Electronic book readers may be the future of publishing, but in one important respect, they're still stuck in 1950: Almost every e-book reader on the market has a black-and-white display. Most can't display more than a handful of different shades of gray.

That's why display makers are racing to bring color to the world of e-books. Their goal is to make Gray's Anatomy and its more than 1,200 full-color illustrations as interesting as the next Dan Brown novel.

The hitch is that color e-ink technologies aren't anywhere near ready for prime time. Amazon chief Jeff Bezos recently told shareholders that a Kindle with a color screen is "multiple years" away.

"There's no doubt color displays can offer much more compared to black and white, which is why we are working on it," says Sri Peruvemba, vice president of marketing for E Ink. "And so far we have hit all the milestones that we had set for ourselves." Last week E Ink was acquired by Taiwanese company Prime View International for $215 million.

E-book readers have become the hottest consumer products of the year. Since the first e-reader was introduced by Sony in 2006, and particularly since the introduction of Amazon.com's popular Kindle in 2008, demand for e-readers has taken off. More than 1 million black-and-white displays have been sold so far, says E Ink, whose black-and-white displays power most of the e-readers on the market. And there are more than 15 e-reader models currently available or in the works.

With the exception of the Flepia, though, almost all e-readers are monochromatic. So what's the technological holdup? To understand that, you first have to understand how E Ink's black-and-white displays work. Electronic ink, pioneered by the company, is composed of millions of microcapsules. Each microcapsule has positively charged white particles and negatively charged black particles suspended in a clear fluid. When a positive electric field is applied, the black particles are attracted to the top and become visible to the user. That makes that area appear black. The reverse is also true: A negative electric field draws white particles to the top, making the area appear lighter. For an electronic display, the ink is printed on a sheet of plastic film, and a layer of circuitry is laminated to it to drive the ink.

For a color display, E Ink needs to put a color filter on top of its black-and-white display. A color filter usually has four sub-pixels – red, green, blue and white – that are combined to create each full-color pixel. That also means reduced brightness of display.

"With four sub-pixels, we get only a fourth of the area that we use today in the black-and-white displays. That means the resolution of the black and white display needs to get higher for the color filter to be effective," says Peruvemba. A 6-inch E Ink black-and-white display has a SVGA resolution of 800 x 600 pixels. To put a color filter on top would require the underlying display to have almost double the existing resolution.

The color filters also block a large amount of light, making the displays look dull and washed out, says Young. "The challenge is to balance the color output of the filter with the amount of light blocked by it," he says. The good news? When E Ink figures it out, its black-and-white displays will be better than ever, says Young.

E Ink says it is on track for large scale production of color displays at the end of next year. At the recent DisplayWeek conference in San Antonio, Texas, E Ink showed off prototypes of its color screen. Meanwhile, E Ink rival Kent Displays has already seen its color screen included in the Fujistu's Flepia, the only color e-reader available today. The Flepia is for sale in Japan only.

Other contenders in the race for color e-reader displays include Pixel Qi, the startup founded by former One Laptop Per Child project CTO Mary Lou Jepsen, and Qualcomm. Qualcomm could improve its existing line of low power displays called Mirasol and introduce a color version next year.

There's a caveat. E-readers with color displays can't match up to the standards set by LCD and now OLED displays. "Color displays for e-readers doesn't have anywhere the contrast ratio of LCDs or OLED," says Barry Young, managing director of the OLED association. "For color electrophoretic displays, the contrast is down to about 20 to 1, while for LCDs it is in the 1,000s to 1 and for OLEDs is 10,000s to 1 range."

"People don't like color screens that are dark," says Raj Apte, manager of prototype devices and circuits for PARC, formerly known as Xerox PARC, "and so far, the displays for e-readers we have seen lack the brightness that makes color screens attractive."

E Ink's rivals are facing their own challenges. Kent's color screens are based on cholesteric LCDs (liquid cyrstals where the molecules are arranged with their axes parallel to each other in one layer and then are displaced a little for each following layer to give them a helix-like structure.) The advantage with cholesteric LCDs is that they consume much lower power than traditional LCDS and are bistable – which means they can retain their image even when the power is lost. These LCDs stack red, green and blue films to create a color display. The trade-off for them is the refresh rate, says Young.

"It operates in three stages, so we are looking at a refresh rate of probably a second for a page compared to say a Kindle 2's 250 milliseconds," he says.

The stacking process also raises questions of whether Kent's displays can be thinner than its competitors. "Thickness is just an engineering issue that can be solved with the use of the right substrate," says Asad Hussain, vice president of technology for Kent Displays.

A problem that won't go away as easily will be in convincing e-reader makers to choose Kent Displays over rival E Ink, which has proven its mettle. A 16-year-old private company, Kent has been showing demos of its color screens for years. But so far, other than Fujitsu, it hasn't found any takers, at least none announced publicly.

Hussain blames the reluctance of e-reader manufacturers to introduce color displays. "Right now black-and-white displays have momentum and though everyone wants color, no one is willing to make the shift."

Check out our detailed comparison of how the four color e-reader display technologies

See also:

Photo: E Ink color screen prototype/E Ink