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About E Paper
E-Paper displays have hundreds of thousands of tiny capsules each with different color e-ink particles, typically black and white ones. Since each color of particle has a different charge, electric fields are applied to the top and bottom of these capsules to move the desired particles to the front or back of the display.
This is a significant simplification of a complicated process but what is important to know is once the particles are moved, they stay in place after the electric field is removed. Once an image is drawn on the display, the display controller cuts power to the display, but the image remains. This makes the displays highly energy efficient. At that point, only the controller is consuming power while it waits for more commands.
The e-ink particles are reflective. That means that no backlight is needed for viewing a display in normal lighting conditions and makes the display easy on the eyes. This is why you can bring your Kindle to the beach, sit in full sunlight and actually see it better than indoors. That is not the case with typical self illuminated display technologies. This too adds to the energy efficiency of the display.
The images will remain on the display with no power for quite a long time. After more than 24 hours, the static charge holding the particles in place will start to decay, and the particles will begin to drift away from their intended location. It is recommended to refresh the display at least every 24 hours.
The static charge holding the particles in place does a very good job. So much so that a simple reversal of the electric fields will not always move all the particles. To address this, the controller will alternate the fields to coax all the particles into their new location. Visually, you will see the display flash black and white during a screen update. This prevents ghost images of previous screens from building up over time.
All of this take a bit of time. Add to that the limited speed of the Raspberry Pi's SPI bus, and the amount of bitmap data contained in a single display image and, depending on the display, it can take several seconds for a new image to be shown.
E-Paper displays also have a limited life in the area of around 1,000,000 refreshes. While this sounds like a lot, it means that updating the screen every minute of every day will bring the display to end of life in just under one year.
There are pros and cons to an e-paper display but in specific applications, they excel over LCD and OLED. Factor in their attractive price, and if you have a circumstance that an e-paper display can be used, the choice is clear.