Nikkei Electronics Asia -- February 2007
Monthly Special
Intelligent Color Engine to Make TFT-LCD High-Definition Display

Nowadays thin-film transistor (TFT)-LCDs find widespread adoption ranging from mobile phones and notebook computers to desktop monitors and TV sets. Nevertheless, there are still many intrinsic problems, including low contrast ratio, serious blue shift varied with grayscales, and narrow viewing angle under color deviation. Among these problems, however, the blue shift caused by gamma-dispersion and the hardware limitations of liquid crystals and the display structure is the one that directly relates to image quality and the visual preferences of viewers.

To provide TFT-LCD panels with high definition without color deviation, it¡Çs necessary to eliminate color shift with grayscales. In normal and common cases, it takes much time and effort for the adjustment process.

Advanced Color Adjustment
New color management solutions such as the ipCE (intelligent picasso Color Engine) have been developed to improve the image quality of TFT-LCD panels. The solution provides three main adjustment functions: Auto Color Shift Eliminator (ACSE), Multi-Axis Color Enhancer (MACE), and Local Area Hue Exchanger (LAHE). Each function corresponds to the three major factors of color quality: brightness, saturation, and hue.

ACSE can automatically solve the color shift issue within ten minutes, which allows customers to further go ahead with other color improvement processes, such as local color saturation enhancement and color tone adjustment. The solution not only improves device performance but also provides it in a friendly, and fully adjustable methodology. In addition, ipCE also allows customers to modify color performance effortlessly, and enables a considerable saving on manpower and the design cycle time, which ultimately leads to a better market opportunity.

When displaying gray levels on LCD panels, the correlated color temperature (CCT) increases very quickly from white to dark. While the color shift is basically caused by the color dispersion of liquid crystal as voltage changes. In fact, there are different tendencies of CCT deviation and color shift among different types of TFT-LCD panel, such as TN, VA, and IPS, etc.

Although the performance of IPS-LCDs is regarded as the best, it still amounts to 1000K CCT, not to mention the worst shift status of TN-LCDs. This intrinsic characteristic causes bluish gray and influences the color accuracy and performance of TFT-LCDs. Many researchers are focusing their efforts on this area. Among their proposals are techniques for different cell gap design for each RGB sub pixel, the three gamma system, etc. Though keeping the same ratio of brightness of RGB codes with various gray levels is the most common solution to eliminate the shift, it doesn¡Çt work very well and effectively for every type of TFT-LCD panel since it takes much time and effort for system makers to adjust the white balance and make gamma adjustment.

For example, an experienced engineer will have to spend at least two days to manually adjust white balance on a definite CCT situation; and if the CCT target changes, he/she will need another two working days.

In summary, this common way requires much manpower, but also delays time to market. Above all, color accuracy is the most important fundamental item to achieve high image quality, and with intrinsic blue shift problem it¡Çs impossible for TFT-LCDs to be good for presenting true color. Furthermore, there are still many difficulties bothering system makers, such as the challenge of adjusting the color performance to meet customers¡Ç requirements. Moreover, there is always a problem with using existing global adjustment tools. When we just want to change the image content of skin color, what we adjust won¡Çt only involve skin color but also includes satisfying other chromaticity of this content.

ipCE Functions
The adjustment functions of ipCE (ACSE, MACE, and LAHE) could be helpful in efficiently eliminating color shift, making TFT-LCD a high-definition display which meets the customers¡Ç requirements. The functions correspond to the three major factors of color quality – brightness, saturation, and hue. Fig 1 shows the function block diagram.

An easy and reasonable way to solve the blue shift is to merge three different gamma curves of RGB into one, since white is contributed by RGB sub pixels, and the different ratio of primary colors affect the chromaticity of the white. To keep the same color coordinates of white at any gray levels, theoretically the brightness ratio of RGB should be contained at the same value, but it is just an ideal case. Making the same luminance ratio of RGB doesn¡Çt guarantee mixing the same chromaticity of color, as the tri-stimulus values of RGB vary with the gray levels.

The only effective way to solve this problem in the past was to merge the gamma curves of RGB into a specific value first, such as 2.2, and then check the smoothness and appearance of all gray levels. If the image of grayscales isn¡Çt smooth, or not fully gray, the designers must fine-tune the gamma LUT repeatedly until a more satisfying result is obtained. Under these circumstances, all judgments are based on human vision, and the results are therefore largely subjective. This is not a scientific approach; it is impossible to set criteria and a logically working methodology.

To solve this problem, the ACSE tool has been designed to automatically eliminate blue shift and finish adjustment for all kinds of CCT targets, gamma curves, and types of LCD panel in a short period of time. If the original tri-stimulus value has been measured, the working time can be dramatically reduced to around three minutes. Fig 2 shows the framework of an ACSE, which builds a close feedback automatic calibration system that comprises adjustment software, implemented hardware, and a set of chromatic detector devices. All the management parameters can be downloaded through the I2C interface to the program LUT.

Color Tone, Saturation
After color accuracy calibration, TFT-LCD can be used to present a high-quality color image and it¡Çs time to take the preferred color adjustment into consideration. The LAHE tool is designed for users to modify the color tone. It is capable of changing the color hue of image content in which users can adjust partially instead of globally. By taking the LAHE tool, designers won¡Çt suffer the side-effects of turning the whole image with color harmonic into the one with specific tone deviation. In short, the LAHE can locally adjust the tone which users want to modify, without affecting the grayscales.

MACE tool provides localized adjustments for color saturation. Global saturation adjustments often lead to trade-offs on image quality. For example, if an image lacks red, a global adjustment will enhance the red saturation at the expense of reddish tint on perfect yellow. The MACE tool, however, enhances the red saturation independently without the involvement of other colors. The MACE enhances saturation on user-preferred color without influencing other colors; natural color will never be affected.

Serious color shift problem at different grayscales is an intrinsic characteristic of LCDs, which is caused by gamma dispersion and hardware limitation of the liquid crystal and display structure. The poor color reproduction is an obstacle to achieving high image quality. To eliminate blue shift on TFT-LCDs and produce high-definition displays, some advanced color management tools such as the ACSE, MACE, and LAHE have been developed. Without initial tri-stimulus measurement, the ACSE can effectively and rapidly overcome the blue shift problem. The LAHE can locally adjust the tone which users want to modify, and the MACE enhances saturation on user-preferred color. In conclusion, customers can utilize new offerings such as the ipCE to adjust the color performance effectively and effortlessly, leading to a shorter time to market without using much manpower, providing a better market opportunity.

by Ching-Hsiang Hsu
Faraday Technology Corp

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