Electronic Design

Color LEDs Take On A New Assignment—Backlighting LCDs

Soon, researchers at Philips' Flat Display Systems (FDS) of San Jose, Calif., will be using LEDs to backlight active thin-film transistor (TFT) LCDs. Other companies have employed monochromatic LED arrays for backlighting before. FDS, however, believes it can achieve some unique benefits by arranging the LEDs in repeating red, green, and blue arrays.

The researchers say it's likely that LEDs will supplant conventional cold-cathode fluorescent (CCFL) tubes. The three LED colors can be individually controlled simply by changing the applied current. In effect, the LED can be "color calibrated," thereby automatically compensating for spectral drift as it ages.

With a conversion efficiency of 98%, the LED die is one of the most efficient conversion devices available. Fluorescents and incandescents don't even come close. Fluorescents emit much of their energy in the ultraviolet range, and incandescent sources spew out most of their energy as heat.

"The crucial challenge is to get the light out of the die in an efficient manner," says Brad Fuller, general manager for Next Generation Lighting Systems at FDS. "Just how the LED is packaged becomes one of the keys to devising a highly efficient light source."

Mounted along one edge of the LCD panel, the outputs from the red, green, and blue LEDs are optically mixed. The composite white light is then applied to a lightguide mounted on the back of the panel (see the figure). A mirror is placed along the opposite edge to enhance the light's uniformity.

As for their output spectrum, CCFL backlights are seldom "tuned" to an optimal wavelength. Designers, then, are pretty much locked into the spectrum governed by the ionization of the mercury and the phosphor that coats the inside of the tube.

But LEDs can be designed to be narrow-bandwidth devices. Fuller says they can select the center wavelength of each red, green, and blue LED. From that center wavelength, the light output falls off sharply—within 20 nm—to each side. As it turns out, over 90% of the light is concentrated in that 40-nm band.

"So now we have a controllable white-light source, since by simply adjusting the current to the LEDs, we can shift to different 'white' points, which for some applications is very nice," Fuller adds.

What has held up the use of colored LEDs as backlights? "Though high-power reds have been in use for some time," Fuller notes, "it wasn't until just two years ago that we were able to make high-power blues and greens. Once we began constructing backlights and lighting systems out of LEDs, it was amazing the number of advantages that popped up."

Using the LED as a backlight has a number of other benefits as well. There's no mercury in the system. There's no tube to break, making it vibration and shock resistant. It can withstand high-temperature operating environments. The LED doesn't require high operating voltages—perhaps 30 V, at most. The LED, then, also may do away with the inverter required to power today's CCFLs. And, LEDs last approximately 75,000 hours while fluorescent sources have a 40,000-hour lifespan.

The company predicts that early LCDs will use an array of approximately 40 LEDs. FDS hopes to release a production display in the first half of 2001. Early versions probably won't exhibit much improvement in efficiency over CCFLs. Even so, FDS expects the efficiency to climb by 2002.

Looking beyond applications as an LCD backlight, Fuller says, "The LED will be at the center of the next major evolution in lighting, if, as we expect, the efficiency gains that have occurred continue to progress. The LED may very well replace all incandescent and fluorescent lighting. Twenty years from now, there may not be any incandescent and fluorescent market. It may be all solid-state LEDs!"

TAGS: Components
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