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

Under the slogan of low carbon and energy saving, LED lighting has been launched vigorously, and under the strong guidance of the government's " 10 cities, 10,000 lights " policy, the development of the LED street lamp industry is particularly rapid. iSuppli data shows that among domestic LED lighting manufacturers, more than 50% of them focus on LED street lamp lighting. "Although the '10 cities, 10,000 lights' project has installed less than 1 million street lamps so far, there are thousands of manufacturers working on this project." Qian Keyuan, deputy director of the semiconductor lighting laboratory of Tsinghua University Shenzhen Graduate School, pointed out. It can be seen that the government policy has given manufacturers great confidence. However, up to now, the test results of LED street lamps have always been unsatisfactory. When analyzing technical issues such as light efficiency , light decay, light distribution, and color temperature , the loopholes in the test methods have gradually been exposed. If the test methods are not scientific, the sunrise industry of LED lighting may be greatly hindered.

Analysis of LED street light problems

"China is very special in the team developing LED lighting technology, because China takes LED street lamp products as the entry point of the industry. In fact, LED street lamps are the most difficult products to design in LED lighting, so there will be many problems that hinder the further development of LED lighting." Qian Keyuan said. At present, the test data of LED street lamps show 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 showed that the average luminous efficiency and energy consumption of LED street lamps were only equivalent to the level of 150W sodium lamps. This result undoubtedly brought a great negative impact on the LED street lamp industry. However, Qian Keyuan believes that through the analysis of the test results, many problems of the test methods and standards themselves have been exposed, and these problems are worth pondering. "If the test standards and methods themselves are unreasonable, it will lead to the wrong guidance of this industry." He pointed out.

"In 2009, we conducted actual tests on the products of dozens of LED street lamp manufacturers in Shenzhen, which was of great significance. Shenzhen is currently a city where LED applications are marginalized. The speed of R&D and production of LED products is very fast, the packaging is also very distinctive, and the industry accumulation is also very strong, but the scale of application is far smaller than other cities.

This comprehensive test of LED street lights was expected to promote the development of LED street lights in Shenzhen, but when the test results came out, it felt like a complete failure. "Qian Keyuan said frankly. However, through technical analysis of the product, experts believe that the results are not as bad as expected, but the test methods and standards have exposed many drawbacks.

In 2009, the time span of the test of Shenzhen LED street lights was 113 days, starting in spring and ending in summer. The LED street lights tested actually ran on the road for 1,200 hours. The test results showed that the average light decay of outdoor tests was 14.1%, while that of indoor tests was 5.3%. "If the sealing of LED street lights is done well, the light decay indoors and outdoors should be similar, unless the product is broken. The only difference is that the indoor temperature is constant, while the outdoor temperature is changing." Qian Keyuan pointed out this key point in the test.

By analyzing the test results, Qian Keyuan summarized six key factors that affect the test results, many of which are related to temperature.

These factors include:

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

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

3. This test started in spring and ended in summer, with a temperature span of about 20°C. A large temperature span is not conducive to reading the final results, especially since the first light decay test was in the cool spring and the last light decay test was in the hot summer.

4. Accuracy of the test equipment. The accuracy of the photoelectric probe used for the test includes parameters such as spectral matching error and temperature coefficient. The spectral matching of this test is good, but the influence of the probe's temperature coefficient cannot be ignored. The current standard probe temperature coefficient is not less than 0.2%. Therefore, during the test period of 2,000 hours and the ambient temperature change of 20°C, the probe will produce a 4% luminous flux test error.

5. The influence 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 light reflected by street lamps on other objects. The influence of stray light should be shielded during the test. Shenzhen paid great attention to the treatment of stray light in this LED street lamp test.

6. Impact of environmental pollution. Surface pollution can have a great impact on the test results, with an error level of 7~8%, especially for some products whose light-emitting surfaces also serve as secondary optical lenses . They have concave-convex structures that are easy to harbor dirt and are difficult to clean, which can seriously affect the luminous flux and further affect the light distribution.

Taking all the above factors into consideration, the total error is greater than 10%. Therefore, he believes that there is still much room for improvement in the LED street light testing methods and standards themselves.

“To accurately test the light decay and life of street lamps, we must first eliminate various influences.

