I Overview The development of LED has been nearly 30 years old and has made great progress. The presentation of white LED has been honed for more than ten years and has developed rapidly. In the era of energy shortages and serious pollution, LEDs came into being and received much attention. In order to alleviate energy shortages, the Japanese government took the lead in the mid-1990s, and the US government financially supported the domestic technology industry and industry to promote the development of solid-state lighting. The Chinese government (including the Ministry of Science and Technology, including the Ministry of Energy, Economic and Trade Commission, Ministry of Information Industry, etc., as well as local governments) also strongly support and fund the research and development of LED, and established the National Semiconductor Industry Alliance to promote China's solid-state lighting (China called Plan for semiconductor lighting , and expects that the luminous efficacy of white LEDs can be increased to 100 lm/W in 2010, and fluorescent lamps with a large replacement efficiency of about 90 lm/W can be widely used in general lighting fields, [1] [2] .
Under such circumstances, a large number of enterprises, investors, researchers and technicians in China have stepped in and invested in large-scale investment in LED R&D and production. So far, it has been estimated that a total of billions of yuan has been invested in less than ten years. It has achieved outstanding results and formed a large-scale industrial chain.
The world's attention to LEDs, especially white LEDs, has its inherent reasons. The current huge energy consumption and the resulting energy shortage, rising prices, and especially the environment have made energy conservation a very urgent task. A large part of the energy consumed by countries is used for lighting. Lighting power in China and developed countries accounts for about 12-13% of total power generation, which is a considerable energy loss. At present, the light source used in the world is still dominated by incandescent light sources with low efficiency, and it is very promising to tap the potential.
The LED field has concentrated many outstanding talents and achieved brilliant results. Some research departments announced that their LEDs have achieved 120lm/W or even 150lm/W light effects under certain conditions. Therefore, most companies have also claimed that the light efficiency of their products has reached 70 lm / W, 90 lm / W or higher, because the light radiation of LED is oriented, its light utilization is only relying on the ordinary light source with uniform radiation output. The directional reflection of luminaires is more efficient, so LED is considered to be the best functional illumination source at present, and it is vigorously promoted to various lighting fields. It has been announced several years ago that LEDs will replace compact fluorescent lamps in large quantities in 2010. Lighting, and asserting that white LEDs are currently the best road lighting source.
Undoubtedly, after more than 20 years of development, LEDs have achieved complete success in the display field and have become unparalleled and irreplaceable display devices. In terms of decorative lighting, it also fully displays its characteristics and achieved half of the country. The outstanding performance of LED at the opening ceremony of the 2008 Olympic Games is even more shocking to the world. But the fact that can not be ignored is that white LEDs still have large limitations in functional lighting. China is already the world's largest LED production base and consumer market. The application field of LED is broad, but due to the current technical level and actual performance limitations, the use of such devices for functional lighting is not something that they use. The so-called high-power LED, the largest power device can be found on the market, but only 3W, even if the 10W device under development can be mass-produced, it is a single particle for the conventional lighting, especially the hundreds of watts used in road lighting. The power loss is too small. The use of white LEDs to design a lighting project must be very large, and it will not be worth the cost of design or cost.
II LED light efficiency <br> The general evaluation of LED is high luminous efficiency and long life. Many manufacturers claim that their products have a luminous efficacy of up to 90lm/W or higher, but these data are only some of the labs in the initial stage of testing. The highest data measured while still in a cold state. The so-called 50,000 hours or 100,000 hours of life is only an early estimate of low-power monochrome LEDs.
The theoretical light effect can be estimated from the luminescence mechanism of the LED. As is well known, LEDs are luminescent by carrier recombination. When the carriers are recombined, their potential energy is completely converted into light energy, and the internal quantum efficiency is 100% in this process alone. However, the energy lost by carriers colliding with the crystal lattice in the medium, the kinetic energy carried by the carriers during recombination, and the electrical energy consumed to overcome the resistance of the external circuit are unlikely to be converted into light energy. Considering the various additional energy consumption, the actual internal quantum efficiency of carrier recombination will not exceed 90%.
The photons generated during carrier recombination have a 50% probability of outward radiation, and some of the 50% of the photons collide with the crystal lattice and are converted into thermal energy for lattice absorption. Part of the photon is reflected back to the original medium at the interface of different media and absorbed. In addition, the metal mesh or transparent conductive film as the outer electrode of the output window will also reflect and absorb part of the photons, which will reduce the LED. The quantum extraction rate is such that the quantum extraction rate of this photon is 80%.
