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Display tech

I was playing with a Kindle the other day. It's got an e-ink display that's quite different from the usual mobile displays. For one, it works as a reflective display, much like the pages of a paper book. That means that you can read it without a backlight, unlike the LCD display you may have in your phone. The second big difference is that it's black & white only. Third, the screen can stay on and display an image without using any power. And finally, the refresh rate is very slow. So what's going on here? Why does it work that way?

E-ink displays have a tiny capsule for each pixel. This capsule is filled with black ink particles and white ink particles. The different colors have different charges. There's a charged plate underneath the capsule that can change the sign of its charge so as to attract one color and repel the other.

Let's say that you have black particles with a positive charge and white particles with a negative charge. When you set the charged plate to a positive charge, the white particles fly towards the charged plate at the bottom of the capsule and the black particles are pushed to the top of the capsule. As you're looking at the display from above, you see that the pixel turns black.

When you flip the charge, the black particles are drawn down to the bottom and the white particles are pushed to the top. The pixel now turns white.

Because the pixels use ink particles, the way they interact with light is much like you'd see with a regular sheet of paper. Light hits the ink at the top of the screen and bounces away towards your eyes. The black ink particles absorb more of the light than the white ones, and you see the resulting contrast. Nice and simple. But kinda slow, as the charging plate needs to flip its charge and the physical ink particles need to swap places for the pixel to change color.

By comparison, LCDs are quite a different beast. An LCD pixel is made out of three layers of material that polarize light. Polarized light is light where all the photons are vibrating in the same direction. In unpolarized light, the photons are vibrating in every which direction. To make polarized light, you either need a light source that produces polarized light or a filter that blocks photons that are vibrating in the wrong direction.

If you put two polarizing filters on top of each other, they do a bit of a trick. If the polarized filters are pointing to the same direction, they let light through. As you rotate them relative to each other, they let less and less light through, and once they're rotated to a 90 degree angle, they don't let any light through. What if you could change this rotation angle with electricity?

That's pretty much what an LCD pixel does. It's got a liquid crystal layer that rotates the polarization of light depending on how much electricity you put through it. To make the trick complete, it has two polarizing filters below and above the liquid crystal layer. The polarizing filters are at a 90 degree angle to each other, so when the liquid crystal layer is in the unrotating state, the pixel doesn't let any light through and appears dark. When the liquid crystal is set to rotate incoming light by 90 degrees, light passes through the pixel, making the pixel look bright.

Usually an LCD screen has a bright light behind it. The LCD in front blocks some of the light, creating the image that you see. Because the filters in the LCD are not perfect, some of the light hitting the dark pixels seeps through, making the darks brighter than ideal. Additionally, the black pixels are lit from behind at all times at the same intensity as the brightest white pixels, which makes LCDs relatively power-hungry. Because the liquid crystals can twist rapidly, LCDs have good refresh rates and work for displaying moving graphics.

OLEDs. OLEDs are tiny tiny lights, printed on the screen surface. The brightness of each of lamp is controllable by the amount of electricity fed to it. Because they're tiny individual lights, they are more power-efficient than LCDs as dark pixels don't use any energy at all. As each pixel is a tiny lamp, and black pixels are lamps that are turned off, the black levels of OLEDs are better than with LCDs. But because each pixel is a tiny lamp surrounded by circuitry, the total brightness achieved by an OLED display is worse than an LCD that can use a large powerful light behind the screen.

Ink, filters and lamps. That's what powers your mobile displays. Could you have something else? Maybe tiny waveguides that absorb a certain color of light - like the ones on butterfly wings - and have a controllable level of absorption? Something DLP-like where each display pixel is a mirror that either reflects incoming light back towards the viewing surface or away from it?

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