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A Sony ebook reader with an E Ink® display

How does E Ink® work?

by Chris Woodford. Last updated: November 20, 2013.

The world's great libraries are stunning stores of knowledge, but sometimes they might as well be in space, because much of the information they contain is so far beyond our reach. There's no easy way to search through billions of pages of old and dusty paper—and the problem gets worse every year as millions more books are added to the pile. Fortunately, the development of computers from the mid-20th century led to better ways of storing information so that it could be indexed, searched, and retrieved more quickly, while the invention of the World Wide Web meant anyone could access computerized knowledge from anywhere. In the last few years, paper books have started to be replaced by ebooks such as the Amazon Kindle™ and Sony Reader. Portable devices that can hold books are nothing new: they've been around since Sony launched its original attempt, the Data Discman, in 1990. What made them finally take off was the development of light and legible displays using electronic ink and paper. How exactly do they work? Let's take a closer look!

Photo: Ebook readers (like this Sony Reader and the Amazon Kindle) use electronic ink displays made by E Ink Corporation. They're lightweight, consume very little power, and are easy to read even in bright sunlight.

What is electronic ink?

Screenshot of an early pixelated space invaders arcade game.

You're probably reading these words in the same way that I am—by staring at a flat, LCD computer screen. For people over the age of about 35, who grew up with computers that used blocky green and black screens with just 40 characters across and 25 down, modern screens are wonderful and amazing. But they still have their drawbacks. Look closely, and you can see jagged edges to the letters. Try to read an LCD screen in direct sunlight and (unless the screen has a very bright backlight), you'll really struggle. But the worse thing is that LCD screens lack the lightness, portability, and sheer user-friendliness of ink-printed paper: you can happily read a book for hours, but try the same trick with a computer screen and your eyes will quickly tire.

Photo: Computer screens as we knew them in the late 1970s and early 1980s. At that time, the best screens could display no more than about 64,000 pixels and often just uppercase text or very crude "pixelated" (square block) graphics. Computer games like Space Invaders, shown here, were very primitive—but still highly addictive!

A comparison of a word printed on an LCD and an E Ink display

Back in the early 1970s, the Xerox Corporation that had pioneered photocopiers a decade earlier became concerned about the threat that computers might pose to its core ink-and-paper business: if everyone started using computers, and offices became paperless, what would happen to a company so utterly dependent on paper technology? It was for that reason that Xerox pumped huge amounts of money into PARC™, (Palto Alto Research Center), the now-legendary campus where modern, user-friendly personal computing was pioneered. Among the many innovations developed there were personal computers that used a graphical user interface (the "desktop" screen featuring icons, later copied by the Apple Macintosh® and Microsoft Windows®), Ethernet networking, laser printers... and electronic paper, which was invented by PARC researcher Nick Sheridon.

The basic idea of electronic ink and paper was (and remains) very simple: to produce an electronic display with all the control and convenience of a computer screen but the readability, portability, and user-friendliness of paper.

Photo: The E Ink® display on a Sony Reader (bottom) is much sharper and easier to read than a typical LCD screen (top). Magnifying by about 8–10 times and zooming in on a single word, you can see why. The E Ink display makes sharper letters with a uniformly white background. The LCD display blurs its letters with anti-aliasing to make them less jagged, though that makes them harder to read close up. The red, blue, and green colored pixels used to make up the LCD's "white" background are also much more noticeable. Unlike the E Ink display, an LCD does not use a true white background: it relies on your eye and brain to fuse colored pixels instead. The resolution of E Ink is also far greater: typical LCD displays use around 90 pixels per inch, whereas E Ink displays use at least twice as many pixels.

How does electronic ink and paper work?

A simple animation showing how electronic ink works through electrophoresis: black and white ink capsules move under the control of an electric field.

Most electronic ink and paper screens use a technology called electrophoresis, which sounds complex but simply means using electricity to move tiny particles (in this case ink) through a fluid (in this case a liquid or gel). Other uses of electrophoresis include DNA testing, where electricity is used to separate the parts of a DNA sample by making them move across a gel, which enables them to be compared with other samples and identified.

