Color E-Paper That Rivals the Real Thing
Despite Amazon’s promise to reinvent the newspaper and magazine industry with its new, large-screen Kindle DX electronic reader, some people may be reluctant to embrace the technology until full-color displays are possible. A new approach developed by Philips now offers fresh hope for color e-paper displays that are so bright and clear that even traditional liquid crystal displays (LCDs) will pale in comparison.
According to Kars-Michiel Lenssen, who headed the work at Philips Research, based in Eindhoven, in the Netherlands, the new approach has the potential to create color images that are three times brighter than displays that use color filters, including LCDs. “This is the closest an electronic-paper technology ever got to printed paper,” he says.
Color displays normally require four subpixels–red, green, blue, and white–to create each full-color pixel. “That costs you in terms of resolution,” says Pieter van Lieshout, head of product research and development for Polymer Vision, which was spun off from Philips Electronics three years ago to develop flexible electronic-paper displays.
The other consequence of using a color filter is that it reduces the brightness of a display, says Sri Peruvemba, vice president of marketing at E-Ink, in Cambridge, MA, which was spun out of research at MIT in 1997. For example, making the entire screen red using subpixels means that only a quarter of the screen will actually be red.
In contrast, Philips Research’s approach involves turning the traditional electronic-paper pixel quite literally on its side, in order to tune it to different shades of the spectrum.
One of the most common e-paper technologies was created by E-Ink and is used for the monochrome screens in a wide range of devices, from Sony’s Reader and Amazon’s Kindle to Polymer Vision’s forthcoming foldable Readius. The technology employs electrophoresis: colored particles dispersed in a liquid that are controlled using an electric field. Each pixel is made of a microcapsule filled with a black oily liquid within which very small white particles are suspended. Because these particles are charged, they can be made to migrate to the top of the microcapsule–the surface of the page–by applying an electric field across them. The presence or absence of these particles at the surface of the screen acts like ink, changing the way that light reflects and giving it a lighter or darker appearance.
Philips’s technique, which is called in-plane electrophoretics, differs in that it involves suspending colored particles in a clear liquid and moving them horizontally instead of vertically. Each pixel is made up of two microcapsule chambers: one containing yellow and cyan particles, the other, below, containing magenta and black particles. Within each microcapsule, one set of colored particles is charged positively while the other is charged negatively.
By carefully controlling the voltages at electrodes positioned on the edges of the pixels, it is possible to spread the colored particles across the pixel or remove them from view altogether by hiding them behind the electrodes, says Lenssen. This means that different shades of color can be achieved by controlling how many of each group of colored particles are visible. To create white, all of the particles are simply shifted to the side to reveal the white substrate beneath the two microcapsules.
“It seems like a good approach,” says Polymer Vision’s van Lieshout. But he notes that the technology is still very much in its infancy compared with more traditional approaches, such as using color filters. Because of this, he believes that the first full-color e-paper displays will use filters.
Peruvemba agrees. E-Ink has explored the use of colored particles in the past, he says, but they are more difficult to manufacture. “Filters are probably the easiest way to get to the market–they are already used in LCDs, so it’s a lower cost solution compared to other approaches.” In light of this, he predicts that it will be at least three years before any nonfilter color e-paper technologies hit the market.
However, one aspect that could make in-plane electrophoretics more attractive: the fact that it relies on cheaper and simpler electronics to address the pixels. According to Lenssen, this not only offers distinct advantages in terms of ease of manufacturing: it also makes the screens more suitable for creating flexible displays.
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