The photons generated during carrier recombination have a 50% probability of inward radiation. When they reach the underlying conductive film that doubles as a mirror, they convert part of the reflection into outward output light. However, this part of the photons is inward or outward. During transmission, it will collide with the crystal lattice and be partially absorbed, and will partially reflect or refract back to the original medium and eventually be absorbed when passing through the interface of different media. The reflection efficiency of the substrate mirror and the limitations of the above process make the quantum extraction rate of the inwardly radiated photons converted to output light not exceed 40%.
According to the above analysis, the total electro-optic conversion efficiency of the LED can be estimated to be about 54%, which is an ideal result under ideal conditions. Any omissions in the manufacturing process, any defects in the material will cause a decrease in its energy conversion efficiency. Compared with incandescent lamps with less than 5% visible light conversion efficiency, even with the highest conversion efficiency of high-pressure sodium lamps and ceramic metal halide lamps (electrical light conversion efficiency is about 30%), it is very high. The reason for the prospects of people.
As is well known, the total luminous flux generated by converting 1W of energy into yellow light of 555 nm wavelength can reach 683 lm/W (683 lm/W, or optical power equivalent), and is about 360 lm/W if all is converted into white light. As estimated earlier in this paper, in a very ideal situation, LEDs with 555 nm yellow light may reach the highest luminous efficacy of about 300 lm/W, and the above estimates are far from being realized. The highest luminous efficacy of the LEDs reported so far is only half of this ideal value, but in fact it is less than a quarter of it. This is precisely why people have great potential to dig and have high hopes for LEDs. The reason is.
The key to improving the luminous efficiency of LEDs is to increase the quantum extraction rate, that is, to reduce the internal absorption of photons as much as possible. This is the reason why LEDs are developing toward ultra-thin. At present, the ultra-thin (thickness tens of nanometers) GaN LED with the highest luminous efficiency, the lower conductive film adopts a high mirror film, and the inner surface of the output window is made into a rough structure to reduce the inward reflection of photons, but even so The highest luminous efficacy reported is only 150 lm/W. [3]
The above analysis is carried out on monochromatic LEDs. The conversion of monochromatic light into white light requires a quantum transformation. Currently, most of the blue LEDs with a central wavelength of about 470 nm in the emission spectrum are used to excite the central wavelength of the radiation spectrum at 560 nm. Broadband yellow phosphors are made into blue-yellow hybrid white LEDs, but this will further reduce the LED's efficacy. Its photon efficiency can be estimated as follows:
If about 20% of the blue light output in the LED radiation is retained, the yellow light phosphor is excited with the remaining 80% blue light. In the best case, about 20% of the photons in the 80% blue light will be absorbed by the phosphor, and the rest At 80%, it is transferred to broadband yellow light with a wavelength of 560 nm. This process produces an average quantum energy loss of 16.1%. Combined with the previous data, it can be estimated that the total energy efficiency of white LEDs will not exceed 40%, and the total luminous efficacy of white LEDs is only 150lm/W when converted to white light. If there is no other breakthrough, this is the current white LED may achieve the highest light efficiency, this light effect is higher than the current highest light source such as high pressure sodium lamp or ceramic metal halide lamp in HID lamp. However, the actual light efficiency achieved at present is not yet half.
We have carried out experimental research on this. We use a 1W high-brightness blue LED produced by a company. The center wavelength of the spectrum is 470nm. See Fig.3. After removing the outer package, the center wavelength of the radiation coated with different thicknesses is 560nm. The yellow phosphor coating was measured and its emission spectrum was measured. Four typical cases were selected from more than twenty samples of different thicknesses. The results are shown in Fig. 4 a, b, c, d and the attached table. In the best case (Fig. 4 c), the lumen output is 38 lm (initial cold state is about 50 lm) and the color rendering index is 78.
A famous multinational company technician introduced that their 1W white LED has an initial luminous efficacy of 70 lm/W. During normal operation, the pn junction temperature rises and the light efficiency drops to 50 lm/W.
