by Jim Anderson
For years, liquid-crystal-display (LCD) research has focused on the challenges of the liquid-crystal panel. Performance improvements have focused on improving viewing angle, contrast, and switching speed by improving the liquid-crystal material, alignment, and electrical driving. The significant advances in manufacturing technology have enabled larger and larger panels while still driving the price down at an amazing pace.
Thanks to creative scientific work in the industry, each component of an LCD module now achieves such a level of performance and price as to enable everything from low-cost gas-pump displays, to energy-efficient mobile devices, to the huge full-HD TV sets with wide viewing angle that are becoming common in households around the world.
The primary focus of the effort has been on the LCD panel itself. Early super-twisted-nematic (STN) LCDs enabled laptop computers into the marketplace, but had poor viewing angle, poor response time, and limited display resolution. Through improvements in manufacturing technology, active-matrix backplanes became cost effective, thus enabling new LC designs. The 90° TN mode, used most commonly today, gave significant improvements in viewing angle and response time, and removed the barriers to high resolution. To continue the improvements to create super-wide viewing angles and high contrast, new LC modes using different alignment (vertically aligned or in-plane-switching) were developed. By increasing the drive frequency of the LCD panel to 120 Hz, researchers have achieved new levels of performance for image-blur reduction. However, performance progress has slowed as the concepts became more difficult to manufacture in the LCD panel and drive electronics.
Similarly, early direct-lit backlights were very simple devices with a large number of CCFL bulbs designed to give uniformity and brightness through the highly inefficient LCDs available then. Researchers at 3M along with other institutions realized brightness gains in the system by placing prismatic and then reflective polarizer films between the backlight and the LCD panel. As these new films became available and work progressed to improve the efficiency of the LCD panel, bulbs were removed. Work in the past few years on wide-color-gamut CCFLs and now LED-based lighting have further improved the performance of LCD modules by making the colors more vibrant.
To enable LCD modules to take the next step in performance, the components must be optimized together. Researchers in the industry have begun doing exactly this and have come up with many creative and exciting new devices. By coupling the way the backlight is driven to the way the LCD image is created, for example, contrast, image blurring, and power efficiency can be significantly improved. The articles in this issue highlight some of these improvements. By utilizing a holistic approach to system design, these researchers have developed amazingly thin, power-efficient, and high-performance designs.
The ability to create an LCD-TV set that is only 10-mm thick by Samsung Electronics was only possible by utilizing the properties of an LED backlight with new techniques in LCD design, as well as overall TV-set design and optimizing these as a system.
In this case, Samsung did not just combine the components needed to meet the goal of a 10-mm-thick TV set; it took advantage of the properties of those components to create an even better product. Samsung utilized an edge-lit architecture with a light-guide plate, and then took the design further by utilizing the other advantages of LEDs to improve display performance. Dr. Jang clearly describes how they used the fast switching of the LEDs to modify the gamma of the device to enable improved picture quality over the entire range.
In another article, the combination of technologies by Toshiba Matsushita Display (TMD), resulting in a very high-performance display, is described. By choosing the most advanced technology for each component, TMD had an impressive toolset with which to create a state-of-the-art display. By choosing the OCB LC mode, TMD was able to have inherent fast response time to reduce the Motion Picture Response Time (MPRT). By using an LED-based backlight, the company improved the contrast ratio to 1,000,000:1 and viewing angle to more than 160° with a CR>50. By using the inherent fast switching of the LCD and the LEDs, TMD coupled the drive scheme of the LEDs to the drive of the LCD panel to further reduce the MPRT down to 2 msec. This combination of technologies along with utilizing the synergistic advantages truly makes this display one of the highest performing LCD modules ever created.
Finally, an article from 3M clearly demonstrates the ability of film technologies, when coupled with the design of the backlight and the LCD, to enable significant efficiency boosts. When a recycling film, such as a prism film or reflective polarizer, is added to a system, the efficiency of the backlight cavity becomes even more important. Realizing this and designing the system as a whole can yield extra performance increases than optimizing the components alone. A 48% increase in the battery life of a mobile entertainment system was achieved by 3M by increasing the reflect-ance of the back reflector and replacing a sheet of prism film with a reflective polarizer. These modifications lead to a total battery life for this 3.5-in. display unit of almost 6 hours. The results in a TV set were even more impressive, reducing the power by 102 W (over 50%) as well as reducing the internal temperature by 10°C.
It is clear that by coupling the technologies available to TV-set designers today, new breakthroughs in display performance are being achieved. I look forward to the creative combinations that will be coming out in the future to enable new benchmarks for TV-set design, power efficiency, and overall set performance. •