Ask The Expert: When Can We Expect Better Color E-ink Screens?

Someone asked a question in the comment section of the blog the other day about color E-ink screens. He wanted to know when we would see a color screen on a note-taking device, but I misunderstood and thought he meant better color screens.

After I realized my mistake, I thought this was a great question that merited getting answered in its own blog post. So here it is:

When Can We Expect Better Color E-ink Screens?

Well first of all, I think we have a decent color E-ink screen now. The Kaleido screen on the Pocketbook Color is a major improvement over the Triton color E-ink screens. I think either ereader with this screen (Poke2 Color, Pocketbook Color) is worth buying if you want a color screen.

There is a chance we could get a larger screen, but I do not expect a better quality screen any time soon.

I would not wait for the next-gen color screen because its launch date cannot be predicted (such as Amazon pulling a rabbit out of their hat). A better screen is going to require a breakthrough in screen resolution and another breakthrough in the number of shades supported by E-ink screens (currently 16).

I will need to address those separately.

Screen Resolution

Kaleido screens are limited to 100 PPI because because the highest resolution screens which E-ink can put on an ereader top out at 300 DPI (dots per inch).

We do not usually use the term DPI with respect to E-ink screens, but I am using it here so I can explain how Kaleido screens differ from Carta screens. The reason I use 300 DPI is to make the point that the highest resolution screen E-ink can put on an ereader squeezes 300 dots or addressable locations into an inch of screen.

Edit: And those 300 dots are measured on the diagonal, not vertically or horizontally. (Check the math, and you’ll find I am right.)

If it helps, you can think of the 300 DPI as 300 boxes which can each contain one value.  Actually, I think that is a great analogy, so I will use it for the rest of this post.

If you wanted to use those 300 boxes for grayscale, you can have a 300 PPI Carta screen such as on the Kindle Paperwhite.

But if you want a color E-ink screen, you’re actually going to need to use three of those boxes for each color pixel, giving you a 100 PPI Kaleido screen.

The thing is, a color pixel is actually made up of 3 different color pixels (red, green, and blue), which means you need to have three different “boxes” for a color screen.

This usually never comes up for LED and LCD screens because everyone just assumes there will be RGB pixels, but with E-ink screens we are used to grayscale, where there is only a single color (shades of gray). We’ve spent years only thinking in terms of grayscale pixels, so it comes as a shock when we gain color but have to sacrifice screen resolution.

(BTW, the older Triton color E-ink screens required four “boxes” for each color pixel. That’s why it had such low resolution.)

In conclusion, we will not get a higher resolution Kaleido screen until E-ink can produce a higher resolution grayscale screen. Since that is by definition a breakthrough, there’s no way to predict when it will happen.

Color

The other major limitation for Kaleido screens is the number of colors supported. It can show 4096 colors, and due to the limitations of E-ink tech that is literally the best it can do at this time.

A pixel on an E-ink screen can display up to 16 shades of any single color. With most ereaders, that is 16 shades of gray (from white to black), which is great for displaying text (and really okay for displaying images).

But with a Kaleido screen we’re talking about 16 shades of red, green, or blue.

The reason the Kaleido screen can do 4096 colors is that the 16 shades of red crossed with the 16 shades of  blue crossed with the 16 shades of  green gives you 4096 colors. (Seriously, check the math.)

We’re not going to get additional colors on a Kaleido  screen until E-ink releases a screen which can do, say, 24 or 32 shades of gray. Since that is by definition a breakthrough, there’s no way to predict when it is going to happen.

* * *

Does that help any?

Nate Hoffelder

View posts by Nate Hoffelder
Nate Hoffelder is the founder and editor of The Digital Reader. He has been blogging about indie authors since 2010 while learning new tech skills weekly. He fixes author sites, and shares what he learns on The Digital Reader's blog. In his spare time, he fosters dogs for A Forever Home, a local rescue group.

10 Comments

  1. SAD16 August, 2020

    “But if you want to a color E-ink screen, you’re actually going to need to use three of those boxes for each color pixel, giving you a 100 PPI Kaleido screen. The thing is, a color pixel is actually made up of 3 different color pixels (red, green, and blue), which means you need to have three different “boxes” for a color screen.”
    Your math doesn’t check out. 300 ppi is 9 times as many pixels per square inch as 100ppi, not 3 times (90k vs 10k). Remember, we’re counting in two dimensions, not one. So each color pixel is made up of 9 subpixels, not 3.

    Reply
    1. Nate Hoffelder16 August, 2020

      I’m sorry, but that is incorrect.

