I hate to say we told you so. No, that’s a lie. I love saying we told you so.

Although LED backlighting for industrial and notebook displays is proceeding at a very healthy clip, progress for large television displays is apparently stalled. And it’s no wonder. Panel-makers are fixated on direct backlighting with RGB LEDs; that is, backlighting the LCD panel with an array of red, green and blue LEDs distributed evenly behind the entire active area of the LCD. For the foreseeable future, that’s simply too costly an approach for any but very high-end consumer displays, as Insight Media made clear in its 2006 Backlight Report.

Ken Werner
Senior Analyst and Editor

At the LED Lighting Taiwan trade show held last month at the Taipei World Trade Center, Lite-On Technology showed a 46-inch LCD-TV with a direct LED BLU containing 400 RGB LEDs. A Lite-On rep was honest enough to acknowledge that cost will make it difficult for LEDs to replace CCFLs for large-size LCD-TVs. In the 46-inch LCD-TV on display, the cost of using CCFLs in the BLU would be US $40, while the cost of the LEDs was nearly US$400. (That’s the cost of the light sources only, not the entire BLU.)

So why don’t the panel-makers give up on the direct RGB approach? Because it is a powerful way to improved image quality. The output of the RGB LEDs leads to a color gamut that is substantially greater than that provided by standard CCFLs; having red, green and blue LEDs permits color tuning of the backlight; and having an array of LEDs permits adaptive control of the backlight area by area depending on the image content being displayed for powerful dynamic contrast enhancement and improved dark-area detail.

But there are much cheaper ways of getting improved color gamut. Sharp was way ahead of its competitors in understanding this and devising a solution. Starting with its 2007 models, Sharp is using CCFLs with one or two extra phosphors in the mix that’s applied to the inside of the lamp tube. These so-called four- and five-lambda CCFLS can produce color gamuts that are as good as that obtained with LEDs, and they only cost about 30% more than conventional CCFLs.

Some Taiwanese panel-makers are looking at white-LED BLUs for large LCD panels, Digitimes’ Max Wang and Carrie Yu reported from Taipei, because production costs are 60% less than for RGB BLUs. But white LEDs produce a color spectrum that is weak in the red and green, and the resulting color gamut is no better than that obtained with conventional CCFLs. Despite the fact that you can get a power saving, this does not strike us a productive way to go for television, although white LEDs make a lot of sense for industrial displays, notebook PCs, and desktop monitors.

The most promising approach, unless you conclude that five-lambda CCFLs are good enough, is RGB LED edge-lighting as being developed by Luminus Devices and Global Lighting Technology, and, less visibly, by Wooyoung of Korea. With this approach, you retain the color-gamut advantages of RGB LEDs, you benefit from power savings, and although you can’t do local-area dynamic backlight control, you can do a scanning version that gives you many of the more complicated system’s benefits. The Luminus Devices prototype shown at SID in June was impressive.

So why weren’t more panel makers showing RGB edge-lighting approaches in Taipei? Maybe they’re not showing the really good stuff to the public, they are worried about some of the other drawbacks of the approach, or maybe they just haven’t figured all of this out yet. If that’s the case, all they need to do is buy one of our backlight reports (p.s. - we have an updated version in the works now).