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."
The light from red, green, and blue LEDs mounted along the edge of
the LCD panel are mixed to create a white light. By controlling the
current applied to each LED, the spectral output may be tuned.
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 above). 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!"
elecdesign.com
VBG, Jim |
