. Let's say they have 4 types of particles: CMYK. If the charges are as follows: C > M > Y > K, you will always have C arrive to the front first. How do you make only Y go the front to display pure yellow?

If it's like the video linked above (https://www.youtube.com/watch?v=N2V9iuTW3sA) it doesn't go straight from Y to C; the "pixel" has to cycle through all the other colours in between.

Maybe some kind of prism that flips one face at a time when it gets a pulse?

The other thought that came to mind would be some sort of resonance system... send the right frequency, and the cyan (e.g.) particles bob up and down hard enough to reach the surface, where they'd stick. Each particle would have its own resonance frequency.

This looks like it may be the patent in question:

https://www.google.com/patents/US8797634

So in summary, there are two layers. Top has white and magenta, bottom has cyan and yellow.

The magenta particles let light through, the others do not.

The top layer can put white on top to make white.

The top layer can pull all the particles to the side to be completely clear. This allows yellow/cyan/green depending on the state of the bottom layer.

The top layer can put magenta on top and white on bottom to make magenta.

The top layer can put magenta on top and pull white to the side to be clear magenta. This allows red/blue/black respectively, when the bottom layer is yellow/cyan/green.

Edit: But the initial article specifically says "The display utilizes a single layer of electrophoretic fluid" so it sounds like it's using a different tech.

Hmm, the press release mentions that only " a single layer of electrophoretic fluid" (and also only a single backplane) is employed. Therefore, the mechanism must be a little bit different than the one described in the patent.

I was wondering the same thing. Just speculating about possible mechanisms: if the different colors have different particle sizes, then they'll move at different speeds. So if you modulate the strength and duration of the charge that you apply, I think you'd have some control over which colors end up at the front of your pixel.

I don't know how you'd do different shades of 3/4 colors though.

That's interesting. Perhaps the colors respond to different frequencies. They did say they use all 8 primaries which would include black and white. So it's a question of how they proportion them at the front of the pixel.

> They did say they use all 8 primaries

No, they say they can display all 8 primaries.

White background and CMYK pigments ?

> too much light attenuation from the fact that the colours are side-by-side and so only let a third of the light through the colour filters

If you mix the pigments, isn't the effect really the same, but on a microscopic level rather than a macroscopic level?

> If you mix the pigments, isn't the effect really the same, but on a microscopic level rather than a macroscopic level?

No. It's the difference between colour addition and colour subtraction.

In a colour-addition (ie LCD) model, the maximum possible light transmission - what you get by displaying 'white' - is 33%, since it involves R, G, B side by side, all on. But in a colour-subtraction model (ie e-ink) you just remove all pigment and get 100% light transmission.

Another example: pure yellow. Addition: R, G on full, B off; total light transmission: (33% * 2/3 =) 22%. Subtraction: all yellow pigment, total light transmission: 66%.

Another example: pure red. Addition: R on full, G, B off; total light transmission: (33% * 1/3 =) 11%. Subtraction: yellow and magenta pigment mixed, total light transmission: 33%.

(to be clear, the numbers are waay over-simplified naive colour theory, not actual measurements or anything)

No, because it lets you toggle each pigment between colored and (nearly) transparent. An LCD needs to be colored or opaque.

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