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About ten years ago there was a startup called PixelQi that developed a combination between an LCD and an e-ink screen. When using such a screen in normal (low-light) conditions, you would not notice a big difference to a normal LCD screen. Only in bright light conditions (e.g. in direct sunlight) or if you turned down the LCD backlight, you could see the black-white e-ink.

I bought two of the prototypes and used them for a long time. They had two main advantages:

  • You could turn down the LCD backlight to save battery. The e-ink screen only uses power when it changes its pixels.
  • You could work in bright conditions. Issues like reflecting sunlight were not an issue anymore. You could also read the screen without any problem in direct sunlight (although the screen appeared black-white then).

I always expected this startup to be bought by a major player like Apple or Samsung, who would then integrate the technology into their hardware lineup.

However, this did not happen, and the startup ceased to exist some years ago. I would like to know if there are technological reasons for that:

  • Does the technology bring any disadvantages that make it unfeasible for high-end laptops, tablets, phones and smartwatches, like for example making the screen too thick?
  • Would the screens become too expensive, even if manufactured in high numbers?

Are there any other disadvantages of the technology that led to it being abandoned?

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    \$\begingroup\$ The poor update speed of e-paper comes to mind. My guess is it would be unusable for device other than e-readers. It's even slower for multi-color screens in retail price tags. In contrast, my ancient laptop (aspire 3810T) already had a somewhat Transflective screen, meaning it works as normal LCD, but needs no backlight in direct sunlight. It's fully readable (yet somewhat greenish) when outside and all the pixels still update at the same speed as a normal LCD. \$\endgroup\$
    – akwky
    Nov 11, 2021 at 12:24
  • \$\begingroup\$ @akwky so your ancient laptop seems to have a technology similar to Pixel Qi's, but that technology seems to be abandonded too. When I hold my Thinkpad, my iPhone or my Apple Watch against the sun, I cannot really read off from them, so there seems to be no transflective material in their screens. \$\endgroup\$
    – cheesus
    Nov 11, 2021 at 13:14
  • \$\begingroup\$ I've stumbled upon a spec for the Qi display. elinux.org/images/3/3f/PQ3QI-01.pdf It seems it's not really "e-ink". Its still traditional TFT, but each pixel composes of 6 subpixels. The top 3 are the traditional transmissive R+G+B type, the remaining 3 are reflective bw only pixels. Thus you only "use" 50% area of your screen at a time, but can switch between "sunlight" 3072*600 bw, or "color" 1024*600. Picture quality always suffers, but is still great in sunlight. \$\endgroup\$
    – akwky
    Nov 11, 2021 at 14:35

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There seems to be a tradeoff between sunlight readability and picture quality in the "indoors" mode.

See this datasheet of a Qi screen. It explains how the pixels are arranged.

It seems the screen technology is not really "e-ink" in the modern sense. It is still a traditional TFT, but with 6 specially arranged subpixels.

The top 3 are common transmissive R+G+B pixels passing light through from the backlight. The remaining 3 are reflective black&white pixels which reflect light, thus only work when illuminated externally, but have a superior performance at direct sunlight. For some (technology) reason the pixels lack color filters, but still can be used to render text well, effectively achieving 3072 x 600 resolution.

In other words, the screen interleaves rows of "normal" LCD pixels with rows of sunlight-only readable LCD pixels (marked 1,2,3 in the picture).

Subpixel arrangement of the Qi display

Therefore you only ever "use" (or "see") 50% area of your screen at a time, but can switch between a "sunlight" 3072 x 600 bw, or a "color" 1024 x 600, whichever happens to perform better at present conditions. The picture quality always suffers compared to having a fully transmissive or a fully reflective panel, but in the end, you still get a better overall contrast across very wide lighting conditions.

(Yes, there was also a combined mode using all pixels, but essentially either the RGB or the 123 pixels will be too dim to really affect the image much.)

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It is my guess that consumer market (sadly?) prefers brighter picture quality over usability outdoors and lower power. At least for laptops.

PS: I saw watches with what looked like reflective (or transflexive) screens.

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  • \$\begingroup\$ So if the pixels were enough small, there would not be any disadvantages? In other words, if someone builds a "super-retina" screen with a pixel density of 4x (i.e. 16 physical pixels for a logical one), then you could change that screen into a Pixel Qi equivalent and get 8 physical pixels for one logical pixel. This would be nearly the same pixel density as a modern iPhone with its 3x "retina screen" which has 9 physical pixels per physical pixel. \$\endgroup\$
    – cheesus
    Nov 11, 2021 at 18:38

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