Electronic paper is a display device designed to look as similar as possible to ink on paper. Sometimes also referred to as electronic ink or e-ink, electronic paper is an extremely low power screen technology. The content on the screen is entirely visible through the reflection of ambient light rather than being lit by a backlight or producing light directly as with other screen types.
Traditionally, e-ink displays attempt to look like paper as much as possible and display dark text on white backgrounds. There is, however, nothing stopping these devices from displaying white text on black backgrounds.
It is possible to create colour e-ink displays using a technology called “electrowetting”. Electrowetting uses an electric charge to manage an oil film above the pixels. Careful management of the oil film can create cyan, magenta, and yellow pixels.
E-ink displays generally work by having millions of tiny capsules. In these capsules, there are two electrically charged pigments, one white, one black. By applying an electric charge across the capsule one of the pigments rises to the top while the other sinks to the bottom. Only the top pigment is visible to the reader. For example, if the black pigment is negatively charged then it will be attracted to the top of the capsule when the top electrode has a positive charge. Many tiny capsules make up a single pixel in an e-ink display.
One of the key power-saving points of electronic paper is that the capsules are bistable. This means that they are stable in both configurations, displaying black or white. This stability means that once a charge is applied to change the colour of a capsule, it doesn’t need to be maintained or refreshed until the capsule needs to change colour again.
In some situations, not all capsules neatly change colour when they are supposed to. This can leave “ghosts” of the previously displayed content. To counter ghosts, e-ink displays can refresh some or all of the screen by flashing it black, then white etc.
Uses of electronic paper
The main market for e-ink is in e-readers, where the minimal power draw can let the devices run for weeks from a single charge. Other more recent examples of places that electronic paper displays have been used are electronic shelf labels and public transport timetables.
Shelf labels are typically only updated occasionally, spending the time and money buying paper and printing them before then regularly throwing them away costs a reasonable amount of money over time. In comparison, an e-ink label can be easily updated and reused, potentially saving money in the long run.
Public transport timetables, such as bus or train schedules, and other types of sign, rarely update. E-ink displays are ideal for this use case as they have an extremely low power usage but can also be easily and quickly adjusted when needed.