One visual aspect of e-paper technology that has not yet been fully achieved is production of an image with the desired color vibrancy. Associated with this is the further desire for greater image brightness. At the recent SID conference, Fuji-Xerox Co., Ltd. (Kanagawa, Japan) reported on and demonstrated the company’s latest efforts to address these problems.
First, a few words of background information. A generic electrophoretic display consists of two substrates. Electrically charged, colored particles are suspended in a liquid contained between the substrates. Internal, typically patterned electrodes are used to apply an electric field to the particles. The particles respond to the polarity of the electric field by being drawn to either the front or the back substrate. Those particles brought into proximity of the transparent front substrate are visible and contribute to the image. The color of the back substrate or the liquid provides a contrasting color to unenergized areas thus producing a two color image.
Many approaches have been investigated to introduce full color into an e-paper display. This includes the use of spatially separated colored subpixels. In some designs this is implemented in conjunction with the use of colored filters. Multi-layer devices have also been proposed. In all cases, the result has been an image that does not have the color vibrancy of a print magazine, let alone a LCD.
In a Distinguished Paper entitled "Novel Color Electrophoretic e-Paper Using Independently Movable Colored Particles" the Fuji-Xerox authors report on three different types of color particles contained in a liquid. The system is designed such that the electric field required to move each type of colored particle is also different. This variation in threshold is accomplished by the selection of the material properties of the particles. When a voltage is applied above the threshold field for the first type colored particle but below that of the second and third type particles, only the first type colored particle will respond. An applied voltage above the threshold for the second type colored particle will move both the first and second types of colored particles but not the third. When a voltage is applied above the threshold for the third type colored particle, all colored particles will respond.
The researchers chose the three absorptive primaries for the colors of the particles. By properly sequencing the applications of appropriate positive and negative voltages, any one, or two, or three or combinations of the three colored particles can be brought to the front substrate. Thus, any color can be produced.
When all particles are on the front substrate, the color will be black.
In addition to the colored particles, white particles are included in the liquid. The white particles are chosen such that they do not respond to the electric field. In this way, a white image can be created by drawing all of the colored particles to the back substrate.
Since the Fuji-Xerox e-paper does not include colored filters, the claim is that the display can produce a brighter and more vivid color image. In addition, the Fuji-Xerox display will have a higher resolution than any type of e-paper based on spatially separated sub-pixels.
The display shown at the SID conference presented an image that had a limited color range. This was because the prototype was based on the utilization of only two types of colored particles. These were the complementary colors red and cyan. The display had a diagonal of 5-inches, a pixel count of 600 x 800, resolution of 200 dpi, reflectance of 30% and contrast ratio of 10:1. The display included amorphous silicon TFTs drivers on the back substrate.
Four shades of gray could be produced by the use of time modulation. The image produced by the prototype display is illustrated in the accompanying photograph of the Eifel Tower.
The Fuji-Xerox e-paper technology is still at an early stage of development. The company’s plans include development of full-color e-paper based on the use of the three absorptive primary colors. To that end, the company has already developed a cell design using cyan, magenta and yellow colored particles. The color bars in the figure illustrate the range of colors that can be produced by the display. The data presented in the accompanying spider graph demonstrates that the color gamut produced by the cell is already wider than that of newspaper. This is clearly a technology to watch and Insight Media will do so - and, of course, report back to our readers -Arthur Berman