In 2007, the Nano Ceramics Center of the Japan Institute of Materials Science received the highest results [3]. They used InGaN blue LEDs with a center wavelength of 450 nm and high photon conversion efficiency, strong absorption peaks in the 450 nm blue range, and low temperature coefficients. Phosphors were tested. These three phosphors were: α-sialon: Yb2+, α-sialon: Eu2+, and Sr2Si5 N8: Eu2+. These phosphors are characterized by a small thermal decay of luminous efficiency and still maintain 85% of the initial value at 150 °C. The emission spectra of these three phosphors are shown in Fig. 5.
The emission spectrum of a white LED made of InGaN blue LED and No. 1 phosphor is shown in Fig. 6, and its luminous efficiency is 55 lm/W, and the color temperature is 4500K. This is the highest light efficiency report of white LEDs that I know (it is tested that the luminous flux of domestic 1W white LEDs is mostly below 40lm). The emission spectrum of white LEDs prepared using mixed phosphors No. 2 and No. 3 is shown in Fig. 7. The color rendering index is increased to 82, and the color temperature is 4200 K, but the luminous efficiency is reduced to 20 lm/W.
As mentioned before, the luminescence of the LED is caused by the carrier (electron) in the n-type semiconductor overcoming the barrier potential under the external potential to enter the p-type semiconductor and to generate complex luminescence for hole trapping. Generally, the temperature coefficient of a semiconductor is large, and the resistivity decreases as the temperature rises, and the potential of the barrier layer also drops to the bottom. At the same time, the center wavelength of the radiation shifts to the long wavelength direction (red shift) and the luminous efficiency of the phosphor decreases.
At present, up to 20% of the energy of white LEDs is radiated in the form of photons, and the remaining 80% of the energy is converted into heat to heat the chip and phosphor coating. For low-power LEDs, 80% of the input power does not increase the temperature of the chip too much, and has little effect on the efficacy, life and operating state of the LED. However, white LEDs with a power of 0.5W or more, more than 80% of the power consumption is concentrated in a very small pn junction region, which will definitely increase the temperature of the chip and the phosphor coating, which not only affects the light efficiency, but also causes rapid Light decay and shortened life.
The long-life judgment of 50,000 or 10 hours is estimated for low-power monochrome LEDs in the early stage of LED development. For high-power LEDs, especially white LEDs, since the chips and phosphors are baked at high temperatures for a long time, the life will be greatly increased. decline. This is why the poorly designed LED lighting project has a sharp light fade after 3 to 4 months of operation, which makes the designer have to change the light source frequently.
III LED and road lighting <br> Due to the huge investment and excessive publicity of white LEDs, some people in this field have high expectations for white LEDs, eager to push white LEDs to conventional functional lighting, many of them The field personnel have directly promoted it to road lighting. In the past few years, there have been dozens of LED road lighting and LED photovoltaic road lighting model projects completed nationwide (the author has examined many road lighting projects using LED as a light source). There are enough experiences and lessons to be summarized. I believe that this test should be temporarily suspended and restarted after the conditions are ripe.
At present, the main source of road lighting in China and developed countries is still high-pressure sodium lamps and a few metal halide lamps. The former has a luminous efficiency of 100~120 lm/W, but the color rendering index is only 20 or lower, and the latter has a luminous efficacy of 80. ~100lm/W, the color rendering index is about 80. Considering the energy consumption of electrical accessories, the actual system light efficiency of using such a light source is between 75lm/W and 90lm/W. Since the light reflectance of lamps used for road lighting is not high, they are usually below 60%. When using such a luminaire, about 40% of the light is directly on the ground, and the remaining 60% of the light is reflected by the luminaire and then shot to the ground. A rough estimate of the total light utilization of conventional road lighting after removal of various losses does not exceed 76%, thus estimating the final luminous efficacy of approximately 60 lm/W to 70 lm/W. In addition, due to the irrational design of the luminaire and the uneven elongated columnar illuminator of the high-pressure sodium lamp, the light distribution is uneven. In order to achieve the lighting standard between the dark line between the road center line and the two lights, the design has to use more power and more. High-light flux sources meet the illumination requirements of dark areas, resulting in a waste of electrical energy.
