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.