In-depth analysis of LED street light detection technology, standards and development prospects

Under the slogan of low-carbon and energy-saving, LED lighting has been vigorously launched, and the LED street lamp industry has developed particularly rapidly due to the strong guidance of the government's “Ten Cities and Ten Thousands” policy. According to iSuppli's data, more than 50% of domestic LED lighting manufacturers focus on LED street lighting. "The 'Ten Cities and Ten Thousand' project has not installed more than 1 million street lamps, but there are thousands of manufacturers doing this project." Qian Keyuan, deputy director of the Semiconductor Lighting Laboratory of Shenzhen Graduate School of Tsinghua University pointed out. It can be seen that the government policy gives the manufacturers great confidence. However, until now, the detection results of LED street lamps have not been satisfactory. When analyzing technical problems such as light efficiency, light decay, light distribution, and color temperature, the loopholes in the detection method are gradually exposed. If the means of detection are not scientific, LED lighting, a sunrise industry, may be greatly dragged.

LED street light problem representation analysis

"In the development of LED lighting technology, China is very special, because China is the LED street light products as the entry point of the industry. In fact, LED street lights are the most difficult products to design in LED lighting, so there will be many problems. Blocking the further development of LED lighting." Qian Keyuan said. At present, the test data of LED street lamps indicates that the reliability, light source life and light decay of the product are not up to standard. In 2009, Shenzhen conducted a comprehensive inspection of 27 LED street lamps. The test results show that the average light efficiency and energy consumption of LED street lamps is only equivalent to the level of 150W sodium lamps. This result has undoubtedly brought a lot to the LED street lamp industry. The negative impact. However, Qian Keyuan believes that through the analysis of the test results, many test methods and standards themselves have been exposed. These problems are worth pondering. "If the standards and methods of testing are unreasonable in themselves, they will lead to wrong guidance in this industry," he pointed out.

“In 2009, we measured the products of dozens of LED street lamp manufacturers in Shenzhen. The significance is significant. Shenzhen is currently a marginalized LED application. The speed of developing and producing LED products is very fast, and the packaging is also very distinctive. The industrial accumulation is also very strong, but the scale of application is much smaller than other cities.

This comprehensive inspection of LED street lights is expected to promote the development of LED street lamps in Shenzhen. However, when the results of the tests come out, people feel that the whole army has been wiped out. Qian Keyuan said frankly. However, through technical analysis of the products, experts believe that the results are not as bad as expected, and the test methods and standards have exposed many ills.

The time span for testing LED street lights in Shenzhen in 2009 was 113 days, starting in spring and ending in summer. The LED street light being tested actually operated for 1,200 hours on the road. The test results showed that the average light decay of the outdoor test was 14.1%, and that of the indoors was 5.3%. "If the LED street light products are well sealed, the indoor and outdoor light decay should be similar unless the product is broken. The only difference is the indoor constant temperature, the outdoor temperature is changing." Qian Keyuan pointed out the test This key point in the middle.

Through the analysis of the test results, Qian Keyuan summed up the six key factors affecting the test results, many of which are related to temperature.

These factors include:

1. The effect of temperature on the luminous flux of the LED chip. This effect is relatively large, about 0.02~0.03%/°C.

2. The effect of temperature on the output current of the drive circuit. The effect is not too large, about 0.01% / °C.

3. The test starts in spring and ends in the summer with a temperature span of about 20 °C. The large temperature span is not conducive to the reading of the final result, especially the first test of light decay is in the cool spring, the last test of light decay is in the hot summer.

4. Test equipment accuracy. The accuracy of the test photodetector includes parameters such as spectral matching error and temperature coefficient. The spectral matching of this test is not bad, but the temperature coefficient of the probe can not be ignored. At present, the standard temperature probe has a temperature coefficient of not less than 0.2%, so the probe will produce a 4% luminous flux test error during a test period of 2,000 hours and an ambient temperature change of 20 °C.

5. The effects of stray light. Stray light includes the illumination of vehicles on the road, the light radiation of adjacent street lamps, the interference of other light sources on the roadside, and the interference of the street light source on other objects. The effects of stray light should be shielded during the test. This LED streetlight test in Shenzhen is very careful about the handling of stray light.

6. The impact of environmental pollution. The surface contamination will have a great impact on the test results, the error level can reach 7~8%, especially the products with some illuminating surface and the surface of the secondary optical lens, which has concave and convex structure, easy to hide dirt and not Easy to clean, which can seriously affect the luminous flux and further affect the light distribution.

Based on the above factors, the sum of all the errors is basically more than 10%, so he thinks that there are still many areas for improvement in the LED street lamp test method and the standard itself.

"To accurately test the light decay and life of street lamps, we must first eliminate the effects.

Specific practices include:

1. The measurement accuracy should be at least 1%, and a higher standard test instrument should be used;

2. The influence of temperature must be controlled or compensated, and the change of ambient temperature within the test time span is preferably less than 5 °C;

3. The collection of light must avoid the interference of various stray light;

4. The serious impact of the surface contamination of the luminaire on the test must be considered. If the street light is installed in the summer, the effect will be better in winter.

