How E-Paper Screens Show Text Without Using Power

An e-paper screen shows an image by moving tiny charged particles with an electric field, and it keeps the image on screen without drawing power. Electricity is used only when the picture changes. That is why an e-reader can show a page for as long as you like at almost no energy cost, and why the screen looks like printed paper rather than a glowing panel.

Reflected light instead of emitted light

A phone or laptop screen makes its own light. An e-paper screen does not. It reflects the light of the room, the way a printed page does. This is the main reason it is easy to read in sunlight, where a glowing screen can wash out.

What is inside

The best-known type is the electrophoretic display, and E Ink is the company most associated with it. In E Ink's description, the ink is made of millions of microcapsules, each about as wide as a human hair. Each capsule holds black and white particles floating in a clear fluid. Wikipedia describes one common arrangement: white titanium dioxide particles with a slight electric charge, suspended in dark oil, with the capsules sitting in a layer between two sets of electrodes. The finished layer is about 80 micrometres thick according to Wikipedia, roughly twice the thickness of ordinary paper.

E Ink's own page and Wikipedia disagree on small details, such as which colour of particle carries which charge. The principle is the same either way: the two kinds of particle carry opposite charges, so an electric field pushes them in opposite directions.

How a pixel changes

• To make a pixel white, the electronics apply a field that draws the white particles to the top of the capsule, where they reflect light.
• To make it dark, the field is reversed. The white particles go down and the dark side shows.
• To make grey, a smaller or shorter push leaves the particles part-way.
• When the field is switched off, the particles stay where they are.

That last point is called bistability. E Ink's page says only the pixels that change need updating, and Wikipedia says power is drawn mainly when the content changes. Static content such as a book page, a price label on a shop shelf or a sign costs almost nothing to keep on show.

Why e-paper updates slowly

Moving physical particles through fluid takes time. E Ink says an update can take as little as 120 milliseconds for the whole display, but that continual updates reduce the power saving and current production modules do not support video-rate updates. That is why e-paper suits text, and why page turns can show a brief flash or faint trace of the previous image. That trace is often called ghosting, and many devices clear it by refreshing the whole screen from time to time.

Adding colour

Wikipedia describes several routes to colour. One places a thin colour filter over a black-and-white layer. Another stacks coloured layers. E Ink describes its Spectra 3000 as using three kinds of particle in sealed cups called Microcups, and says its Spectra 6 and ACeP platforms use coloured particles directly, so they need no colour filter. E Ink shows the ACeP platform in 2016. Colour e-paper generally has fewer colours and slower updates than a normal screen, a trade-off the sources describe only loosely, so check a specific product's specifications.

A short history

The first electronic paper came from Nick Sheridon at Xerox PARC in the 1970s. It was called Gyricon and used tiny two-coloured balls that rotated, which is not the same principle as electrophoresis. The microcapsule version was developed by students at MIT in the 1990s, and a Nature paper by Comiskey and co-authors appeared in 1998. E Ink was founded in 1997 to bring it to market. Sony's Librie, released in Japan in 2004, was the first e-book reader with an E Ink screen. Amazon began selling the Kindle in late 2007, according to Wikipedia.

Where it is used

Wikipedia lists e-readers, shelf labels and signs as typical uses, all places where the image changes rarely. Wherever content must change quickly, like video or fast scrolling, an ordinary backlit screen is the better fit.

Sources

• How E Ink displays work, E Ink
• Electronic paper, Wikipedia