Physics Photo of the Week

October 17, 2014   

2014 Nobel Prize in Physics - Blue LEDs

Light 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
3
Al
5
O
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.




The top image shows the spectrum of colors emitted by the white LED.  The horizontal axis of the image represents the wavelength.  Red wavelengths, being longer than blue, are displayed further right on the image.

The second image shows the simultaneous spectra of 3 separate blue, green, and red LED's.  The LED's form a vertical column to the left of the image.  The separation of the wavelengths for the three different colors is clear.

The graphs at the bottom show the intensity vs. the wavelength for the four LED's.  The top trace shows the white LED spectrum.  Notice there is a large intensity at that of the blue LED indicating that a blue LED is the basis of the light.  The rest of the white LED spectrum is a broad band that includes both the green and the red.  This broad band is the emission of the phosphor.  A white-light LED for head lamps, reading lamps, and flashlights, typically look rather "blue".  The spectrum trace shows why: there is still a peak in the blue color emitted, unlike ordinary "white" light that has no gaps between the blue and red - unlike the white-light LED.

The bottom trace shows the spectral intensity that would result if the white were made with separate blue, green, and red LED's.  Notice that there are 3 distinct separated bands for the blue, green, and red with three separate LED's.  The bottom trace is the equivalent of the sum of pixel values in each column.  Each column represents a unique wavelength.

This year's Nobel Prize in Physics is the third popular consumer item that has been featured in Physics Photo of the Week.  The others are the transistor (PPOW for November 18, 2005), and the CCD detectors for digital cameras (PPOW October 9, 2009).  Physics Photo of the Week would not be possible without these two earlier inventions.



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 dcollins@warren-wilson.edu.

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