LEDs attempt the jump from small screen to large screen

Some display manufacturers have already gone as far as developing products with LED backlights. In May, NEC-Mitsubishi Electronics Display of America unveiled prototype 21.3 and 23 inch LCD monitors with LED backlights incorporating Lumileds DCC technology. And in August, Samsung unveiled 17 and 21 inch LCD monitors.

The Korean company says that the LEDs used in these models deliver greatly improved brightness: the 17 inch monitor has a brightness of 500 cd/m2 (compared with other models that average around 250 cd/m2). In addition, the high color saturation of 92% means that the colors are much closer to appearing natural than those produced by a standard monitor.

Also in August, Sony became the first to unveil LCD televisions that use an LED backlighting system. It said that in Japan, from November, it would start shipping 40 and 46 inch screen televisions from its high-end Qualia range that features LED devices. However, the technology doesn t come cheap: the two Sony televisions will reportedly be priced at ¥840,000 (around $7500) and ¥1.1 million (about $10,000), respectively.

LED manufacturers are also getting involved, with Nichia, Toyoda Gosei and Osram Opto Semiconductors having demonstrated LCD displays with LED backlights at various shows throughout this year.

LEDs have a number of advantages over conventional light sources such as CCFLs, including long lifetime, ruggedness and the absence of mercury. More importantly from a user s point of view, LEDs provide extremely good color quality.

The use of LEDs significantly enhances the color gamut (the range of colors) that a display can produce. The display system used in Sony s Qualia televisions has a color gamut that is 105% of the color space defined by the US National Television System Committee, while conventional technologies only produce 65-75% of the same color space. The more hues and tones that can be produced means that finer and more precise color definition can be achieved, so that on-screen color definition is more lifelike. The use of LEDs can also introduce real-time, dynamic color and brightness control, as well as a reduction in the blurring effect of fast-moving images.

Inevitably, the use of LEDs presents certain challenges. Since each LED acts as a point source, rather than a line source, like the CCFL, new optical designs are necessary to take advantage of the attributes of LEDs and to maintain luminance uniformity across a screen. Most computer LCD panels use an edge-lit light guide as the backlight. Simply replacing the CCFL with discrete red, green and blue LEDs on a linear PCB (which would require several tens of LEDs for a 15 inch screen used in typical notebook computer) would mean that the colors of the individual devices could be seen near the edge of the screen. To solve this problem requires the development of a mixing zone to combine the colors before they enter the light guide.

The main drawbacks of using LED backlights in LCD displays are cost and power consumption, both of which increase as the display size increases. Table 1 lists the attributes of backlighting sources, as described by Jong-Tae Lim of Samsung Electronics at the recent Intertech LEDs conference. Lim discussed Samsung s designs for backlighting units (BLUs) using LEDs, which incorporate a heat pipe to maintain the metal-core PCB at the target temperature of 45°C.

There are a number of ways to produce backlights for LCD screens. Table 2 summarizes three approaches that have been pursued by Lumileds, which are all suited to different market segments. One approach, termed "white", is suitable for smaller screens, such as those used in automotive displays (figure 2). It involves coupling the light from side-emitting white LEDs into an acrylic light guide positioned around the edge of the display.

For larger display applications, such as computer monitors, the use of RGB LEDs provides the benefit of a wide color gamut. In Lumileds DCC approach, used in NEC-Mitsubishi monitors, the light from LEDs with a Lambertian emission profile is mixed inside a folded light guide. The optical system mixes and distributes the white light evenly over the whole panel.

For still-larger displays, such as LCD televisions, the edge-lit system s light guide becomes too heavy, and other factors also point toward an alternative approach using direct backlighting - in other words, the LEDs are placed on the backplate of the display, rather than around its edge. Color mixing takes place within the cavity (or light box) behind the LCD panel. A series of diverters and diffusers helps to spread the mixed light over the entire panel, and prevents the light from the front of the individual LEDs from forming color spots on the screen.

According to Norbert Hiller of Cree Lighting, color quality is the main reason to use LEDs as backlights, and this determines the requirements passed on to the LED firms. "Display manufacturers want a wide color gamut, so they are looking for a precise combination of red, green and blue wavelengths," he said. "Also, they want high light output and low power consumption." Now that Cree has commenced its XLamp packaged LED activity, Hiller says that Cree s products are being designed-in by LCD-monitor and television manufacturers. "We have had good feedback for the XLamp package in this application."

In terms of market growth, Hiller believes that demand for LCD televisions and monitors is growing rapidly. "The question is how quickly will LEDs replace CCFLs," he said. "The markets haven t made up their mind yet, and end-users haven t seen many LED solutions, so the situation is unclear." Equally, there are no design standards - for example, regarding the choice between edgelights and true backlights for different applications. Nevertheless, this is a technology that is being widely evaluated. "I m pretty sure that all the main LCD manufacturers are pursuing LED solutions at the moment," said Hiller.

Despite this interest, Jagdish Rebello of market research company iSuppli Corporation, who recently wrote a report on the LED industry (see LCD backlighting continues to thrive as pricing pressure bites), does not think this market will grow tremendously quickly. "There are no overwhelming advantages driving the demand for LEDs in this application," said Rebello. "Benefits such as wide color gamut are not essential, except in some niche markets such as medical monitors, and the [extra] cost puts this technology out of reach for most customers - the cost-benefit analysis is not favorable."

LEDs typically cost about 10 times as much as the CCFL alternatives they seek to replace. Even though the price of LEDs will inevitably fall in tandem with rising performance, Rebello thinks that this market is under tremendous pricing pressure. "People are always demanding free monitors with their computers," he said. "Any technology adding several percentage points to the cost is something they need to look hard at."

As shown in figure 1, iSuppli expects the large-screen LCD backlighting market for LEDs to grow from zero in 2004 to around $3 million in 2005, eventually reaching $160 million in 2008. Most of this growth will come from applications in LCD televisions, especially for large-screen (> 40 inch) models. These are big-ticket items costing several thousand dollars, so adding LED backlights that may cost an extra few hundred dollars is acceptable to these particular consumers.

There is not expected to be any significant take-off for smaller LCD televisions with LED backlights until about 2007. The market for desktop monitors and for industrial, financial and medical monitors will each amount to only a few million dollars by 2008, says Rebello. "Besides cost, there are issues with heat dissipation, and the need to obtain uniform illumination across large displays," he said. "We really see this being a niche market for some time."