emitting diodes (LED's) have been very common in the past
several decades as pilot lights, early calculator displays,
and as invisible infrared emitters for remote
controls. The early LED's were predominantly red
and infrared due to the nature of common and inexpensive
semiconductors. Blue LED's were finally developed in
the early 1990's. That paved the way to produce white
light from highly efficient LED's. The photo here
shows blue LED pilot lights on my data router.
The recipients of the 2014 Nobel Prize in Physics -
Isamu Akasaki and Hiroshi Amanok from Japan, and
Japanese-American scientist Shuji Nakamura - had discovered
recipes for semiconductor compounds Indium Gallium Nitride
that were successful in emitting blue light. With the
advent of blue LED's it soon became possible to produce
white light emitting diodes for illumination purposes.
How does a blue LED produce white light? One
particular compound can only produce light of a single color
determined by its energy band gap. White light
requires all colors.
Because red and green LED's have been around for awhile,
white light may be created by the combination of
closely-packed red, green, and blue LED's. A similar
technique is used in color televisions, but LED's are too
expensive compared to back-lit colored liquid crystal
displays. The older technology for white light on
color TV's used tri-color cathode ray tubes, where the white
light is caused by closely-spaced glowing phosphorescent
dots that are excited with electron beams.
However, the common method for producing bright white
light LED's for
headlights and flashlights is to coat a powerful blue LED with a phosphor coating. The phosphor is Y
12:Ce, or "YAG" (no phosphorus in a "phosphor"). The YAG phosphor absorbs the blue light and re-radiates a broad band of light composed of all colors (white). With equipment loaned to me by Dr. David Coffey of the Warren Wilson Chemistry-Physics Department I determined that typical white-light LED's use the phosphor. I made photographs of the spectra emitted by a white light LED and compared that spectrum with separate Blue, Green, and Red LED's.
Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department. These photos feature interesting phenomena in the world around us. Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation. Atmospheric phenomena are especially welcome. Please send any photos to firstname.lastname@example.org.
All photos and discussions are copyright by Donald Collins or by the person credited for the photo and/or discussion. These photos and discussions may be used for private individual use or educational use. Any commercial use without written permission of the photoprovider is forbidden.