Development of Wide-Color-Gamut Mobile Displays
As content for mobile electronics devices such as cell phones and PDAs continues to flood the market, next-generation LCDs for such applications will require wider color gamuts. Here, a new four-primary-color LCD technology for mobile electronics is described.
by Eiji Chino
LIQUID-CRYSTAL DISPLAYS (LCDs) are widely used in applications where power consumption is critical, such as mobile phones, personal digital assistants (PDAs), and laptop computers. The growing demands for mobile broadcasting, visual telecommunications, and viewing photographs have made wide-color-gamut characteristics of mobile LCDs – along with contrast and brightness – the most important features of next-generation mobile products.
Various types of LCDs have been demonstrated to meet these requirements and to realize a world of "Color Image Matching," where images taken with a camera are equivalent to printer performance levels and where print output has the same image quality as the image viewed in an LCD viewfinder of a digital still camera (DSC). For example, displays that use more than three primary colors per subpixel were reported for TV applications1,2 or spectrum-sequential LCDs.3,4 However, these prototypes have not been developed for commercial use, due to the rising costs, gaps in the technology, and excessive power requirements.
To meet these demands, we have developed a new, mobile direct-view wide-color-gamut four-primary-color LCD technology refered to as Photo Fine Chromarich (PFC). In Fig. 1, the color points of PFC (red line) and a typical color-reproduction area of an ink-jet printer (yellow line) are shown in combination with the NTSC color gamut (large triangle). It is clear that this technology enables the normal LCD color gamut (small triangle) to be filled in, while almost all the NTSC colors and colors such as emerald green and blue can be reproduced to the same level as that of an ink-jet printer – this was not previously possible in small- and medium-sized LCDs (screens on current mobile phones cover an average of about 50% NTSC). PFC creates higher-resolution images than standard displays, accommodates a wide gamut for color reproduction, and reproduces clean, crisp images with good color rendition. To achieve these characteristics, the technology employs a four-primary-color filter system and fine procession techniques for high definition, along with a color-management algorithm that Sanyo Epson Imaging Devices originally developed on the basis of Seiko Epson's ink-jet-printer technology.
Eiji Chino is the Manager of the System Development Department at Sanyo-Epson Imaging Devices Co., 6925 Tazawa, Toyoshina, Azumino, Nagano 399-8285, Japan; telephone +81-263-72-1447, fax +81-263-72-8787, e-mail: chino.eiji@sanyo-epson.com
Fig. 1: The color gamut of PFC technology compared to that of an ink-jet printer, NTSC color gamut, and a "typical" LCD.
System Overview
Standard LCDs employ red, green, and blue (RGB or three-primary-color) filters to produce the color subpixels that are combined to produce color images. These conventional RGB-primary-color LCDs cover only about 40–70% of the NTSC gamut and are not good at displaying cyan or emerald colors. However, the market demands displays that utilize deeper, richer, and more-vivid overall color gamuts. In conventional LCDs with RGB subpixels, the color gamut can be increased by making color filters with narrower spectrums, but this leads to lower transmission of the LCD panels. The color gamut of LCDs can also be extended by combining the LCD panel with an RGB-light-emitting-diode (LED) backlight,5 but this system has low power efficiency, is thicker than conventional white LED systems, and is white-point-temperature dependent. Alternatively, color-sequential LCDs6 do not require a color filter and can have both a relatively large gamut and high transmission, but very high frame rates are needed to avoid flicker and to eliminate color break-up.
To overcome these problems, we have developed a new four-primary-color filter. LCDs can reproduce a wide color gamut of more than 100% of the NTSC gamut by using newly developed red, blue, yellow-green (YG), and emerald-green (EG) color filters, combined with a typical white LED backlight. Figure 2(a) shows the basic concept of the four-primary-color filter. The conventional green color is divided into YG and EG colors. By adopting YG, a rich golden yellow color (i.e., golden autumn leaves) can be achieved, which conventional three-primary-color LCDs have difficulty in displaying. Also, EG enriches the emerald and cyan color areas found in images of steep mountains, dark valleys, and/or deep water. In addition, thickening red and blue primary colors make it possible to have a far wider reproduction gamut, in excess of 100% NTSC. Figure 2(b) shows a microphotograph of the newly developed four-primary-color filter in which four subpixels are formed into a regular square pixel that is the same as that found in conventional three-primary-color LCDs.
To minimize the occurrence of edge pseudo-colors in the new four-primary-color system, we studied the relationship between all 12 candidates of subpixel arrangements and the occurrence of edge pseudo-colors using an S-CIELAB7–9 simulation. We applied the simulation to a test image of one black line on a white background. Edge blur occurred for all cases of the 12 candidates due to the low-pass spatial filter used in the S-CIELAB model. However, no pseudo-colors arise at the edge area for the reference non-subpixel case. Figure 3(a) shows the non-subpixel reference case and two of the subpixel arrangements: the B-YG-R-EG and the R-B-EG-YG.