Based on the above situation, LED road lighting designers believe that using the directional radiation performance of LEDs for road lighting design can greatly reduce energy waste. They believe that white LEDs are more energy efficient than high pressure sodium lamps. They designed a number of road lighting solutions that use high-power (1W, 3W) white LEDs and made prototype projects. However, most of these sample projects have not been carefully tested and certified. One of the lighting design schemes is to distribute 120 1W white LEDs in a rectangular pattern on a 200×250mm rectangular planar circuit board. The board is wired and then mounted in a conventional street light fixture. This kind of design is extremely unscientific. The LED single illumination of this scheme is not more than 40lm. It is estimated to be 40lm. Considering the loss of supporting electrical appliances (10%) and 90% of light utilization, the actual system light utilization The rate is less than 32 lm/W, and the total effective luminous flux of 120 LEDs is about 4,300 lm. If the light emission angle of the LED is 120°, according to this design, 4300 lm of light will be emitted downward at an emission angle of 120°. It is conceivable that such a design would make the ground light distribution extremely uneven and the street and the light poles completely dark. These facilities do not consider effective cooling measures for LED substrates. So many LEDs are concentrated in such a small space. The continuous accumulation of more than 80 watts of power will make the chip temperature too high, and the blocking laminate will drop. The blue light drifts in the long-wave direction, which inevitably causes a serious drop in its light efficiency. Inspected a photovoltaic lighting model road in the suburbs of Beijing (the total power of the light source is 105W). The illumination under the lamp is only 3.5 lux. According to the previous two and a half months of the cloud, the illumination under the lamp was 7 lux when the project was just completed. There is no lighting at all under the lamp 15 meters in diameter. There are many such engineering models in Jiangsu, Zhejiang, Shandong, and Guangdong. For example, somewhere in Shunde, Guangdong, and somewhere in Shenzhen, the LED road lighting demonstration street is not only dimly lit, but also needs to be replaced every 3 to 5 months.
LED road lighting test must be carried out under the guidance of experts, after strict certification and design.
Some enterprises specializing in LED road lighting engineering in Guangdong have learned the lesson. The measures they take are to make the 1W LED work in the 0.5~0.6W state, or to design the 3W LED as 1W, which is costly and light. The effect is very low, the lighting effect is not good, but the problem of light decay and life is basically solved.
In view of the fact that China's white LEDs are very hot and specially developed for China, a white LED for road lighting, they designed the packaging material of the 1W LED front end as a specific convex lens structure. It is said that it is not required to use such LEDs. The secondary and tertiary optical design can obtain uniform road illumination and is convenient to use. The company claims that the LED's cold light efficiency is 70 lm / W, and in normal use it is 50 lm / W.
A company in Zhejiang designed an integrated white LED for road lighting. They assembled multiple blue LED chips on the same substrate, using a specially designed large convex lens as the output window and coating the inner wall of the output window with phosphor. All LED chips are co-excited with the same phosphor coating. The illuminating feature of the combined LED is modified to be similar to that of a conventional spherical light source, and is convenient to use, but the problem of substrate heat dissipation of such a structure LED still exists.
It should be noted that when multiple LEDs are grouped together for road lighting design, in addition to sufficient luminous flux and reasonable optical design to ensure a reasonable light distribution, more importantly, heat dissipation. As mentioned above, about 80% of the current energy consumption of LEDs is converted into heat, and semiconductor devices are not resistant to high temperatures. If the heat generated cannot be immediately removed, it will cause serious consequences.
The road lighting conditions with LEDs are not yet mature. This requires both the improvement of LED light efficiency and the increase of power. It is more important to solve the effective LED heat dissipation scheme and measures. Of course, more reasonable optical design is also very important.
IV US Department of Energy Assessment of the Effectiveness of Commercial LED Products (CALiPER)
In terms of LED efficacy and life indicators, there are currently large differences of opinion. One important reason is that there is no uniform standard and test method. For example, the lighting industry has always used luminous flux output and light efficiency as its product parameter indicators in stable operation. LED industry generally adopts the initial (cold) maximum luminous flux and light efficiency as the standard. There are also many differences in the measurement methods and standards of color rendering index. The life standard and measurement method are also different, and the relevant data are far from each other. Evaluations are also very different.
