LED light-emitting semiconductor chip as the use of solid materials , when both ends with a positive voltage, the semiconductor carrier recombination occurs , the excess energy released due to the visible light photons emitted . In recent years , LED lights has been rapid development , some products have entered the commercial stage , bring revolutionary changes to the indoor and outdoor lighting.
The early 1960s , people have mastered the method of the semiconductor material can produce light : forward biased GaAs junction diode can be used as a radiation source . In 1964 , the use of GaAsP materials, developed a red light-emitting diode (λp = 650n m), luminous efficiency of 0.1lm / W, the drive current of 20mA , the luminous flux is only a few thousandths of lumens.
To the 1970s , developed a green (λp = 550nm), yellow (λp = 590nm) and orange (λp = 610nm) of the LED panel lights, luminous efficiency of 1lm / W. Monochrome LED booming , making it on the big screen display , traffic lights , instrument displays and other fields are widely used . Unfortunately achieve breakthrough full-color display Blu-ray yet .
In the early 1980s, developed the GaAIAs of the LED, making the red LED spot light efficiency of 10lm / W.
1994 , the development of GaN based blue and green light-emitting diode according to the synthesis of three -color theory, the full color display and semiconductor lighting becomes possible. For general lighting purposes, people need more white light. In 1996, Japan's Nichia Corporation pioneered the development of white light-emitting diodes, brought great hope for the LED tube lighting into a wide range of fields, as a milestone in the history of the development of LED lighting .
After several years of development , such as performance -color LED light efficiency has been further improved, making its decorative lighting and other fields has been widely promoted and applied. Several key features colored LED 's shown in Table 1 .
Currently, the white LED light efficiency is not high, but growing fast. According to the relevant information on: 1999 Japan developed a luminous efficiency of 15lm / W of LED; 2001 United States developed a luminous efficiency of 40 ~ 50lm / W of LED, even more than incandescent and halogen , fluorescent light proximity effect . Single LED power is not big enough , only 1W more , it can not be widely used for indoor and outdoor lighting, but IPL LED decorative lighting applications in the field has matured. Experts believe : white LED light efficiency is only reached 50lm / W or more, and to enhance the single white LED power , in order to truly be widely used in the lighting field . Currently, the white LED is moving towards high efficiency, high brightness , high power, high color rendering index of the direction of development . Currently , the world focused on the research and development of white LED in the near future , white LED light efficiency is expected to reach 100lm / W, and even upgrade to 200lm / W or more, then , LED will occupy a dominant position in the field of lighting .
2, LED structure and working principle
Mainly by the PN junction LED chip , an electrode, an optical system and other components and accessories . Called LED chip emitting body , which area is 10.12mil (1mil = 0.0254mm2).
In a clean epoxy resin -encapsulated semiconductor chips. P -type semiconductor PN junction end of the positive voltage , the N-type hole will flow terminal electron flow from the P-type N -type terminal end . When electrons pass through the wafer , the negatively charged electrons with positive holes move to the region of holes and electrons can be combined directly in the binding process, the release of energy in the form of a photon of light , which is the LED light-emitting principle . In addition , after some LED electrons accelerated in the crash , the process can also be free to release energy and light. Energy ( band gap ) between the electron and the hole larger , the higher photon energy produced . Most conventional light emitting diodes using gallium arsenide (GaAs), gallium phosphide (GaP) , or combinations thereof crystals (GaAsP) , etc. group III-V semiconductor.
LED luminous efficiency, color and optical properties of semiconductor materials and processing technology related. Incorporation of P-type and N -type material in the different impurity materials , it can be a different single wavelength of the light emitting diode LED , and electrical properties will be different. According to the optical principle , by a continuous white light spectral composition , therefore , impossible to produce a continuous spectrum emitted directly single white LED. In the lighting field, the most widely used is white , so white LED research and technological advances related to LED bulb lighting in the area of application and promotion.
A single light emitting diode LED optical system comprising a transmitter parallel polarizers, etc. through the mask . The optical system of the LED light beam or a parallel light beam emitted angle ( Figure 1 ) . Thus, changing the geometry of the enclosure dome , the LED can change the beam angle .
Two kinds or three kinds of monochromatic LED may be formed of a synthetic white light generated by the LED to be mixed together by a few monochromatic LED , synthesized according to the principle of the three primary colors white light , as shown in Table 2 . Currently, there are three main commercial white LED light way :
( 1 ) coated on the InGaN blue wavelength 460nm YAG fluorescent material layer of the grain , using the blue LED irradiating the fluorescent substance , a yellow wavelength of 555nm can be generated , and then use the principles of the lens can be complementary to blue, yellow mixture , the can emit white light (see Figure 2 ) . However, a strong blue spectrum spectrum , the lack of red spectrum , color rendering index Ra to reach 70. This is currently the most widely used white LED production principle .
( 2 ) excited by ultraviolet chip RGB trichromatic phosphor luminescence mechanism with three -color fluorescent similar. Changes in the ratio of phosphor composition , you can get a variety of color temperature of white light , the color is good.
