Electronic paper gets set for market

30 Jul 2002

Europeans will start to see electronic-ink displays in railway stations and mobile devices by this time next year, says the US company E Ink. Michael Hatcher examines the latest candidate to challenge the pencil and paper.

From Opto & Laser Europe July/August 2002

It's one of the great paradoxes of the modern age: more electronic information devices are available than ever before, but despite such supposedly paper-saving technology as PCs, mobile phones and personal digital assistants (PDAs), we are using up an increasing volume of paper.

There are several reasons why our portable electronic devices have failed to stem the paper flow. As well as not needing a power source, paper offers portability, low cost and excellent optical characteristics - paper and ink gives a good contrast, wide viewing angle and high brightness. No other portable information-display system can boast all of these qualities.

If we are to avoid churning up forests and generating reams of waste, we need to start using less paper. The ideal substitute is electronic paper - a technology offering all of paper's useful characteristics, but with the additional benefit of being reconfigurable.

Electrophoretic liquidIn the spring of last year, US company E Ink announced its development - in conjunction with Bell Labs - of electronically-activated flexible plastic sheets.

The "ink" in these displays is an electrophoretic liquid that migrates towards an electric charge. The liquid is sandwiched between two thin, flexible electrodes - the "paper". The application of voltage across the two electrodes attracts the contents of a liquid "microcapsule" (which acts like a pixel) towards one side or the other. This makes that area of the paper appear either white or black (see box for more details).

E Ink claims that the optical qualities of its electronic devices give them the edge over other technologies. E Ink's Darren Bischoff explained: "Unlike liquid-crystal displays (LCDs) and organic light-emitting devices, electronic-ink displays have a paper-like look with readability under all lighting conditions, even bright sunlight." This is because, in contrast with LCD technology, no additional optics such as polarizing layers or optical films are needed.

At the recent industry-leading Society for Information Display (SID) symposium in Boston, an E Ink research team led by Gregg Duthaler announced that it had made another leap forward - a colour display using electronic ink technology.

Colour versionsE Ink made its first rudimentary colour device last year - an 8-colour demonstration module with a segmented backplane. The more advanced colour display unveiled at SID works in the same way as the initial monochrome devices. The only difference is that a colour filter array (CFA) is carefully aligned on top of the display.

The CFA comprises red, green and blue stripes that run parallel to the columns of a thin-film transistor (TFT) array, which drives the patterns of pixels seen. Alignment between the CFA and these columns is crucial and requires precise control. Duthaler and colleagues first applied the electrophoretic optical film to the CFA and then thermally laminated it onto the TFT array.

The resulting colour display measures 5 inches on the diagonal and comprises a 320 x 234 pixel array, corresponding to a resolution of 80 pixels per inch. The CFAs are manufactured by the Japanese company Toppan, which is another of E Ink's strategic partners. Toppan has invested $30 m in E Ink, and will distribute the company's products in Japan.

According to Bischoff, this prototype device can display 4096 colours with a 4-bit greyscale. The switching speed is 150 ms. Bischoff says that the key innovation has been integrating the colour display with a high-resolution active-matrix backplane from Philips Electronics.

Travellers will see E Ink's monochrome displays around Europe from mid-2003. A distribution deal with Germany-based Vossloh System-Technik (VST) will result in information displays in airports and railway and bus stations across the continent. John Thorn, E Ink's director of business development, told OLE: "VST will produce two kinds of signs: alphanumeric character types, and dot-matrix types. They will be designed as modules that allow the final products to be scaled according to the job."

Thorn says that a key advantage of the new signs will be their extremely low power consumption (under normal use, this is expected to be at least one order of magnitude better than LCDs). Another is that the package is much easier to integrate owing to its reduced weight, its flexibility and the absence of any light sources to attach.

As far as passengers are concerned, the displays should be brighter and easier to read because of the improved contrast. This will be especially noticeable in bright daylight, which tends to "wash out" displays that use light-emitting technologies.

VST is currently in talks with a number of transport authorities about deploying E Ink signs, and there are said to be several pilot candidates in Europe.

E Ink is also partnering with Netherlands-based electronics giant Philips to produce monochrome displays that measure 3-8 inches on the diagonal by next year. Resolution will be at least 125 pixels per inch, with 40% reflectivity, a 10:1 contrast ratio and a 150 ms response time.

The target end-user applications for these displays are PDAs, mobile phones and portable computers. Philips and E Ink are also collaborating on the commercialization of the new colour devices from Duthaler and colleagues, expected in 2004.

The ultra-low power consumption of these displays should prove to be a great advantage over current technology. Typical LCD screens need a constant flow of energy to hold an image in place and, as everybody with a mobile phone knows, this means that frequent recharging is necessary.

The low power consumption of electronic ink arises because energy is only needed to change the display pattern, which should help batteries to last substantially longer. A recent study by E Ink and Philips researchers estimated that a 7 inch monochrome e-book based on the technology would consume around 1.5 mW. A colour display would demand three times that power (1.5 mW for each of the primary colours), but even this would represent a massive improvement over reflective active-matrix LCD devices.

So much for the technology - what about the cost? Conventional wisdom would suggest that a disruptive technology like this inevitably comes at a high price. Not so, says E Ink. The company reckons that its electronic paper can be made with easily scalable printing processes that do not require the hugely expensive fabrication facilities demanded for LCD manufacture.

But will electronic paper bring any reduction in our consumption of the real thing? Next year, we will begin to find out.

How E Ink displays work

The secret of E Ink's technology is chemistry. The "ink" is a layer of electrophoretic liquid sandwiched between a pair of flexible electrodes. Suspended in this liquid are millions of microcapsules. Each capsule is about 100 µm wide, representing a pixel, and contains a mixture of a dye and a pigment. The polarity of the electrodes causes the pigment to migrate to the top or bottom electrode, making the pixel appear light or dark. The display is fixed onto a TFT array, which drives the patterns of pixels seen.

E Ink says that its electronic paper appears six times brighter than reflective liquid-crystal displays. The contrast ratio of the monochrome devices is said to be better than newsprint, allowing readability in both dim and bright light.

Unlike LCD screens, the display can be read at any angle without a change in contrast. No backlight is required to illuminate the screen, as normal ambient light is sufficient. The thinnest device that has been made so far is a monochrome display with a thickness of just 0.3 mm - less than half that of a credit card.