The results of the S-CIELAB model are shown in Fig. 3(a) and demonstrate how the S-CIELAB model exhibits the same low-pass spatial-filtering function found in the human visual system (HVS). The S-CIELAB is a model that takes into account how the HVS processes spatial-frequency-dependent color-difference information and provides a way to quantify the edge pseudo-colors8 for the various subpixel arrangements in the proposed four-primary-color system.
Fig. 2: (a) Shown is the basic concept of a four-primary-element color filter. The conventional green color is divided into yellow-green (YG) and emerald-green (EG) colors. (b) A microphotograph of the newly developed four-primary-color filter in which four subpixels are arranged into a regular square pixel that is the same as found in conventional three-primary-color LCDs.
Figure 3(a) also illustrates the luminance and color changes across the line profiles of the best and the worst subpixel arrangements, as well as for the reference non-subpixel-type case. In the S-CIELAB color-difference system, L* is related to the luminance and is frequently used as a perceived brightness scale. a* and b* are coordinates along the red-green and yellow-blue axes, respectively. For the reference non-subpixel line profile [Figs. 3(a) left], the L* versus position value has a minimum at the central position of the line profile, and the a* and b* values do not vary across the profile; i.e., the brightness of the black line shows a minimum at the central position, and color does not change across the spatial profile of the line, as expected. For the worst case, as shown in Fig. 3(a), the L* versus position value has a relatively symmetrical shape about the center-of-line position, but the a* and b* versus position values have an inverse behavior relative to each other around the central position and have a relatively large fluctuation. This means that the brightness of the black line shows a minimum around the central position, but it also exhibits large chromatic variations across the line profile. The high-chroma color subpixels, such as R and YG, exhibit their component colors when they are used to form a black line. Also, when YG is next to EG, a higher chroma in green in the R-B-EG-YG order is exhibited; this combination exhibits the worst edge pseudo-colors. In contrast, the best arrangement is the B-YG-R-EG order as shown in Fig. 3(a) (left and center), although the L* value has almost the same shape, as does the worst case, and a* and b* have comparatively small value changes across the line profile, resulting in a relatively minor exhibition of pseudo-color. The order of subpixels is arranged to eliminate the worst-case properties. B and EG have lower chroma compared to that of R and YG and are arranged at both edges on the sub-pixel set. YG is also placed far from EG to prevent high chroma in the green color. The same simulation result is shown in Fig. 3(b) by the word "New" and where the B-YG-R-EG order was adopted as the best subpixel arrangement for four-primary-color filters.
In order to develop displays, we adopted a new ultra-precise processing technology for the latest low-temperature-polysilicon thin-film transistor (LTPS-TFT) and amorphous-silicon (a-Si) TFT-LCDs. This processing method thins the width of the driver's wire connection and diminishes the size of the transistors. As a result, PFC has the same subpixel structure as the conventional system for three-primary colors. Furthermore, the system improves the dot density. While standard displays for mobile phones offer a pixel density of about 200 pixels per inch (ppi), PFC attains a density of 280 ppi by using LTPS.
To expand the color-gamut values, RGB-LED or color-sequential backlight systems can be used. However, for an RGB-LED, the white balance must be adjusted by coordinating the current intensity for each LED in order to create a good white image and to maintain the balance by controlling the intensity changes caused by adding an extra circuit. Color-sequential backlight systems also require the addition of an extra circuit and the tripling of the driving frequency to avoid flicker and color break-up. Both methods result in a relatively high power consumption that is not feasible for mobile applications. From the outset, the PFC was developed for the mobile-displays market in mind, and, therefore, a white LED backlight with a proven track record was utilized. We were able to use a conventional white LED backlight system that is almost equivalent to the backlight system of commercially available mobile phones.
In order to compensate for the green-tinged appearance of the newly developed four-primary-color filter [obvious from Fig. 2(b)], we adjusted the peak wavelength of the white LED backlight system to a shorter wavelength.
Fig. 3: (a) An S-CIELAB7–9 simulation was applied to a test image of one black line on a white background to study the relationship between all 12 candidates of subpixel arrangements and the occurrence of edge pseudo-colors. Shown is the non-subpixel-reference case (left) and two of the subpixel arrangements: the B-YG-R-EG (center) and the R-B-EG-YG (right). (b) The same S-CIELAB simulation as shown in (a) applied to the word "New."
