# Why not use monochrome LCDs instead of E-ink?

Recently I've been reading about E-ink displays and the benefit over LCDs, as I understand it, is that there is no backlight in E-ink displays and thus, physical light is reflected, making reading under direct sunlight possible.

However, this got me wondering, is there any difference, apart from power consumption, that would prohibit the use of monochrome, reflective LCDs (like the ones found in calculators, watches etc.) in eReaders, replacing the slow eInk screens?

This would enable us to have a nicer UI with animation.

• Contrast and power consumption. – PlasmaHH Jul 30 '15 at 12:36
• Power consumption is a major factor. Especially when you throw in animation. – Ignacio Vazquez-Abrams Jul 30 '15 at 12:36
• @IgnacioVazquez-Abrams I see... – Jason Barmparesos Jul 30 '15 at 12:39
• @IgnacioVazquez-Abrams, an LCD consumes quite a lot even when there's no animation, backlight being a major contributor. I wouldn't dismiss the question on consumption alone. – Dmitry Grigoryev Jul 30 '15 at 12:47
• @DmitryGrigoryev: The backlight isn't a factor in reflective LCDs, and you need to drive the segments harder for animation in order to prevent ghosting. – Ignacio Vazquez-Abrams Jul 30 '15 at 12:52

Thanks to a different recent question, I found that it has been tried and is called "memory LCD" and is used in the Pebble smartwatch.

The key is that normally LCDs are driven by a pulse every refresh period, and will fade if voltage is not maintained across them. Sharp have addressed this by adding to the conventional thin film transistors a memory element of some sort. Their power consumption for static images is very small, although not zero.

I think the LCDs are still necessarily glass, while e-ink displays can be slightly more flexible. This is a fragility disadvantage.

• E-ink displays also have better contrast. Since ther's no polarizing films as in LCD, the light has less obstacles to go through. Also, no polarization means the picture is visible from all angles, unlike on LCDs. – Dmitry Grigoryev Jul 30 '15 at 13:44

Check your calculator: the blacks are good, but the whites are very bad. This happens because the LCD is good at blocking light, not very good at letting it through. You don't notice this in normal LCD screens because the manufacturer can just ramp up the backlight until the whites are acceptable. Since eReaders are meant to imitate paper, bad whites are a big problem.

If you think about how LCDs produce images, this makes a lot of sense: light always passes through a polarizing filter that absorbs a lot of light, leaving only the "correctly" polarized part. The result is less light being reflected.

As I understand it, the advantage to E-Ink (besides improved contrast as mentioned above) is that e-ink displays only require power while refreshing (changing) the content on the page. Once the image appears, it will remain indefinitely without consuming any electricity (there may be a fade time or something, but I'm not aware of one). Thus the batteries in e-ink based readers can last a very long time between recharges.

True e-ink doesn't allow for animation, but I don't believe its initial intention was to be used as a "screen" so much as it was intended to be an alternative to disposable paper. My opinion is that e-readers were the first practical use for the tech. (Though, there was a cellphone a while back that used e-ink in the buttons to change them dynamically depending on whether the phone was in phone or texting mode -not sure if this was out before or after the e-reader explosion)

E-paper is only low power when it's off. Every time you update the image it draws a lot of power. For example, if you wanted to make a 7.5" e-paper clock, you'd need 38 mW for 6 seconds every minute, and that is:

$$\frac{365 \cdot 24 \cdot 60 \cdot 6 \cdot 38 \cdot 10^{-3}}{3600} = 33.28~Wh~(per~year)$$

That's a lot of energy!

On the other hand, there are lots of big LCD clocks on amazon, that run for years on a single AAA battery, and an AAA battery only contains 1.3 to 1.8 Wh.

So, the e-paper eats 25 times more!

• Welcome George - where does your "6 seconds every minute" come from? – awjlogan Oct 28 '18 at 13:45
• Two problems: You're not answering the question, you kinda agree with OP. The main problem is that if what you say is a major problem, every eBook reader would have an LCD screen - they don't. – pipe Oct 28 '18 at 16:10
• awjogan from: waveshare.com/7.5inch-e-paper-hat.htm "Full refresh time: 6s Refresh power: 38mW(typ.) " – George Of The Jungle Oct 29 '18 at 13:23
• pipe: Ereaders use epaper because it's got better contrast and is better to read, but if you were flipping pages like crazy it would eat much more energy than any LCD, yes. On the other hand, as the epaper is bistable, the ereader can enter deep sleep except for page flips, that's a plus. – George Of The Jungle Oct 29 '18 at 13:31

There are a lot of good answers already. I would like to add on to them the fact that eInk screens are not inherently slow. There was this Chinese company around 2010, that made eInk screens that can show 48fps videos -- however, they were not able to advertise themselves right, and they went bankrupt; their technology was bought out by Sony I believe for around \$1.2M, and then promptly discontinued. One example of a similar quick video playback on an eInk screen is https://www.youtube.com/watch?v=24srQXX81Oc (though that's not the one from that company).

The idea is, that the lag that's incurred is happening because the USB interface is using the "naive" way of sending of the image. The actual update speed of an eInk screen is close to 300 Hz (as measuring using an oscilloscope), but you have to update the screen in the correct order to remove the aliasing effect. Therefore, most eReader companies simply flash the entire screen black, white, black, white, and display the image -- which works, but it makes the image update very slow. If the USB driver is programmed right, eInk is capable of very quick updates.

One company that sells relatively quick eInk monitors which is not bankrupt right now is Dasung. In the 3rd generation of their displays, they have been able to reach approximately a 20fps update rate (with the exact rate depending on the complexity of the image). So eInk is not inherently slow: it can definitely do animation. But since people are buying them the way they are already, there is not enough incentive to really put in the development to make them display animation quickly.