In short, the test should be based on the above considerations to correctly measure the light decay, do not mistake the good things. Qian Keyuan further pointed out.

There is no need to develop a reasonable evaluation standard for LED street lamps.

The industry has reached a consensus on the problems in the test link pointed out by Qian Keyuan. According to the "Opinions on the Development of Semiconductor Lighting Energy-Saving Industry" jointly formulated by the National Development and Reform Commission and the Ministry of Science and Technology, the current development of semiconductor lighting has the drawbacks of testing equipment, testing methods and standards development work that cannot meet the requirements of rapid industrial development. The construction of standards and testing systems needs to be improved. As the authoritative testing platform has not been established, it is impossible to conduct quality evaluation or certification of existing semiconductor lighting products.

Before formulating the standard and inspection system for LED street lamps, it is necessary to fully understand the factors affecting the life of street lamps. Qian Keyuan believes that these factors mainly include: the life of each module of the street lamp, whether the structural parts can meet the requirements of the street lamp in the all-weather climate, and the performance of the drive module. Other factors affecting the life of the street lamp include: the life of the chip itself, the impact of the package, the quality of the drive components, and the circuit structure.

In addition, four factors that affect the reliability evaluation of street lamps, including test standards, test equipment, test environment, and test methods, need to be considered. As far as the test standards are concerned, the government and enterprises are currently competing to develop some standards. For example, Guangdong Province, Jiangxi Province, Shandong Province, Jiangsu Province, Sichuan Province and Shenzhen City have their own standards.

"Guangdong Province LED street light standards stipulate some detailed testing methods, but it also attracted a lot of controversy, such as whether it is 2,000 hours or 5,000 hours? Because LED street lights are new things, many contents are in the exploration stage, so the dispute "Inevitably," Qian Keyuan said. "But setting standards is not the goal of the company, but the responsibility of the government or related institutions. Enterprises should study more technical issues."

In the process of setting standards, Qian Keyuan emphasized that since the most important point of the standard is that everyone can abide by it, there are three issues to be considered when formulating:

First, how much credibility is there in local standards, whether it can be used in a wide range, and how long it is applicable.

Second, the timeliness of the standard is introduced. Because LED is a developing technology, the parameters of parameters such as luminous flux should be judged according to the development of technology. “It used to be 65 lm/W for LED lighting equipment. This parameter is obviously not suitable now,” he pointed out.

Third, we must start from the long-term planning to set standards. 

Thoughts on the Future Development Direction of LED Street Lights

In addition to the lack of standards and equipment, the "Opinions on the Development of Semiconductor Lighting Energy-Saving Industry" also pointed out that due to the blind transformation of road lighting in some local governments regardless of economic benefits, excessive investment in landscape lighting has led to disorderly competition in the industry, and the quality of products is mixed. Waste is serious, affecting consumer confidence and not conducive to the healthy development of the industry.

At this point, Wu Chunhai, director of the Planning and Design Office of Shenzhen Lighting Environmental Management Center, also pointed out that through the investigation of some pilot cities in the north, it was found that some of the secondary roads and branch roads are very old and suitable for LED street lighting renovation, but the reform The main road should be replaced by LED street lights, and the situation is very common. He said that under the existing technology, LED street lights have been matured on roads with two lanes or less. Moreover, when applied on narrow roads, the power saving rate can reach 50% or more through reasonable design. "But the most important problem in the current product is quality. LED street light technology has become more and more mature, and its application is getting wider and wider, but the resistance is getting bigger and bigger. Semiconductor lighting is promoted due to obvious energy-saving effect, due to poor lighting quality. And stop. Only under certain quality assurance of lighting products, it is meaningful to talk about energy saving.” Wu Chunhai emphasized.

For the future development of LED street lights, Xie Junyan, director of Shanghai Street Light Management Center, pointed out that from the perspective of maintenance of LED street lights, it is pointed out that there is no serialization of LED road lamps, no refinement standards, products between factories and factories cannot be replaced, and even The same manufacturer's products are also not interchangeable. These drawbacks cause inconvenience in operation and maintenance, and the site cannot be repaired. In addition, due to the rapid replacement of LED street light products, it also leads to the lack of continuity of the product.

"If you want to use it on a large-scale streetlight project, it will inevitably cause maintenance difficulties and waste of equipment." Therefore, he stressed that the manufacturer's road lighting accessories should be universal or relatively uniform, and leave room for upgrade. At the same time, the lighting industry management department should actively collect feedback on the actual application of LED street lights, compare various solutions, and make corresponding improvement or update requirements in stages, and form a unified standard when the technology is mature. "For some LED road lamps that lack interchangeability and are difficult to repair on site, we must pay attention to limiting the current range of use." Xie Junyan said. 

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