This problem is not only in China but also in foreign countries. The US Department of Energy has already discovered that they have conducted 7 rounds of market product surveys from December 2006 to September 2008. In 2008, they only continued in January. Three spot checks were conducted in May and September, and “Commercially Available LED Product Evaluation & Reporting†was published in September. The data and conclusions they published were negative. Only two of the 24 types of products tested (downlights) are in line with the product specifications. Except for a few LEDs with a luminous efficacy of 62 lm/W, the average is only 32 lm/W, and the low is less than 10 lm/W. The standard of the star. Very harsh and negative conclusions were made about the confusion in the propaganda and the exaggeration of the data, including light efficiency, color temperature, color quality, light distribution, and the power factor of the lighting system. In particular, the light effect, they found that the promotion is often exaggerated 2 to 3 times, and the average user does not understand this. One conclusion of the US Department of Energy is that uniform standards must be established to regulate product quality and indicators. The above problems are more prominent and important in China.
V Conclusion LED is an excellent promising source of light, and its specific illumination mechanism gives it a range of features and characteristics. Because of its small illuminant, high density of media, very concentrated light, and high brightness. Even LEDs with low power and low luminous flux can give the eyes clear, bright and error-free optical signals. They are used for indication and display with unparalleled superiority, so they have been widely used in the display field. omnipresent. The low-power monochrome LEDs also show unique features and charms in decorative lighting and art lighting. In 2008, the Beijing Olympics gave full play to this function of LEDs.
White LEDs are outstanding members of the LED family and have many features and characteristics. One of the important purposes of developing white LEDs is to replace conventional electric light sources such as incandescent lamps, fluorescent lamps, high-intensity discharge lamps, etc., but it seems that their development is not as fast as expected. At present, the actual light efficiency of white LEDs on the market is about 40 lm/W, and the maximum is no more than 50 lm/W. For functional lighting, these parameters need to be improved. More important to promote white LEDs is the development of inexpensive, efficient cooling systems.
As is well known, all illuminants other than LEDs operate at high temperatures, and high temperatures of several thousand degrees are conditions for ensuring efficient illumination, but semiconductor-based luminescent devices such as LEDs have extremely high temperature rise in the center of illumination. Harmful, if this problem can not be solved, the improvement of LED light efficiency and power increase is limited.
Low-power white LEDs (1W and below) have shown unparalleled advantages in a wide range of applications, such as local lighting in small spaces, garden lights, close-range low-power flashlights, desk lamps, and emergency lighting. There is room for development. At present, white LEDs do not need, nor have conditions and medium and high power conventional illumination sources compete for the road lighting market.
Different from indoor lighting , there is no reflective object around the road lighting in open space, so the luminous flux required for road lighting is larger. This is the reason why most of the current road lighting uses 250W, 400W high luminous flux high-pressure sodium lamp. The luminous flux of this kind of lamp is 28000lm. In the range of 48000 lm, if it is replaced by a white LED of 1 W or 3 W, the LED is required to be too large, the cost is high, the design difficulty is large, and the effect is difficult. In recent years, LED has gained a lot of experience in road lighting. At present, the light efficiency of white LED is not high and the life is not long. I believe that the use of LED for road lighting test should be suspended, and the research and development should be more efficient and more powerful. White LEDs and inexpensive and efficient cooling systems are then advanced.
Road lighting consumes huge energy and has great potential to be tapped. Its transformation is imperative. Recently, Philips and Fudan University have cooperated and conducted very useful research [5]. Their conclusion is ceramic metal halide lamp. It is a very good replacement for high pressure sodium lamps.
Participation [1] Hans Nikol etl; “Worldwide National Solid State Lighting Programs and Standardization Efforts. An Overview from Industry's Perspectiveâ€; “Proceedings of the 11th International Symposium on the Science and Technology of Light Sources May 20th-24th, 2007, shanghai , china"; P.475 – 478
[2] Wu Ling; “National SSL Program in Chinaâ€; “Proceedings of the 11th International Symposium on the Science and Technology of Light Sources May 20th-24th, 2007, shanghai, chinaâ€; P.479 – 485
[3] Volker Härle, B. Hahn, J. Baur; “With ThinGaN to 1000lm white†; “Proceedings of the 11th International Symposium on the Science and Technology of Light Sources May 20th-24th, 2007, shanghai, chinaâ€; P .487 – 488
[4] R.-J. Xie, N. Hirosaki; “White Light-Emitting Diodes Using (Oxy) nitride Phosphorsâ€; “Proceedings of the 11th International Symposium on the Science and Technology of Light Sources May 20th-24th, 2007, Shanghai, china"; P.491 - 492
[5] Journal of Lighting Engineering. 2008 VOL. 19 NO.3 Back Cover
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