Pittsburgh, Rainbows and LEDs
I just returned from a short trip to Pittsburgh, possibly the most
underrated city in the U.S. Good friends from Mars took me to
lunch at a restaurant on Mt. Washington, overlooking the city. I
had forgotten how breathtaking is the close-up view of Pittsburgh
and the surrounding hills, with the joining of the Allegheny and
the Monongahela to form the Ohio River. As appropriate when
you''re dining with someone from Mars, we discussed the news
report on Moon''s problems with overly eager developers.
Speaking of development, you may have read about the pros and
cons of the building and public financing of new sports arenas.
Since I left Pittsburgh in 1950, they tore down Forbes Field, built
Three Rivers Stadium, tore down Pitt Stadium, and now are
about to demolish Three Rivers. From the restaurant, I saw the
partially completed new football and baseball stadiums on either
side of the soon-to-be-no-more Three Rivers. This works out on
average to be one teardown and one new build every 16-17
years. They take their sports seriously in Pittsburgh!
What prompted my trip was the commemoration of the 125th
anniversary of the founding of the Chemistry Department of the
University of Pittsburgh. This happens to coincide with the 50th
anniversary of getting my Ph.D. from Pitt. One of the recipients
of the distinguished alumni awards presented at the banquet was
my research professor, Ed Wallace, whom I credit with teaching
me how to think properly about a scientific problem. Among the
other distinguished award recipients were Paul Lauterbur,
inventor of MRI (magnetic resonance imaging), and Allen Roses,
who led the effort that identified a genetic basis for one form of
Alzheimer''s. I could heartily applaud all of them.
The day of the celebration was marked by strange weather
involving a series of brief downpours of rain alternating with
brilliant sunshine. After one of these storms there appeared the
most spectacular double rainbow I have ever seen. Bits of it
were even shown on the evening news programs. What''s more,
the rainbow, which spanned the sky, ended on a lawn about a
hundred feet or so outside my hotel window. I had never before
seen the end of such a rainbow but, unfortunately, didn''t have the
time or tools to dig up the pot!
The bright colors in that rainbow bring me to my trip to
Pittsburgh on Continental Airlines. My plane was two hours late
in departing Newark Airport and I took advantage of a very wide
selection of complimentary magazines offered by Continental in
their lounge area. I couldn''t believe that among these were
copies of the National Geographic magazine that inspired last
week''s column on mass extinctions. I chose the September-
October issue of MIT''s Technology Review and was rewarded
with an article by Neil Savage titled "LEDs Light the Future". In
my fifth stocksandnews.com column, I discussed my work and
that of others on red and green light emitting diodes (LEDs).
In that column, I considered various applications of LEDs in
stoplights on automobiles, traffic lights and their use as indicator
lights on computers, clocks, phones etc. In the article, Savage
describes his visit to a company called Color Kinetics in Boston.
He was, as I would be, fascinated with a couch that was bright
red but before his eyes changed color from red to crimson to
navy blue and back again. Actually, of course, the couch wasn''t
turning color but the light illuminating it was. The couch was
illuminated light from little spotlights containing LEDs. Each of
the lamps contained three LEDs that emitted red, green and blue
light. With these three colors, you can create virtually any other
color by mixing the three in the proper proportions.
Let''s say you have the three types of LEDs in a single lamp and
each LED is connected to a dimmer switch. By dimming or
brightening the individual LEDs you can manually control the
color of the light coming from your lamp. Feeling passionate?
Turn up the red or whatever you think will turn on your partner.
Just want a relaxing evening with a good book? Perhaps a grassy
green is to your liking. To read that book, however, chances are
you''d like a reading lamp giving off white or slightly yellowish
light similar to that given off by your incandescent bulb.
All this variety of light from an LED lamp was just a dream only
a decade ago. Although red, green, yellow and orange LEDs
were common there were no practical blue ones. The search for
a viable blue LED began back in the 1960s and I recall being the
chairman of a session in which the hot topic was blue light from
silicon carbide. The silicon carbide LED never flew, however,
and it wasn''t until about 1990 that a real breakthrough occurred.
To set the stage for this development, you should know that what
determines the color of an LED is the particular chemical
compound you use to make the LED.
