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Mirasol screens, which are expected to appear in e-readers later this year, are composed of Interferometric Modulator (IMOD) elements. Each element is made of two conductive plates. One is a thin film stack on top of a glass substrate, and the other has a reflective membrane. The height of the air gap between the plates determines the color of light that is reflected from the IMOD. When a voltage is applied, the plates are drawn together by electrostatic forces and the element goes black. When the voltage is removed, the plates separate and color is reflected off the IMOD. A single pixel is made up of several IMODs; adjusting the height of each affects the overall color of the pixel. The plates stay in place, using almost no energy, until the color needs to change again. A plate only has to move a few hundred nanometers to change color and can do it in tens of microseconds–fast enough to show video.

The LCD-based screens from Kent Displays feature technology that is very different. “Our material is transparent, so we can put three layers on top of each other,” explains Asad Kahn, the company’s chief technology officer. “One is red, one is green, and one is blue.” In contrast, IMOD elements have to be placed side by side. Kahn says the layering approach ultimately leads to a brighter display. And unlike Qualcomm, Kent’s technology is already on sale. The Fujitsu FLEPia color e-reader, released last spring, features the screens. Unfortunately, the refresh rates aren’t yet fast enough for video.

Liquavista announced two color e-reader screens of its own this week. Both the LiquavistaColor and the LiquavistaVivid are readable in sunlight, but the latter will also include a backlight for more vibrant hues. The screens are slated for release in 2010 and 2011, respectively. The LCD devices are based on a technique called electrowetting, in which a voltage is used to modify the surface tension of colored oil on a solid substrate. In the absence of a voltage, the oil forms a film over the substrate and is visible to the viewer. When a voltage is applied, the pixel becomes transparent. By controlling the voltage of each pixel independently, a picture can be displayed. Unlike E Ink’s technology, electrowetting pixels can be switched in a few milliseconds, making them suitable for showing video.

With so many video-capable e-reader screens on the horizon, E Ink has decided to focus solely on one application: reading. But its upcoming devices will feature color screens. Sri Peruvemba, the vice president of marketing at E Ink, says the company will have color devices out by the end of next year. Unfortunately, the refresh rates are too slow for video. “We have animation that we can do today, but we can’t do full video speed,” Peruvemba says.

So while their competitors will likely slice up the market for smart devices with Internet and video capabilities, E Ink plans to go after the education market. The company will make “dedicated” e-readers for computer textbooks, Peruvemba says, adding that the color should add to the experience. But the devices will intentionally omit any distracting applications, such as a phone or Web browser.

“If I give one of these devices to my daughter and I know she’s going to make phone calls on it and surf the Internet on it, I’m not going to be motivated to buy it for her,” he says.

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Credit: Qualcomm MEMS Technologies
Video by Stephen Cass, edited by Brittany Sauser

Tagged: Computing, displays, LCD, E ink, e-reader, screens

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