White LED Project members: Rim Hur
Thu Pham
Introduction
The idea of a “White light emitting diode” ems not to be inherently unusual or surprising. What is so special about white light, which is all too common since the invention of the light bulb? But it was not until the recent successful creation of high frequency light LEDs in the blue/ultraviolet region when the White LED made its debut. With the perspective of a highly efficient, cold light source, the LED market will have an enormous growth and the White LED will be a likely candidate for the replacement of the light bulb once production costs fall as the technology advances. Performance and Applications
幼儿英语的启蒙Compared to conventional incandescent light sources, the White LED is superior in lifetime, robustness, and efficiency. Operation cycles of 100,000 hours and an efficiency of 80-90% are today’s industry standard. After red, green and yellow LEDs have begun replacing traffic lights at interctions, White LEDs are next to substitute for light sources in areas where low maintenance requirement are desired. How fast they will find their way to our homes and general lighting depends however on our acceptance of the yet quite unusual white of the White LED. White light
As commonly known white light is compod of various colors of the whole range of visible electromagnetic spectrum. In the ca of LEDs, only the right mixture of complementary monochromatic colors can cast white light. This is achieved by having at least two complementary light sources in the proper power ratio. By adding more colors a “fuller” white is obtained that rembles more sunlight or light from conventional light sources that we are ud to. A chromaticity diagram reprents the result of the color mixing. The goal is to hit the center of the chromaticity diagram as best as possible.
高中英语教材
Fig.1
Illustration of
different colors
in the
stonehenge
chromaticity
diagram.
Monochromatic
colors are
located on the
perimeter.
2014amaWhite light is
located in the
center of the
chromaticity
diagram.[2]
Phosphors and Luminescence
Phosphors are widely ud in CRT display, UV lamps, and recently flat panel displays. When expod to some kind of excitement –e.g. electrons, photons, current etc. - they reemit the absorbed energy in form of light, usually in the longer wavelength region. This effect is known as luminescence.
As LEDs are monochromatic by nature, phosphors are also ud in most White LEDs to achieve white light in a simple and efficient manner. Phosphors are lected according to the absorption and emission requirements of the purpo. In order to get sufficient brightness, a high intensity LED is needed to excite the phosphor to emit the desired color which will combine with other colors to light that is perceived as white by the human eye.
Fig.2 Absorption and Emissions spectrum of a yellow phosphor [1]
Phosphors are usually made of Zinc Sulfide or Yttrium Oxides doped with certain transition metals (Ag, Mn, Zn etc.) or rare earth metals (Ce, Eu, Tb etc.) to obtain certain colors. The dopants have als
mike jacksono an important role in inrting point defects into the crystal. They provide the intermediate energy states for electrons in order to emit light in the visible spectrum after they fall back from the conduction band to the valence band. The mechanism for fluorescence, the luminescence that terminates after the causing effect depletes, depends also on temperature-related energy fluctuation of the lattice. The oscillation caus the displaced electron to escape from the potential traps created by the imperfections and fall back their initial state to emit light. The effect is prominent in rare earth and transition metals.
Nichia’s First Solution
The first commercially available White LED bad on phosphors was produced by Nichia Co., which was also first to manage to make the blue LED. Nichia ud a blue light emitting Gallium Indium Nitride and coated the chip with yellow fluorescent phosphor.
Fig.3 (a) Structure of white LED consisting of a GaInN blue LED chip encapsulated in a phosphor-containing epoxy
b) Conversion of blue light to yellow due to phosphorescence [1]
Fig 4 Emission spectrum of a commercial phosphor-bad white LED Note the peak in blue wavelength – LED will have a blue glimmer Another way to achieve white light is using a UV LED with RGB (red, green, blue) phosphors for a higher power output. Problematic is however the color rendering due to the inconsistency in the chromaticity of the colors and the harmfulness of UV-light itlf. High Efficiency LEDs –
The Photon Recycling Semiconductor LED (PRS-LED)
Another promising development us the effect of a condary emitting layer, which absorbs the light created by the primary light source and reemits yellow light. The principle is analog to the phosphor encapsulated LED, however, the condary light source consists of a fluorescent miconductor material, AlGaInP, bonded to the primary source wafer. It is therefore possible to u the same fabrication method ud to produce the LED chip.
Fig.5
Structure of the photon-recycling miconductor light-emitting diode.
The LED consists of a primary LED emitting in the blue wavelength range and a photon-recycling wafer emitting the complementary color. Materials Processing
A small concentration of dopant is introduced into the substrate through chemical and electrostatic process. After diffusion of the impurities through the crystal, the material is annealed to reestablish the crystal structure.
References
[1] The rearch group of E. Fred Schubert at Boston University
www.lightemittingdiodes
(Physics of LEDs with many figures, comprehensive but rather technical)
[2] E. F. Schubert, X. Guo, and J. Graff,
Photon Recycling Semiconductor Light Emitting Diode (PRS-LED) people.bu.edu/efs/PRSLED-WebSummary.pdf
[3] David N. Jamieson PhD., The University of Melbourne,
Light without heat – Luminescence in Moonlight, Video screens and other stories www.ph.unimelb.edu.au/~dnj/jl/jl95/jl95.ps
[4] David N. Jamieson PhD., The University of Melbourne,
Quantum Mechanics - Demonstration with your Cathode Ray Tube Video Monitor www.ph.unimelb.edu.au/~dnj/quantumdemo/tvmaster.htm
[5] The University of Greenwich, the Faculty of Science
Centre for Phosphor and Display Materials
uk/luminesc.html
[6] - Phosphors
facia
[7] LumiLeds Lighting, (Technical Documentations page)
/technology/tutorial/slide1.htm
[8] Phosphor data table, phosphors of different colors
/~speff/ufulinfo/phosphor.htm
Resources
Shuji Nakamura’s story
/2000/0800issue/0800profile.html
/interview/2000/070500nakamura/index.html
Everything about LEDs of all colors
ledmuum.home.att/menutop.htm
Manufactures
(Industry leader and most innovative producer of blue and white LEDs) jp (Japan homepage)
俄亥俄大学
/opto.index.html
, (-> Agilent Technologies and Philips Lighting) (-> Siemes AG)
jp/top_e/index.html
新东方背单词LED industry and market
Exhaustive list of manufacturers and retailers
/led_busdir.htm
Where to get White LEDs (Note: Quality might vary considerably, for best quality choo Nichia models) , ~$2
法规英文
, ~$2.50
, general electronic retailer ~$3
, ~$3
, ~$3
, ~$3
Al Lasher’s, 1734 University Ave, Berkeley, CA 94703, electronic store, ~$6
LED flashlights
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