great article for anyone interested in CREE
techreview.com
LEDs Light the Future
Roll over, Tom Edison. Drawing on new semiconductor technology, muscular offshoots of
those dainty colored dots could shine bright white light that illuminates the world.
By Neil Savage
A riot of light assaults a visitor walking into
the lobby of Color Kinetics on the 17th
floor of a downtown Boston office
building. Swirled designs on posters
change from orange to green, clear plastic
shapes glow blue, purple and red in quick
succession. And a question soon arises:
What color is that couch? It shines cherry
red, fades to crimson, turns navy blue, then
begins the cycle again.
In fact, the couch is red. It’s always red,
and only the light shining on it from dozens
of tiny spotlights changes, as Color
Kinetics demonstrates the effects possible
with its digital lights. Each little lamp
contains red, green and blue light-emitting
diodes (LEDs), which light up in varying
combinations under computer control.
“We’re revolutionizing the lighting industry with what we consider a disruptive technology,”
enthuses company president George Mueller, tall and ponytailed with the Gen-X standard goatee.
“It’s a new way to create light.”
Mueller and his co-founder, Ihor Lys, have married computer software to a decade of advances in
LED technology. LEDs have become ubiquitous in daily life, glowing from the faces of VCRs,
clock radios and microwave ovens. But these LEDs have been humble indicator lights on all
manner of electronic appliances. Once limited in brightness and stuck at the red end of the
spectrum, LEDs have become more powerful in the past dozen years. And a breakthrough in the
early 1990s created blue LEDs, suddenly making the whole rainbow available and holding up the
promise of white-light LEDs—either by blending the output of colored LEDs or by more exotic
techniques. Color Kinetics buys LEDs from device makers such as Agilent and Cree and
incorporates them in lamps that give off virtually any color—changing a white wall or a store
display from pale green to hot pink at a whim.
Their devices, aimed right now mostly at the retail and entertainment markets, take advantage of
some of the special characteristics of LEDs: small size, light weight, low power consumption,
nearly infinite selection of colors. But lighting experts say this is only the beginning. Ahead lie
entire buildings that light up, traffic lights that last a decade, headlights that won’t exhaust your car
battery if you leave them on and perhaps even cheap, long-lasting lamps that will drive
incandescent and fluorescent bulbs to extinction.
Making Light Work
Thomas Edison is acclaimed, above all else, for inventing the light bulb. While his other
inventions—such as the phonograph, the mimeograph and the tickertape machine—have been
displaced in recent decades by digital technologies, the light bulb continues to shine on. Now,
after 12 decades, technological advances are finally threatening to dethrone it. Anticipating the
transition, the major lighting manufacturers are forging alliances with LED makers. General
Electric Lighting joined forces with chip-maker Emcore last year to form a lighting division called
GELCore, based in Independence, Ohio. Philips Lighting and Agilent Technologies, a
Hewlett-Packard spinoff, created LumiLeds in San Jose. And Osram Sylvania has teamed with the
semiconductor business of its parent company, Siemens. “You’re seeing a big drive by light bulb
manufacturers, who are in some sense eating into their own business, but with the recognition that
if they don’t do it, someone else will,” says Makarand Chipalkatti, marketing and technical
manager for LED light services at Osram Sylvania in Danvers, Mass.
Not all the players in this field, however, are big companies. Startups are getting into the act as
well. Mueller and Lys of Color Kinetics met at Carnegie Mellon University, where Lys earned a
doctorate in electrical engineering and Mueller majored in computer and electrical engineering,
with a minor in fine art. Their first venture into lighting, in 1992, was to build a novelty sign like
one Mueller had seen in the Detroit Science Center. A single vertical row of LEDs displayed slices
of an image, one column at a time. The human brain, responding as if the viewer’s eye were
scanning across an unbroken image, would reassemble the picture. The first sign Mueller made
said “LOVE”; he gave it to his mother. Then, for the roommates who bet him he couldn’t do it, he
built a sign that read “BEER.”
“We wrote a business plan around it because I was taking business school classes,” recalls
Mueller. He had three goals for the resulting firm, Stone Age Technologies. The first two were
typical student desires: earn some beer money and get a couple of freebies (in this case, free
signs). “The third goal was to be on the cover of In Pittsburgh magazine,” he says, picking
through a basket of designer potato chips to find the orange ones. “My new goal is Rolling
Stone.”
“I’ll settle for the Wall Street Journal,” puts in Lys, the shorter-haired, less kinetic half of the
pair. The technical guru, Lys is an engineer who, like his counterparts at computer startups,
comes to work around noon and stays late.
Although Lys and Mueller came up with the idea for digital lighting in 1993, they put it aside
when Mueller and his brother Gary went off to found an economics research firm, Internet
Securities, Inc. The brothers sold 80 percent of the Boston-based company last year for $43
million. Mueller disdains the traditional “brass, glass and gas” lighting industry. “It’s boring,” he
declares. “There’s no technology involved.”
