In the short term, it is unlikely that pigment-based electronic paper or e-paper displays (such as the Kindle) will provide the high-quality, full-color performance of conventional notebook PC displays (such as the iPad). But Jason Heikenfeld, associate professor at the University of Cincinnati (Cincinnati, OH) is betting that if e-paper displays can achieve adequate color, they can take advantage of their inherent strengths such as superior energy efficiency, perfect contrast even in sunlight, and their rollable, flexible formats to win out over emissive or transmissive displays. Toward this goal, Heikenfeld has developed a new bi-primary color system that cooperatively displays two complementary colors inside a single pixel, doubling the white-state reflectance and color gamut of single-layer e-paper.1 Furthermore, the new technology is applicable to several different types of e-paper pixel technologies.
Bi-primary color
Currently, e-paper displays typically use traditional red-green-blue-white (RGBW) or RGB color-filtering schemes; unfortunately, this method only provides color at 25% of the pixel area (for RGBW) and a maximum theoretical white-state reflectivity of 50%. The other traditional path of RGB or cyan-magenta-yellow (CMY) filter stacking is typically not used as it induces significant optical losses and does not allow for video operation, even though it theoretically claims the best white-state reflectivity and color gamut. But in the bi-primary color method, each sub-pixel shares two complementary primary colors: That is, each RGB primary is matched with its CMY complement in three sub-pixels. For example, red is the visible-spectrum complement of cyan (see figure).
The bi-primary method can be implemented in two forms: mixing and nonmixing. In the nonmixing implementation, two colors are displayed side-by-side in the pixel but cannot overlap or mix. As a result, when no color is displayed in the pixel, a third black colorant or black background is required. Candidate pixel technologies for the nonmixing bi-primary method are electrowetting or electrofluidic types where colorants are moved in a fluid (not through a fluid). In the bi-primary mixing implementation, colors can mix to create black or be removed to create white. Electrophoretic displays are candidates for this bi-primary mixing method.
In a case study that applied the bi-primary method to the most common e-Ink vertical electrophoretic e-paper display technology on the market today, as well as to horizontal electrophoretic displays and single-layer RGBW displays, color gamut and white reflectance were improved for all techniques using the single-layer bi-primary method. "Regardless of how good a particular pixel technology is, color systems like RGBW drastically reduce reflective image quality," says Heikenfeld. "Bi-primary provides a new option to display developers—a true leap toward bright color e-paper without having to move to complex and costly three-layer e-paper architectures."
REFERENCE
1. J.C. Heikenfeld, SPIE Photonics West 2011, San Francisco, CA, paper 7956A-07 (Jan. 26, 2011).
