Electronic paper that reflects light, instead of filtering it from a backlight, as most conventional displays do, is easy on the eyes and saves on battery life. But this reliance on ambient light becomes a handicap when trying to make a bright, beautiful color display. Researchers at HP are addressing the problem by developing new materials that use ambient light to create a more vibrant color for video-capable, low-power screens.
Conventional displays, including LCDs, use a backlight to produce light, and layers of optics to filter it to create different colors. This type of display needs a lot of power because most of the light is lost during filtering.
Reflective displays need no backlight. For example, the pixels in the displays made by E Ink, the dominant electronic-paper company, are filled with black and white capsules of opposite charges; when the pixels are switched, the white or black particles move to the surface, reflecting or absorbing ambient light.
Making color electronic paper is a major challenge, and the prototypes made so far look muddy and dim compared to conventional displays. Adding color filters over black-and-white pixel arrays–the approach taken by E Ink–introduces the same light-loss problems that LCDs suffer from. But in an LCD, the backlight can be pumped up to maintain brightness. Reflective displays are limited to ambient light, and that loss can’t be recovered. Another problem is that the colored subpixels used in color displays typically sit side by side, with one-third of the area of each pixel given over to each color: red, blue, and green. When the pixel is reflecting red light, two-thirds of the incident light is simply lost, no matter how good the filter is.
Gary Gibson, a scientist in the information surfaces lab at the company’s Palo Alto, CA, is involved with a project aimed at addressing the dimness problem using brighter, luminescent materials. The company has developed a composite material that converts blue and green light into red and another that converts blue light into green. It isn’t practical to make a blue luminescent pixel. A fast-switching liquid-crystal shutter sits above each pixel and lets light in and out; mirrors below also help light escape.
Developing luminescent materials that convert the color of light is a major materials-science challenge. “There aren’t any materials in nature that do all the things we’d like,” Gibson says. The group has developed composites for each color. In the red composite, for example, blue and green light is passed along from dye molecule to dye molecule, gradually converting it to the red wavelength with as little loss as possible. Blue remains a challenge because there’s not enough higher-wavelength light in sunlight or ambient room lighting to convert to blue. So the company’s prototypes either use a conventional, larger blue sub-pixel or rely on blue light in a white subpixel to achieve sufficient brightness.
In theory, the HP materials should be brighter than a perfect color reflector, says Gibson. So far, Gibson says, they’ve made materials that are stable over time, and have demonstrated these materials in optical systems similar to those that could be used in a display. As they continue to tinker with the materials, HP researchers are developing manufacturing systems for complete displays. Gibson says they should be compatible with high-volume production processes such as ink-jet printing.
The popularity of the iPad shows that there is “clearly an appetite for color electronic gizmos for reading magazines, books, and other content,” says Nick Colaneri, director of the Flexible Display Center at Arizona State University. “Vibrant, color e-paper will feed off that, and will multiply the market,” he predicts.
Down the road, HP may combine reflective displays with flexible, rugged plastic electronics being developed as part of another project from the Palo Alto labs. “That would be really innovative,” says Paul Semenza, a senior analyst at industry research firm Display Search. “A flexible, low-power color display is the Holy Grail,” he adds. “The key thing is, can they identify and manufacture all the materials and get it to work as it seems it should?”
Meanwhile, E Ink product manager Lawrence Schwartz says the company’s color electronic paper will be in products at the end of the year. The company is compensating for some of the light loss through the color filters by capitalizing on improvements in its ink formulations to produce higher contrast between white and black. The company is also improving the switching speed of its displays, which will eventually mean more animation and video.
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