Maybe not, but there certainly is money involved. The United States buys $3.5 billion worth of
incandescent bulbs, fluorescent tubes and halogen lamps each year; globally the market is $11.5
billion. So far, the market for bright, visible-light LEDs is relatively puny—about $680 million,
according to research firm Strategies Unlimited. But advances in LED technology are moving
these devices into an increasing number of applications, and the market is expected to grow to
$1.8 billion in five years.
The White Answer
The first LEDs were built in the early 1960s. The tiny chips of semiconductor material, encased in
a clear epoxy, give off a single color of light when electricity runs through them. Negatively
charged electrons move to fill positively charged regions in the material, called “holes,” where
electrons are missing. The combination of an electron and a hole produces a photon of light. The
greater the difference in energy between electron and hole—the so-called bandgap—the higher the
energy of the photon that emerges. The energy of the photon corresponds in turn to the color of
the light; within the visible spectrum, blue and violet photons carry the most energy, orange and
red photons the least. Different materials naturally have different bandgaps, so to change the
energy level and hence the color of the photons, engineers grow the crystalline semiconductors out
of different alloys (see sidebar: “They Come in Colors”).
High-brightness monochromatic LEDs are already making headway in the marketplace. About 10
percent of the red traffic lights in the United States have been replaced by LEDs. They are more
expensive than conventional light bulbs but have other advantages that outweigh the cost issue.
One is efficiency: A red LED traffic light uses only 15 watts of electricity instead of the 150 watts
consumed by traditional stoplights. Another is longevity—the LEDs are expected to bring traffic to
a halt without burning out for a full decade. Single-color LEDs’ compactness, low power, intense
colors and low heat also have them popping up as car taillights, airplane warning lights on radio
towers and runway lights at airports. But the minds of researchers and the eyes of the lighting
industry are focused on white.
And that creates a challenge: How do you get white light out of devices that are, by nature,
monochromatic? One method involves mixing LEDs of different colors so they appear white. Just
as a television set makes all the colors it displays—including white—out of glowing red, green
and blue phosphors, the right combination of LEDs can give the appearance of white. The
standard way to mix is with three separate red, green and blue diodes, but the right combination of
just two—say blue and orange—can also produce white.
A second method uses an LED to power something else that emits white light. Start with a device
made from gallium nitride (GaN), which emits blue light. Coat the inside of its epoxy casing with
a phosphor—a material that emits white light when struck by blue or ultraviolet photons. The
phosphor thus turns the blue LED into a white-light emitter. (Ordinary fluorescent bulbs work by
a similar principle; ultraviolet emission from an electric discharge in the tube causes the phosphor
to shine bright white light.) Such devices have become possible only with the arrival of practical
blue LEDs; photons at the blue end of the spectrum are more energetic than red ones and have
enough punch to trigger a phosphor’s emission.
Researchers at the Boston University
Photonics Center have recently come up
with a white-light diode that combines the
mixing and exciting methods (see
illustration, Full Spectrum). Their
“photon-recycling” device essentially
consists of two LEDs stacked one on top of
the other in the same chip. The bottom LED
is made of blue-emitting gallium nitride.
This light strikes a layer of a different
material—a complex alloy of four materials:
aluminum, indium, gallium and
phosphorus, or AlInGaP (pronounced “allengap”). Some blue photons give up their energy to
create yellow photons; others slip through unimpeded. Choosing the right material and the right
thickness produces photons that mix to give white light.
Fred Schubert, a professor of electrical and computer engineering at BU and one of the designers
of the system, says any color of light should be possible with this setup. He is working on a
system that uses differing concentrations of another material—indium gallium nitride—to produce
the blue and the yellow. By skipping the AlInGaP layer altogether, this approach would simplify
manufacture. That advantage would come at a cost, though. In a multi-chip design, the color of
the emitted light could be tuned at will by applying more power to one diode and less to another.
In a photon-recycling device, the color is forever fixed by the materials chosen.
The Photonic Squeeze
Whichever methods are chosen to make white light, the LEDs involved have to put out more light
and become more energy efficient if they’re to replace Edison’s bulbs. White LEDs produce about
10 lumens of illumination per watt of electricity consumed, which is comparable to the
performance of incandescent bulbs (a lumen is a measure of how brightly the eye perceives light).
Roughly 10 percent of the electricity that they consume gets turned into light—marginally better
than the 7 to 8 percent figure for incandescent bulbs. But LEDs are still too expensive to challenge
your average GE Soft White. On sale at the local discount store, 100-watt incandescent light bulbs
run about a dollar for a package of four and deliver 1,500 lumens of illumination apiece. “I cannot
make an LED that gives you 1,500 lumens for 25 cents,” says Roland Haitz, research and
development manager of the semiconductor product group at Agilent. He predicts that in “a few
years,” his group will be able to make a 1,500-lumen LED that could sell for $150. He doubts the
average homeowner will be rushing out to buy such a product.
