Three-Minute Tech: Electrophoretic ink

[In our Three-Minute Tech series, we tell you everything you really need to know about a technology in three minutes or less.]

If you have a dedicated e-reader, such as a Kindle or a Nook, you’re using a technology called electrophoretic ink, made by E Ink, to read your favorite books. Unlike other display technologies, such as LCD or CRT (remember them?), electrophoretic ink uses no backlighting.

The lack of a backlight has some notable benefits. For one, it means that electrophoretic ink can use much less power than other types of displays. Devices using E Ink-based displays can run for weeks without recharging, and their batteries are much smaller and lighter. That leads to smaller and lighter readers. For comparison, the latest iPad weighs 652 grams (nearly 23 ounces). The smallest Kindle, with a display about half the size of the iPad, only weighs 168 grams (less than six ounces).

How it works

Picture a screen covered with marbles, where one side of each marble is black and the other white. When the marbles are turned with the black side facing outward, you see black on the display; turn them the other way and you see white. Now, picture millions of tiny marbles, each with a diameter around that of a human hair. These marbles, or microcapsules, are so small that some 100,000 of them fit in a square inch.

1. Upper layer; 2. Transparent electrode layer; 3. Transparent micro-capsules; 4. Positive charged white pigments; 5. Negative charged black pigments; 6. Transparent oil; 7. Electrode pixel layer; 8. Bottom supporting layer; 9. Light; 10. White; 11. Black

Each microcapsule contains black and white pigment chips floating in oil. The white pigments are positively charged, and the black are negatively charged. An electric field under these microcapsules is positively or negatively charged, attracting or repelling the pigments to make the microcapsule appear while, black, or grey. The current E Ink Pearl display (used in all non LCD-based e-readers) offers 16 levels of gray, but the technology hasn’t yet managed to display actual white, because of the other layers of plastic the display contains. Once a page is rendered, no power is required to keep displaying it; those miniature marbles stay in position until you change the page again.

Trade-offs

There is a trade-off, though, between the low power required for E Ink-based displays and a backlit screen, such as on an iPad. While electrophoretic ink is fine for reading in situations where you have bright light—outdoors during the day, or with a reading lamp—it’s much less usable in low-light environments. Because of this, companies can advertise that you can read your ebooks on the beach, something you’ll find difficult with an iPad.

On the other hand, electrophoretic ink displays can’t hold a candle to the iPad if you’re reading in bed and don’t want a bright reading light shining on your screen. To counter this limitation, Barnes & Noble released its Nook Simple Touch with GlowLight, which integrates a reading light into the case design. And of course, makers of e-readers (and lots of third-party accessory makers) offer add-on lighting products as well.

The individual elements in an E Ink display also change shades far more slowly than the pixels of an LCD screen, so they're not well suited to moving graphics or video.

(E Ink-based displays aren’t only for e-readers: there are watches and other types of devices that use this technology for their displays. You may also find them in supermarkets, as signage on shelves showing product prices.)

The future

Electrophoretic ink hasn’t yet made the cut for color, though researchers are working on it. So right now, if you want to read books that contain pictures, even in black and white, a backlit screen is optimal. E Ink-based displays can form simple black and white or grayscale pictures or graphics, but that’s all.

What can we expect from electrophoretic ink in the future? LG is finally producing a flexible display that could lead to e-readers less than 1 millimeter thick that we can just toss into our pockets and not worry (too much) about breaking. As these devices get cheaper, they may even become the razors to the blades that are ebooks and magazines: companies may just give them away when you subscribe to a magazine or commit to buy a few ebooks.

While some users will find that the contrast is insufficient unless they’re in bright light—or buy a model with a built-in light—electrophoretic ink displays beats an LCD hands down in bright sunlight. An E Ink-based ebook reader is much less expensive and lighter than a tablet like the iPad, but is only designed for one use: reading books with mostly text. If you’re a heavy reader, then electrophoretic ink may be your best friend.

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