No fewer than half a dozen different technologies are emerging from laboratories to compete to be the e-reader screen of the future. The stakes are high: Research firm DisplaySearch estimates that the market will near US $10 billion by 2018, powered by a compound annual growth rate of 41 percent.
Illustrations: Emily Cooper
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To understand the technical challenges, first consider where we are today. Today’s electronic readers, such as the Amazon Kindle and the Sony Reader, meet two of my criteria for the ideal e-reader: They’re easy to read in bright light and use minimal power. These monochrome displays, sometimes called electronic paper or e-paper, use a kind of electrophoretic technology developed by E Ink Corp., a company in Cambridge, Mass., that was spun out of the MIT Media Lab in 1997. An electrophoretic pixel comprises numerous tiny capsules that contain a mixture of oppositely charged pigment particles, typically carbon for black and titanium dioxide for white. A voltage attracts or repels the pigment particles within the capsules from the screen, depending on whether a white or a black pixel is needed at that spot. Like mixing paints, with the right voltage control the system can also leave the particles in a partially mixed, or grayscale, state. It doesn’t need much power, because the pigments simply reflect—or don’t reflect—the ambient light, and they don’t need any power to maintain their most recent state. An electrophoretic display takes 200 milliseconds to switch images. So if the image on the display changes every 60 seconds, in 1000 hours of continued use the display would effectively draw power for only about 3 hours.
E Ink has spent over a decade getting to this point and is still refining the basic technology. But already these displays are really simple to produce. Manufacturers purchase ready-made film containing the pigment-filled capsules and simply laminate it to an underlying panel that carries the drive circuitry. The first generation of E Ink displays used silicon transistors and glass panels; the second, due this year, will use organic transistors and plastic panels. This second generation includes Polymer Vision’s Readius and Plastic Logic’s Que; the Readius is literally paper thin, and it can be rolled and unrolled tens of thousands of times.
Now for the downside. Electrophoretic technology has limited potential for displaying full-color images. That’s because it hasn’t really solved the brightness challenge. Imagine that you’re going to paint a wall white that’s now a very dark brown. You’ll need at least three coats of white paint to cover that brown. Electrophoretic pixels have a similar problem, because the black particles are never fully hidden by the white ones. So the white reflectance is only about 40 percent, compared to 80 percent for a sheet of paper.
If you try to get around this problem by using more particles, you run into problems with switching speed. Electrophoretic pixels already switch slowly because the layer of electrophoretic ink is relatively thick, about 40 micrometers, and the voltage applied to the pixel must be spread across the entire thickness. The level of liquid crystal material in LCDs is only a few micrometers thick, and that’s one reason they’re so much faster. Electrophoretic technology also can’t do video; the switching speed is just too slow.
Want color? The current approach is to add a red-green-blue color filter array over the pixels. The problem is that this reduces the brightness by a factor of three, because each primary color filter passes through only one-third of the visible spectrum of light. So, at each color pixel the display can reflect only 10 to 15 percent of the available light. The first color electrophoretic displays, expected to reach consumers late in 2010, will use very weak color filters; this will crank up the brightness at the cost of color saturation.
For bright, full-motion color images on portable screens, LCDs dominate. First developed in the early 1970s and almost continuously improved since then, LCDs are hard to beat for almost any characteristic except efficiency. That’s why Time Inc. recently presented its futuristic concept version of an electronic Sports Illustrated on a standard LCD, and Apple’s new iPad sticks with this established technology.
LCDs are energy hogs for several reasons. For one, an LCD works by polarization, which means that at least 50 percent of available light is lost because it doesn’t pass through the polarizer. It loses more light to color filters, ultimately wasting about 90 percent of the light from its backlight. So the backlight has to be intense, and it saps power, but that’s the only way you can get a bright, crisp, vivid image. The upshot is that LCDs convert electricity to viewable light with pitifully low power efficiency—just 2 to 3 percent.
Worse yet, the readability of both LCDs and the newer organic LED displays, which must also rely on electrically generated light, dramatically deteriorates outdoors. The displays simply cannot compete with direct sunlight, which is about a thousand times as bright as typical indoor lighting. Even a slight sunlight reflection is far brighter than the light coming out of an LCD screen.
The final blow against LCD as the ultimate display technology is that for many people, long-term viewing of an LCD strains the eyes. E-paper displays generally don’t cause eyestrain because they automatically reflect—literally—the brightness of your surroundings.
So today’s e-paper has readability and low power, and LCDs have brilliant colors and full video motion. Is there a technology that can do it all? A few of the contenders are bistable liquid crystal, cholesteric liquid crystal, microelectromechanical systems (MEMS), electrowetting, and electrofluidic technology, as well as new generations of electrophoretic technology.
These technologies exploit radically different principles and offer varied features. None of them yet provide the ultimate 2020 display experience of low power, readability, bright color, and full-motion video. But at least a few of them are getting close to providing color e-paper that would be as bright as the monochrome Kindle.