When mass-produced, white LEDs have several color classes ranging from yellow to bluish white. This time, we chose a bluish-white LED for commercial production. By controlling the peak wavelength of the white LED, we were able to achieve a good white image despite using green-area-doubled color filters. The use of this backlight system led to a simple LCD module with a low power consumption suitable for mobile applications. The reproducible color gamut of standard displays for mobile phones is about 50% of the NTSC color space, whereas PFC offers a color gamut that is 108% of the NTSC color space.
Controlling Color
A color-management system constitutes an essential technology for electronic-display devices. Low NTSC ratios form the basis of the color signals that are output by electronic devices. Therefore, problems may occur when a PFC display attempts to use such color signals without modifications. For instance, the gradations may have an unnatural appearance or the colors may seem artificial. To prevent these problems, we designed an LCD driver IC with a color-conversion algorithm that converts RGB color signals into four-color signals suitable for PFC.
Generally, input signals from the main system to the LCD modules consists of R, G, and B signals. Some three- to multi-primary-color conversion algorithms have been reported for multi-primary-color displays10,11; however, a simple algorithm that can achieve low hardware cost and low power consumption was needed for multi-primary-color mobile LCDs. To optimize these signals for the four-primary-color system, we developed a three- to multi-primary-color conversion algorithm for the driver IC in a mobile LCD. Figure 4 shows the outline of the newly developed color-conversion algorithm from three to four primary colors. The algorithm includes a gamma conversion, a three- to four-primary-color conversion, and an inverse gamma conversion to calculate four primary colors in linear space. To obtain a simple conversion algorithm, we selected a three-grid three-dimensional look-up-table (3DLUT) conversion as a three- to four-primary-color conversion algorithm. Figure 4(a) shows the structure of the three-grid 3DLUT algorithm. The converted data (R2,YG2,B2,EG2) are located at each grid of the cube. For the conversion (R2,YG2,B2,EG2), data sets are loaded, and the interpolation using these sets is performed according to the location of the input. Details of the color-conversion circuit will be revealed at IDW 2006.12
We also developed a driver IC using this color-conversion algorithm [Fig. 4(b)]. This IC contains the gamut-expanding algorithm in compressed form, so there is no need to increase the main memory of the products. For this color-conversion system, when a conventional three-primary-color signal is inputed, the system embedded in the LCD drive circuit can automatically convert it to a four-primary-color signal.
Fig. 4: (a) The structure of the three-grid 3DLUT algorithm. The converted data (R2,YG2,B2,EG2) are located at each grid of the cube. (b) The newly developed driver IC using the same three-grid 3DLUT algorithm.
Color Performance
Figure 5 shows the color points of PFC panels (trapezoid) and Pointer's Gamut13 (colored lines), which is a well-known standard of natural and real surface colors. The magenta coordinates are somewhat in disagreement with each other, but almost all the points of Pointer's Gamut are covered by our four-primary-color LCD. In other words, our four-primary-color LCDs with a white LED backlight can reproduce almost all natural and real colors.
Volume production of this new display began this summer. Displays are now available in small (~ 2 in.) to medium sizes (~10 in.), and we are targeting manufacturers of mobile phones, digital cameras, PDAs, digital viewfinders, and handheld video-game consoles, as well as vehicle dashboards. We will also aim at the emerging market for mobile satellite TV.
References
1Y. C. Yang, et al., SID Symposium Digest Tech Papers 36, 1210–1213 (2005).
2S. Roth, et al., SID Symposium Digest Tech Papers 34, 118–121 (2003).
3S. J. Roosendaal, et al., SID Symposium Digest Tech Papers 36, 1116–1119 (2005).
4M. J. J. Jak, et al., SID Symposium Digest Tech Papers 36, 1120–1123 (2005).
5G. Harbers, et al., SID Symposium Digest Tech Papers 32, 702–705 (2001).
6F. Yamada, et al., SID Symposium Digest Tech Papers 31, 1180–1183 (2000).
7T. Aragaki, et al., Proc. Imaging Conf. (in Japanese), 41–44 (2006).
8X. Zhang, et al., SID Symposium Digest Tech Papers 27, 731–734 (1996).
9M. Kanazawa, et al., Proc. 10th Color Imaging Conf., 261–266 (2002).
10H. Motomura, et al., Proc. 10th Color Imaging Conf., 267-271 (2002).
11H. Kanazawa, et al., Proc. 12th Color Imaging Conf., 65–69 (2004).
12H. Moriya, et al., "A color-conversion circuit of wide-gamut-color spaces for multi primary color LCDs," Proc. IDW (2006) (to be published).
13M. R. Pointer, Color Research and Application 5, No. 3, 145–155 (1980). •
Fig. 5: The color points of PFC panels (trapezoid) and Pointer's Gamut.