( 3 ) of the RGB three primary colors chip package together , changing the light emission intensity ratio of the three, can be obtained at any temperature of the white ( Figure 3 ) , but the product quality is difficult to control . Consistency of product performance is poor , vulnerable to environmental impacts.
" Figure 4 , Figure 5" shows a schematic view of the white LED and the light emitting structure : GaN chip and the yttrium aluminum garnet phosphor (YAG) package together, blue LED chip mounted on the bowl-shaped reflection chamber , the cover substrate a thin layer of resin mixed with YAG phosphor. After power , GaN chip emits blue light (λP = 456nm, Wd = 30NM). After part of the Blu-ray excitation YAG phosphor emits yellow light , yellow light mixed with part of the blue , you get white light. For InGaN / YAG white LED, the YAG phosphor by varying the chemical composition and adjust the thickness of the phosphor , it is possible to obtain a colored white 3500 ~ 10000K .
3 LED 's performance
High luminous efficiency of 3.1 and has great room for improvement
Monochrome LED luminous efficiency has reached 100lm / W or so, with the highest light efficiency sodium, metal halide fairly . Currently white LED light effect , although only 15 ~ 50lm / W, but it has gone beyond incandescent, halogen light efficiency, close to fluorescent light efficiency . Experts predict that in the most promising areas of illumination white LED luminous efficiency , as well as a great room for improvement . Will set off a revolution in the lighting field .
3.2 Response time is short
Incandescent response time in milliseconds , LED response time places nanoseconds . This feature makes the LED decorative lighting as a whole can change color in an instant consensus reached dreamy effect .
3.3 Energy saving
LED supply voltage DC 6 ~ 12V, single-tube power 0.03 ~ 0.06W, current 15 ~ 18mA. In the same lighting effects , the power consumption of incandescent 1 / 8 , Fluorescent 1/2 . Such as the use of LED replacement of existing incandescent and fluorescent lamps, will get a very considerable economic benefits. After further improve LED light efficiency , energy saving effect will be more significant.
3.4 long life
LED small size , fast response , light weight, epoxy resin , durable , with an average life of 100,000 hours , you can use 5 to 10 years. Easy to install concealed , while maintaining a small amount , low maintenance costs .
3.5 Application flexibility
Can be made as points, lines , surfaces and other forms of light products .
3.6 beam directional
Each LED light bulb can be controlled with a certain directivity. No glare and soft light, the built-in micro-processing system to control light intensity , adjusting the light way .
3.7 Variable Color
LED light color can be changed with the current size . LED can be easily modified by chemical methods , adjust the material's band structure and band gap , to achieve red, yellow , green, blue , orange, purple and other multi-color light . This feature is very useful in decorative lighting .
3.8 Safe and reliable pollution
LED low voltage, safe to use, especially for public places. Because it is a cold light source , while working low heat, no infrared and ultraviolet radiation. LED mercury , sodium and other metallic elements endanger human health , is a truly green lighting.
4 LED problems to be solved
Although the LED has many excellent properties, but there are still some problems to be solved .
( 1 ) improve the efficiency of white LED light
So single white LED light effect as soon as possible from the current 50lm / W or less raised to 100lm / W or more, to get more energy-efficient economies ;
( 2 ) increase the power of single LED
The current application of the LED, single 1W less power in general , in order to meet the needs of more than just a combination of the LED decorative lighting, traffic signals and lighting needs. To really make LED lighting is widely used in the field , we must improve the single white LED power . Especially in outdoor lighting , the widespread use of high-power light source , get a higher flux. Therefore , LED lighting to really enter the field, especially into the outdoor lighting field, there is still a long way to go .
( 3 ) improve white LED color
Compared to the white light emitted from the LED with natural light , there is still a gap. Ingredients containing blue light emitted from the LED white light , giving visual unnatural feeling. Incandescent light , yellow light contains a strong component , gives a warm feeling. Therefore , to further improve white LED color rendering index, reducing white blue component , due to increase yellow component , is one of the future to improve the quality of the white LED light an important issue .
( 4 ) reduce the price
Reduce LED prices , is also a problem that must be addressed . Currently use a variety of LED decorative lights, marker lights , traffic lights , etc. , are made on more than just adding accessories LED combination lamps made . LED lights set the price is the same set of general lighting lamps lighting effect several times , it is difficult to be accepted by the majority of consumers .
Status of white LED and improved methods shown in Table 3 .
5 LED Applications
Currently , spectral color LED products , has been covering the entire visible spectral range , and high luminous efficiency, color purity , high brightness , good direction , color variable , such as excellent performance , widely used for decorative lighting and signal flags fields.
The white LED technology performance to be further improved, so white LED indoor and outdoor lighting has not been widely used, but experts expect : According to the current trend of rapid development of LED technology , white LED is expected in the near future , become the incandescent a new generation of lamps, fluorescent lamps , gas discharge lamp lighting together accounted for hegemony dominance .
LED has two external power supply ways: one is LED and other high-grade electronic components together into a reliable light-emitting line is directly connected