In one of the best-known electronic ink products, called E Ink® and used in ebooks such as the Amazon Kindle, there are millions of microcapsules, roughly the same diameter as a human hair, each of which is the equivalent of a single pixel (one of the tiny squares or rectangles from which the picture on a computer or TV screen is built up). Each capsule is filled with a clear fluid and contains two kinds of tiny ink granules: white ones (which are positively charged) and black ones (which are negatively charged). The capsules are suspended between electrodes switched on and off by an electronic circuit, and each one can be controlled individually. By changing the electric field between the electrodes, it's possible to make the white or black granules move to the top of a capsule (the part closest to the reader's eye) so it appears like a white or black pixel. By controlling large numbers of pixels in this way, it's possible to display text or pictures.

Animation: Electronic ink works through electrophoresis. Each pixel (microcapsule) in the display (the gray circle) contains black (negatively charged) and white (positively charged) ink granules. When a positive field (shown in blue) is applied to the top electrode, the black capsules migrate to the top, making the pixel look black when seen from above; switching the field over makes the granules change position so the pixel appears white.

Advantages and disadvantages

If you've tried reading an electronic book, you'll know that electronic ink and paper is much easier to read from for long periods than an LCD computer screen. Since the microcapsules stay in position indefinitely, with little or no electric current, electronic ink displays have extremely low power consumption. A typical ebook reader with an E Ink display can be used for something like 2–4 weeks of average everyday reading on a single charge. Low power consumption means low energy use and that translates into an environmental benefit; in other words, electronic ink and paper is environmentally friendly.

The disadvantages are less obvious until you start using electronic ink in earnest. First, although the displays work excellently in bright indoor light and daylight, including direct sunlight, they have no light of their own (unlike LCD displays, which have backlights shining through them from back to front so you can see them). That makes them hard to use in poor indoor light, especially in the evenings, which is why many ebook readers are sold with clumsy addon lamps. Electronic ink also takes much longer to build up the image of a page than an LCD screen, which means it's unsuitable for everyday computer displays using any kind of moving image (and completely unsuitable for fast-moving images such as computer games and videos). Sometimes parts of a previous page linger on as "ghosts" until you've turned another page or two. You've probably noticed that, when you "turn the page" of an electronic book, the entire screen momentarily flashes black before the new page is displayed? That's a rather clumsy compromise to prevent ghosts, in which the screen tries to erase the previous page before displaying a new one (a bit like Etch-a-Sketch®!).

Another major disadvantage is that most electronic ink displays are currently black and white. Color displays do exist (E Ink produces one called Triton, in which a layer of red, green, and blue color filters is mounted over the usual black-and-white microcapsules) and better ones are in development, but they're much more expensive than their black-and-white electronic paper (or LCD equivalents). In time, we're bound to have color electronic books and magazines; according to Amazon's Jeff Bezos, however, speaking in mid-2009, a color Kindle ebook reader is "multiple years" away: "I've seen the color displays in the laboratory and I can assure you they're not ready for prime time." (Amazon's color Kindle Fire™ product, announced in September 2011, uses an LCD display.)

A comparison of an E Ink ebook display and an LCD screen in bright sunlight A comparison of an E Ink ebook display and an LCD screen in dark, indoor evening light

Photo: Night and day, are you the one? Here I've propped a Sony Reader against the screen of a conventional laptop and photographed it in different light conditions. Left: In bright light or daytime outdoors, electronic ink displays are much easier to read than backlit LCD displays, which become virtually invisible. Right: In dark indoor light in the evenings, things are reversed: LCDs are much easier to read and electronic ink displays are a struggle to decipher unless you sit in strong light (or use a clip-on light attachment).

What is electronic ink used for?

Electronic books are still by far the biggest application, but there are as many potential uses for electronic paper and ink as for their ordinary equivalents. Billboards, point-of-sale shop displays, grocery store labels that change their prices automatically, and street-side bus and railroad timetables that can be updated remotely from the depot are just a few of the likely applications we'll see in time, especially when color technology becomes more affordable.

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Text copyright © Chris Woodford 2012. All rights reserved. Full copyright notice and terms of use.

E Ink is a trademark of E Ink Corporation.
Etch a Sketch is a trademark of The Ohio Art Company Corporation.
Kindle is a trademark of Amazon.com, Inc.
Kindle Fire is a trademark of Seesaw LLC.
Macintosh is a trademark of Apple Inc.
PARC is a trademark of Palo Alto Research Center Incorporated.
Windows is a trademark of Microsoft Corporation.

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Woodford, Chris. (2012) Electronic ink. Retrieved from http://www.explainthatstuff.com/how-electronic-ink-works.html. [Accessed (Insert date here)]

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