Mass production of LCD television sets that use light-emitting diodes as the light source for the backlight looks set to begin in 2005. If practical application can be achieved, color expression will be greatly boosted.
One engineer at a manufacturer of light-emitting diodes (LED) commented, "An engineer over at a TV company was complaining the competition stole a march on them."
In November 2004, Sony Corp of Japan released the 40- and 46-inch QUALIA 005, the first liquid crystal display (LCD) televisions to use red (R), green (G) and blue (B) LEDs for the backlight. Engineers throughout the industry responded with wonder and acclaim. As one LCD panel manufacturer put it, "Sony is flying ahead... I still can't believe they were able to commercialize it so soon!"
Backlights illuminate the LCD panels from the rear, and generally consist of a light source and optical components such as spreaders. Large-size LCD TVs have used cold-cathode florescent lamps (CCFL) for this purpose, but now Sony is switching to 3-color (RGB) LEDs. 
It seems certain that a number of LCD TV manufacturers will follow Sony's lead, launching models with LED backlights at volume production in 2005. Samsung Electronics Co, Ltd of Korea, for example, was reported in the Korean press as launching volume production in the first half of 2005. Many TV manufacturers are eagerly amassing expertise in the field, striving to develop LED backlights as soon as possible. The key reason is that LED backlights are expected to provide a major boost in the value-added content of LCD TVs (Fig 1).
Color Performance
There are a number of advantages to using RGB LEDs as the backlight light source. Most TV manufacturers are especially intrigued by the heightened freedom when it comes to color (Fig 2). It will become possible to achieve color reproduction ranges in excess of 100% of National Television Standards Committee (NTSC) specs, and freely adjust white color expression.
Not only do RGB backlights offer a significantly wider color reproduction range than conventional CCFL backlights, they also make it much easier to reproduce colors difficult to achieve with phosphors in cathode ray tube (CRT), plasma display panel (PDP) or Surface-conduction Electron-emitter Display (SED) technologies. A number of LCD panel manufacturers agree that as far as color is concerned, LCD panels offer the best performance of any of the devices available, and that the key technology is the LED backlight.
Sony was the first to commercialize a product based on this view. The firm is considered a late-starter in the thin TV market, and many observers in the industry believe that it pushed ahead with productization of the LED backlight as one means of boosting its brand name recognition in the TV receiver market. "It's pretty easy to imagine that Sony is looking at this LED backlight technology as the next Trinitron," a number of LCD panel engineers have said.
Sony is not the only manufacturer to position color as a key point for competition, of course. The competition in LCD TVs until now has emphasized higher luminance and definition, larger screen size and lower cost, and all manufacturers would love to add a new characteristic for competition to these existing indices. A source at an LCD manufacturer claimed that Samsung Electronics has been hiring color experts recently, evidence of its determination to attack the color issue full-scale. Already the spotlights are beginning to illuminate the fierce competition shaping up in color.
Spur to Competition
Along with these activities of the equipment manufacturers, peripheral component manufacturers involved in the LED backlight sector are also getting interested. A number of engineers working at backlight or LED firms agree that Sony's bold move will spur development competition. The initial target dates of 2007 or beyond will no doubt be moved up, they have commented. The component manufacturers are swinging into action to prepare to cope with expected demand.
Most manufacturers working on LED backlights are using LEDs from Lumileds Lighting, LLC of the US. At present, they offer the highest output and are available in all three (RGB) colors.
Competing LED manufacturers are of course taking action. Toyoda Gosei Co, Ltd of Japan, for example, plans to ship a high-output LED suitable for LED backlight use in 2005, offering all three colors. According to Ota Koichi, director, Optoelectronics & Intellectual property at the firm, "The LCD television backlight market is a very exciting one, and we want to make it one of our major business sectors." Showa Denko KK of Japan, a chip manufacturer which only just recently entered the blue LED market and began to ship samples in November 2004, is also hot, as Kazuhiro Mitani, project leader, GaN Project, Electronics Sector explained: "We plan to add green LEDs to our product line-up in 2005, which will give us all three colors."
Possible Content Changes
The adoption of LED backlights could lead to the release of TV receivers capable of exceeding 100% of the NTSC color reproduction range specification. If that happens, it could easily lead to changes in the content itself, because the TV receivers would then have richer color expression than the content.
Color information for video content such as TV broadcasting, or packaged media saved as Motion Picture Experts Group (MPEG) imagery, is created to match the range of color expressions of CRT televisions. In other words, it only contains the sRGB standard color information equivalent to about 70% of the NTSC spec. The Sony QUALIA 005, therefore, uses a proprietary algorithm to convert video signals from broadcasts or other sources, expanding the range of displayed color outside the sRGB standard. If many TV sets with this capability appear on the market, it will lead to increasing demand for content offering a wider color range.
In fact, the trend is already picking up steam, as evidenced by the new video color space standard called xvYCC now being discussed by the Japan Electronics and Information Technology Industries Association (JEITA). It is intended to define a color space wider than the sRGB standard. A comparison with the Munsell Color Cascade, an extensive series of color samples, shows that the sRGB standard achieves only 55%, while xvYCC will hit 100%. JEITA proposed the xvYCC standard for international standardization to the International Electrotechnical Commission (IEC) in October 2004, and it could be adopted as early as 2006. If the standard is officially adopted, it could well mean new color information for content.
Color Mixing
Backlights using 3-color RGB LEDs are the most effective means at present of expanding the color reproduction range, but they will not be easy to utilize effectively.
