Electrophoretic displays (EPDs) were pretty much ideal for the first generation of eBook readers; indeed, they were the key enabling technology for the category. Reflective EPD display technology consumes very little power, is easily visible in sunlight, and has the general appearance and human-factors advantages of ink on paper — which is why it is called electronic paper (ePaper). But EPD is a black-and-white technology, and there is now huge pressure from both consumers and content providers for color eReaders.
Since LCDs are an inherently monochrome technology too, it was reasonable to try the same approach to the color that’s used with LCDs: placing a matrix of red, green, and blue (RGB), or red, green, blue, and white (RGBW) color filters over the monochrome pixels to provide color. This is being done, and in November Hanvon — which dominates the Chinese eReader market — introduced the first color EPD eReader.
The approach is inefficient however, since an RGB matrix color filter (MCF) absorbs roughly two-thirds of the incident light. RGBW absorbs about half. This has been tolerable in transmissive LCDs, because you can build in a backlight that’s bright enough to give you a bright image despite the losses. With reflective displays, that option isn’t available, and you have to work with whatever ambient light there is. The result has been color EPDs that are rather dark. Alternatively, the manufacturer can intentionally use a low-density MCF, which produces a brighter image with unsaturated colors. In either case, the color gamut is in the range of 12% (NTSC). This is probably enough color for many uses, at least in the short run, but like Oliver Twist, we all want more.
In a paper delivered yesterday to a packed room at Photonics West in San Francisco, Jason Heikenfeld (Director, Novel Devices Laboratory, University of Cincinnati) proposed a new system for doubling the reflectance and colorfulness of ePaper. Basically, said Heikenfeld, better color depends on a better choice of color system. Single-layer RGBW color filtering offers greater brightness than single-layer RGB, as indicated above, but stacked RGB or CMY theoretically gives the best white-state reflectance and the best color gamut, said Heikenfeld. Unfortunately, stacking three layers gives rise to appreciable optical losses, particularly at high resolutions, and is generally not compatible with video rates.
As a result, Heikenfeld proposed “a compromise between the simplicity (but poor performance) of RGBW and the complexity (yet higher performance) of stacking three…display panels.” His compromise is “a new bi-primary color system that can display two complementary colors inside a single pixel, and therefore double the white state reflectance and color gamut for single-layer e-Paper. Although the color performance is not perfect, it has the potential to be the best among single layer color systems.”
In Heikenfeld’s bi-primary display, each of the three different kinds of subpixels contains a pair of complementary colors — red and cyan, green and magenta, blue and yellow. In the “mixing” version of the display, the complementary colors can overlap each other, producing black. Or both color regions can be clear, producing white; or only one can be clear so the other complementary color is presented.
One benefit of the system is obvious. Since each subpixel can be white, the absorption can be zero, and the white can be twice as bright as in an RGBW display. It takes a bit more study to realize that the use of six primaries — RGBCMY — produces greater brightness for individual colors.
In addition, a greater percentage of the area of the display that’s supposed to be showing a particular color is actually reflecting that color with the bi-primary display. In the case of an RGBW display, only 25% of the area reflects red when a pure red is called for. When this percentage, called the color fraction (CF), is greater, colors are more saturated and appear brighter. The figure shows the red state for a two-layer, bi-primary electrowetting display. This architecture allows both the red sub-pixel and the complementary yellow and magenta sub-pixel to be used to create red, thereby increasing the color fraction. For the details, refer to Heikenfeld’s paper, “A New Bi-primary Color System for Doubling the Reflectance and Colorfulness of e-Paper.”
Heikenfeld notes that the system is compatible with both electrophoretic and electrowetting display technologies. But he acknowledges the system is still in the R&D stage, and there are still materials and fabrication challenges to be overcome.
Although a lot has been written — and is being written, including this Display Daily — about ePaper display technologies, there hasn’t been an extensive, neutral, and critical review of ePaper technologies in the open literature. Starting next week there will be, when a 28-page review paper appears in the Journal of the SID. That paper is “A Critical Review of the Present and Future Prospects for Electronic Paper” by Jason Heikenfeld, Paul Drzaic, Jong-Soouk Yeo, and Tim Koch, and it combines readability with 145 references. I have a copy of the article proof on my second monitor right now. It is highly recommended.
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