Today''s stars in the LED game are the so-called III-V (three-five)
compounds. These are compounds made from elements in
groups III and V of the periodic table. For nonchemists, some
group III elements are aluminum and the less familiar gallium
and indium; group V includes arsenic and phosphorus. My own
work in the ''60s was on gallium phosphide (gallium and
phosphorus). Today''s red and green LEDs are made typically of
compounds that combine the four elements aluminum, gallium,
indium and phosphorus. Varying the relative amounts of these
elements in the compound produces different colors.
But no blue light. That was left up to an unknown worker in a
small Japanese company, Nichia Chemical Industries, a
manufacturer of phosphor coatings for TV and computer monitor
screens. The researcher, Shuji Nakamura, chose a III-V
compound of gallium with nitrogen, another group V element.
The compound, gallium nitride (GaN), was known but was very
difficult to make and control. However, Nakamura persevered
and managed to make high quality material suitable for practical
blue LEDs. It''s not just enough to make an LED that emits blue
light. The LED has to withstand passing the electrical current
through it to generate the light and, very importantly, last long
enough to make the LED economical. How often do you have a
bulb burn out at an inopportune moment?
With blue at hand, why the concern for white LEDs? For such
applications as traffic lights, a red LED uses only about a tenth
the power needed for an ordinary red traffic light and lasts for 10
years or so. However, 20 percent of the electricity in the U.S. is
used for lighting. For lighting, you want the white light of a
fluorescent lamp or the near-white light of your incandescent
bulb. And cost is important. A typical 100-watt bulb costs about
25 cents and gives off 1,500 lumens, a lumen being a measure of
how bright something appears to the human eye. I won''t define a
lumen but now you know what 1,500 of them look like. Sadly,
you wouldn''t want to pay a couple hundred dollars for a white
LED lamp that would give off 1,500 lumens, even if one were
available and it would last a long time. In other words, the white
LED reading lamp isn''t here yet.
Here''s one of the problems in a nutshell. Ideally, when you turn
on the current through an LED, you''d like each electron to
generate a photon of light. For the red and green LEDs, almost
all (90 percent) of the electrons do produce photons. In the blue
gallium nitride LEDs, only 30 percent of the electrons generate
photons. But this isn''t the whole story. Once you''ve made the
photons, you want them to escape from the LED to light up your
room. Enter the refractive index of the LED material. This is a
measure of how much light bends and is the reason that when
you look into a pool of water, the fish isn''t where you think it is.
The red LED materials bend the light so much that most of the
photons just bounce around inside the LED and don''t get out.
Only about a third get out for you to see. The bending is less for
the blue gallium nitride LED but not as many photons are
generated in the first place. Bottom line - today''s white LED
lamps deliver about 10 lumens per watt compared with 15
lumens per watt for ordinary supermarket light bulbs. By
working with the shape of the diodes themselves a lot can be
done to get more photons out, especially for the red and green
LEDs but the blues still have a long way to go, at least for
Nevertheless, the progress has been fantastic. In my day, we
were putting out less than a lumen per watt and we managed to
put LEDs into telephones and the like. There is a move afoot to
get Congress to fund a major effort to advance the LED field
sufficiently to replace half the lighting market with LEDs by the
year 2025. If I were younger, I would love to be a part of that
There is another problem. Today''s incandescent light bulb gives
off light that promotes a warm fuzzy feeling. One reason that
more people haven''t embraced the more energy efficient
fluorescent bulbs on the market is the quality and harshness of
the typical fluorescent light. Ideally, as we mentioned at the
beginning, the LED lamp should provide a dial-up solution to
light of any desired tone or intensity. Savage ends his article
with the optimistic thought that someday, when LED lamps have
taken over, instead of a candle lit for special dinners one would
place an old fashioned incandescent bulb on the table.
Back to lunch, it just occurred to me that you might think that
either I was dining with aliens or I had flipped my lid. Mars and
Moon are localities in the Pittsburgh area, an area that embraces
such places as my wife''s hometown of Mutual, as well as
Pleasant Unity, United and Norvelt. The latter is named after
Eleanor Roosevelt, who visited the area during the Depression
era. My "Martian" friends were Al, my best man at my wedding
nearly 50 years ago, and his wife, Mary Louise. I owe a lot to
Al. A certain coed invited him to take her to a dance when we
were at Pitt. Al refused but suggested me as a substitute. That
coed became my wife. Thanks, Al.
Allen F. Bortrum