      What you missed is that both the 300 PPI and 100 PPI stats are expressed in terms of the diagonal, not horizontal and vertical. I know that print resolution works the other way, but for E-ink screens we have always measured the resolution along the diagonal.

      Carta HD has a screen resolution of 1080 by 1440, right? Well, here’s the math on that:

      ((1080^2)+(1440^2))^.5 / 6 = 300 PPI

      Reply
      1. SAD16 August, 2020

        Hmm, I didn’t know that.
        But that doesn’t invalidate what I’ve said. If you’ve got 3 subpixels per color pixel along the diagonal, then that’s still a total of 9 subpixels per color 1 color pixel.
        Imagine a tic tac toe board. 3 fields diagonally, 9 fields total.

        Reply
        1. Nate Hoffelder16 August, 2020

          No?

          Reply
      2. Tom S17 August, 2020

        This is a really confusing way to put it.

        We have 1080 pixels horizontally, 1440 vertically. The screen is approximately 3-9/16” inches wide (my ruler only has resolution of 1/16”). So: 1080×16/57 = 303 PPI. Vertically, it is about 4-13/16”, so 1440*16/77 = 299 PPI. Both of these are effectively 300 PPI, without having to claim it is actually ‘diagonal’ resolution. (I hate the convention that screen size is reduced to ‘diagonal’ dimension)

        The color filter extends over these same dimensions, but at 100 PPI instead of 300 PPI. So instead of 1080×1440 it is 360×480. So there are 9 BW pixels for every color pixel. The number of colors has nothing to do with eInk but it determined by how many intensities R, G, B can assume (16). 4096 colors is theoretical only, color gamut is determined by physical properties of the color filter material and intensity and spectrum of LED and ambient lighting. There is no reason they could not increase the resolution of the color filter, but as the saturation and color gamut is limited, it probably doesn’t really make photos and color diagrams look that much better. It is not there so you can have color text.

        The question I have is whether color images only use color filter, or whether eink gets mixed in (so you could have bright and dull reds etc.).

        Reply
  2. Brian T17 August, 2020

    When you get the new color e-reader, can you try to take some zoomed pictures of the screens? I am curious what they are using for their subpixel layout.

    To answer SAD; PPI is a bit harder to understand in this context. Probably DPI (dots per inch, https://en.wikipedia.org/wiki/Dots_per_inch) is an easier way to understand it. The e-ink screen can make 300 dots per inch. Those dots can either be greyscale or a single color.

    One color “pixel” needs a red, a green, and a blue subpixel to be able to make any color. So you need three dots per color pixel. You can see it more clearly in an example picture: https://commons.wikimedia.org/wiki/File:LCD_RGB.jpg

    So if you have 300 dots in a square inch and you split them equally into 3 groups of color subpixels (red, green, and blue) you end of with 100 blue, 100 red, and 100 green subpixels per inch. This means you can only have at most 100 full color pixels in the square inch.

    Reply
    1. SAD17 August, 2020

      Brain T, I think you’re confusing inches with square inches. 300 pixels per inch does not equal 300 pixels per square inch.

      Reply
      1. Brian T17 August, 2020

        You are correct, it is not pixels per square inch. I don’t know why I switched to that at the end. I think the user “tired” below did a better job explaining that part than I did.

        I still think that dots per inch is probably a better way to describe the e-ink screen. Converting from DPI to Pixels Per Inch can be a little complicated when you actually look at the subpixels. An example is the pentile layout commonly used by OLED screens. These use “2.5” subpixels per pixel which makes the screen seem like it has less detail compared to the square pixels in a traditional LCD screen. If you want to see all the variations of subpixel layouts in modern screens, do a google image search for “subpixel layout”.

        (I don’t know why people have so much trouble typing Brian since it is a common name. I always just take Brain as a compliment.)

        Reply
    2. SAD17 August, 2020

      And sorry for mistyping your name just now. Not intentional.

      Reply
  3. tired17 August, 2020

    “Edit: And those 300 dots are measured on the diagonal, not vertically or horizontally. (Check the math, and you’ll find I am right.)”

    Pixels are square so if the pixel density is 300 ppi across any path on the screen, it is also 300 ppi for any other path. But hey let’s do the math.

    Screen width: .6*6 = 3.6 in
    Width resolution: 1080 pixels

    ppi = 1080/3.6 = 300

    Screen height: .8*6 = 4.8 in
    Height resolution: 1440 pixels

    ppi = 1440/4.8 = 300

    Reply

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