LEDs made from AlInGaP are pretty efficient at turning electricity into light. About 90 percent of
the electrons that enter the diode generate photons. Unfortunately, this semiconductor alloy also
has a high index of refraction (a measure of how much a material bends light rays). Instead of
shining out for all to see, therefore, most of the photons bounce around the interior of the diode
and turn to waste heat; only 30 percent of them emerge as visible light. GaN has a lower index of
refraction, so more light escapes from LEDs made from that material. Only 30 percent of the
electricity fed into a GaN device is converted to light in the first place, though, so the final
efficiency is still only about 10 percent. That’s plenty bright for something like a traffic light, but
not for general illumination.
This is not an insurmountable problem, says George Crawford, chief technical officer at
LumiLeds. Researchers there have been experimenting with new structures of the diodes to let
more photons escape. Conventional LEDs consist of cube-shaped crystals. But by arranging the
layers of the semiconductor differently and cutting the wafers to create sloping sides, LumiLeds
has created an inverted pyramid that results in a shorter optical path for the photons to traverse. In
the lab, LumiLeds has managed to get half the photons out of an inverted pyramid LED made of
AlInGaP, and they hope to have such LEDs in commercial production this year. “Fifty percent is
plausible, but hard,” Crawford says, but adds: “It’s hard for me to envision doing much better
than that.”
Getting half the photons from AlInGaP may be enough to compete with fluorescent lights, but not
by itself. Devices made of that material provide only the red and yellow light. The complementary
blue photons needed to produce white light must come from gallium nitride, and there the
technology is still embryonic. “We really don’t understand the fundamentals of how to build a
better crystal in gallium nitride,” says Steve Johnson, leader of the lighting research group at
Lawrence Berkeley National Laboratory.
Researchers are looking for a shot of government funding to help their quest. Arpad Bergh,
president of the Optoelectronics Industry Development Association, wants a major R&D effort to
bring LEDs to the point where they can compete with traditional light sources. His group is
working with Johnson at Lawrence Berkeley to develop a research plan for more efficient
white-light LEDs and intends to ask Congress for a five-year, $50 million per year funding
program that would start as soon as 2002. Meanwhile, Haitz has written a white paper with
Sandia National Laboratories calling for the government to pour $500 million into research over
10 years. Such an expenditure is needed, Haitz argues, to lift LED lighting over the hurdles that
are now impeding progress. Left on its own, he argues, the lighting industry will advance LEDs
only enough to take about one-tenth of the lighting market by 2025. But with government help, he
says, the devices could by that year account for half the market. Because lighting accounts for
about 20 percent of the electricity consumed in the United States, a shift to the more efficient LED
technology could render significant energy savings.
An Eerie Glow
But if LEDs are to capture a large share of the illumination market, they will have to produce light
with the right tone. As anyone who has ever taken an indoor picture with outdoor film knows,
incandescent light has a strong yellow cast, and designers say it has a warm feel. White phosphor
LEDs, on the other hand, emit a distinctly bluish glow. “If you’re trying to illuminate a red object
with a white LED that only has blue and yellow in the spectrum, you’re not going to get a very
nice-looking red,” warns Kathryn Conway of the Lighting Research Center at Rensselaer
Polytechnic Institute. That can be a problem with human skin, for instance, which looks unnatural
under light that doesn’t approximate daylight. Renowned New York lighting designer Howard
Brandston points out: “You don’t want someone to wake up in the morning and look in the mirror
and say, ’Egads! I could audition for ”The Addams Family“ without makeup.’”
But the technology will definitely keep improving, since the payoff is large—in part because with
LEDs, you’ll be able to turn a dial to get lighting with just the right feel for the situation at hand.
Brett Andersen, a senior designer at Focus Lighting, a New York-based lighting design firm,
envisions a day when people will be able to set the color and brightness of the light in their homes
according to their moods. This kind of control will make the old-fashioned dimmer switch a
primitive tool for creating ambience. Beyond that, LEDs offer new possibilities that raise more
fundamental questions about how people think about lighting, says Chipalkatti of Osram Sylvania.
“How would things look if the building itself was a light fixture?” he asks. “You could have your
floor or your ceiling light up.”
In the offices of Color Kinetics, software writer Mike Blackwell sits on the chameleon couch and
demonstrates the program he’s developed for lighting designers to create effects with the
company’s lights. He sets a row of lamps to run through the spectrum, repeating the cycle every
10 seconds. Then he adds a white pulse that moves down the row once a second. The effect is
jarring, reminiscent of the psychedelic light shows of the 1960s. It also suggests artifacts of
another era: those desktop-published newsletters of the mid-1980s, brimming with clashing fonts.
But if lighting designers are right, more sophisticated users will be able to create subtler effects or
repaint their walls with light. And perhaps the incandescent bulb will join an earlier lighting
standard, the candle, as a quaint accent for special occasions, while our days and nights are lighted
in the glow of tiny chips.
Neil Savage is a freelance writer living in Lowell, Mass. He has written for Astronomy,
The Scientist, and the Discovery Channel. |