The first shock for engineers hoping to utilize LEDs is unevenness of luminance and color. According to one backlight manufacturer, luminance variation is about five times that of CCFL and color variation five to seven times. One engineer working for a backlight maker said, "I was so shocked to see the prototype actually lining up LEDs and turning them on for the first time. I believed this technology would never succeed." Sony has yet to disclose technical details, but appears to have resolved the problem in the QUALIA 005 through stringent LED selection procedures. Lumileds, which supplies the Sony LEDs, has commented that its shipments are now required to meet considerably tougher standards than originally imagined.
The difficulty of using LEDs, unfortunately, cannot be entirely resolved through stringent selection procedures and similar measures. They suffer from a host of issues entirely different from those encountered in CCFL backlights. A variety of efforts are under way to find solutions to these problems, by not only the equipment and panel manufacturers eager to get LED backlights into products, but also the backlight and LED manufacturers themselves. 
There are three major problems encountered by both equipment and panel manufacturers in using LED backlights, namely (1) reducing unevenness of color in the display, (2) reducing color shift, and (3) reducing power consumption (Fig 3). Of these, many engineers have pointed out that reducing color unevenness (1) is the most difficult. Color unevenness is usually caused because three (RGB) LEDs are used. Unlike the CCFL design, which uses white light to start, the three RGB colors must be mixed to produce a uniform white, which then illuminates the LCD panel.
One effective method of rectifying color unevenness is to increase the distance between the LED light sources and the LCD panel being illuminated (the path length). This makes it easier for the light to mix. Several manufacturers are using dual light guides to increase the path length, reflecting the light to accomplish this. NEC-Mitsubishi Electric Visual Systems Corp (NM Visual) of Japan, for example, uses this approach in its LCD monitors for desktop publishing (DTP) and other commercial applications, including a 21.3-inch model. This structure alone will not eliminate color unevenness completely. NEC LCD Technologies, Ltd of Japan, which designed the backlight for the LCD monitor, reveals that other changes were required as well, including changes to the LED positioning pattern and spacing.
Direct-LED Designs
While light guides are one effective method of enhancing color uniformity, they lower light utilization efficiency because of losses during propagation through the guides. When the target is consumer TVs, unlike the commercial LCD monitor manufactured by NM Visual, luminance must be at least 400 to 500 cd/m2, and as a result many people in the industry feel that TV receivers require illumination by LED directly under the LCD panels, rather than via light guides. Dr Hiroaki Sugiura, manager, Imaging Color Group, Imaging Engine Technology Dept, Advanced Technology R&D Center of Mitsubishi Electric Corp of Japan, said, "Backlights for televisions will almost certainly be direct-LED designs, because they make it easier to increase luminance." Sony's QUALIA 005 is a direct-LED design (Fig 4).
Even with direct-LED backlights, however, color uniformity is essential. Normally LEDs are tightly grouped to facilitate color mixing, but the tighter they are the more heat sources are also concentrated, requiring complex thermal radiation structures. If the LEDs are spaced out a bit, though, it becomes much easier to handle the generated heat; but color unevenness becomes much more of a problem. The QUALIA 005 tries to achieve a balance between color and heat by packing the LEDs tightly in the horizontal direction and loosely in the vertical.
Thinning Direct-LED
Direct-LED backlights are generally about 50mm thicker, because of the need to provide an adequate distance between the LED outputs and the LCD panel for color mixing.
One firm emphasizing thin designs is Tama Fine Opto Co, Ltd of Japan, which developed a direct-LED backlight only 30mm thick. It was revealed at FPD International in October 2004. It mounts multiple light source units (developed by Omron Corp of Japan) in a lattice. The light from the RGB LEDs in the light source unit, which is only 6mm thick, is reflected inside the unit itself and is already mixed to a degree when output from the unit, making it possible to reduce the path distance to the LCD panel somewhat.
Tama Fine Opto believes that the sizes of the light source unit and the reflecting mirrors can be optimized for an even slimmer design. Mitsuhiro Suzuki, general manager of the Development Dept at the firm, commented, "This prototype measured 30mm thick, but we hope to slim that down to 25mm when volume production starts in fall 2005." The firm also intends to work on eliminating seams between units, made visible by differences in light source unit color and luminance.
Power: CCFL-Level, 2007
Reducing color shift (2), another key problem with LED backlights, will probably require the use of color sensors and temperature sensors.
Color shift occurs when the three LEDs exhibit different emission characteristics, due to temperature or temporal change. LED manufacturers point out that resolving the problem of LED emission characteristic shift will not be trivial, and suggest it will be necessary to use a sensor to detect LED fluctuation and automatically adjust individual pixel intensity. LCD monitors like the QUALIA 005 and designs from NM Visual use this approach.
Resolution of point (3), reducing power consumption, can only be solved by improving the performance of the LEDs themselves, not through any innovations by the equipment manufacturers. This is a transient problem that will remain with us until the emission efficiency of LEDs reaches a sufficiently high level. The emission efficiency of CCFL is about 60lm/W, but LEDs achieve only about 30lm/W. Simple arithmetic indicates that the LED will consume about double the power to achieve the same luminance as a CCFL backlight.
It is likely to take another two or three years before LED backlight power consumption drops to the level of CCFL backlights. Lumileds claims that improvements in internal quantum effects and light extraction efficiency have resolved the major technical obstacles to boosting emission efficiency to the level of CCFL by 2006 or 2007.
by Takuya Otani
Websites:
Lumileds Lighting: www.lumileds.com
Samsung Electronics: www.samsung.com
Showa Denko: www.showadenko.com
Sony: www.sony.com
Tama Fine Opto: www.tama-fo.co.jp
Toyoda Gosei: www.toyoda-gosei.com/led
(March 2005 Issue, Nikkei Electronics Asia)
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