Specific practices include:

1. Measurement accuracy should be at least better than 1%, and a higher standard test instrument should be used;

2. The influence of temperature must be controlled or compensated. The change of ambient temperature within the test time span should be less than 5°C.

3. Light collection must avoid interference from all kinds of stray light;

4. The serious impact of lamp surface pollution on the test must be considered. If the street lamp is installed in the summer and the light decay is measured in the winter, the effect will be better.

In short, the above considerations should be used to correctly measure the light attenuation during testing, so as not to mistakenly kill a good thing. "Qian Keyuan further pointed out.

It is urgent to formulate reasonable LED street light evaluation standards

The industry has reached a consensus on the problems in the testing process pointed out by Qian Keyuan. The Opinions on the Development of the Semiconductor Lighting Energy-saving Industry jointly formulated by the National Development and Reform Commission, the Ministry of Science and Technology and other six departments pointed out that the current development of semiconductor lighting has the disadvantages that the testing equipment, testing methods and standard formulation work cannot meet the requirements of the rapid development of the industry, and the construction of the standards and testing system for semiconductor lighting products needs to be improved urgently. Since the authoritative testing platform has not yet been established, it is impossible to conduct quality evaluation or certification on existing semiconductor lighting products.

Before formulating the standards and testing system for LED street lights, it is necessary to fully understand the factors that affect the life of street lights. Qian Keyuan believes that these factors mainly include: the life of each module of the street light itself, whether the structural parts can meet the requirements of the street light in all-weather climates, and the performance of the driver module. Other factors that affect the life of street lights include: the life of the chip itself, the impact of packaging, the quality of the driver components , and the circuit structure.

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

"Guangdong Province's LED street lamp standards have some detailed testing methods, but they have also attracted a lot of controversy, such as whether to check for 2,000 hours or 5,000 hours? Since LED street lamps are new, many contents are still in the exploratory stage, so disputes are inevitable." Qian Keyuan gave an example, "But setting standards is not the goal of enterprises, but the responsibility of the government or relevant institutions. Enterprises should study more technical issues."

In the process of formulating standards, Qian Keyuan emphasized that since the most important point of standards is that everyone can abide by them, three issues need to be considered during the formulation:

First, how much credibility do local standards have, whether they can be used on a large scale, and how long will they be applicable?

Second, the timeliness of issuing standards. Because LED is a developing technology, the regulation of parameters such as luminous flux should be determined according to the development of technology. "In the past, it was stipulated that LED lighting equipment must reach 65lm/W, which is obviously not appropriate now," he pointed out.

Third, we need to set standards based on long-term planning.

Thinking about 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 some local governments blindly transformed road lighting without considering economic benefits and over-invested in landscape lighting, resulting in disorderly competition in the industry, uneven product quality, serious waste of resources, affecting consumer confidence, and being detrimental to the healthy development of the industry.

On this point, Wu Chunhai, director of the Planning and Design Office of Shenzhen Lighting Environment Management Center, also pointed out that through the investigation of some pilot cities in the north, it was found that the existing sodium lamps on some secondary roads and branches were very old and suitable for LED street lamp transformation, but the ones that should be transformed were not, and the main roads were replaced with LED street lamps, and this situation was very common. He said that under the existing technology, the application of LED street lamps on roads with 6 lanes or less in both directions has been relatively mature. Moreover, when used on narrower roads, through reasonable design, the power saving rate can reach more than 50%. "But the main problem with the current products is still quality. LED street lamp technology has become more and more mature and its application has become more and more extensive, but the resistance is getting greater and greater. Semiconductor lighting was promoted because of its obvious energy-saving effect, but it was stopped because of poor lighting quality. Only when the quality of lighting products is guaranteed to a certain extent, it is meaningful to talk about energy saving." Wu Chunhai emphasized.

Regarding the future development of LED street lights, Xie Junyan, director of the Shanghai Street Light Management Center, pointed out from the perspective of LED street light maintainability that there are currently drawbacks in LED road lights, such as no serialization, no detailed standards, and products from different factories cannot be replaced, and even products from the same manufacturer cannot be interchanged. These drawbacks make operation and maintenance work inconvenient, and on-site maintenance is impossible. In addition, due to the rapid replacement of LED street light products, there is also a lack of product continuity.

"If it is to be applied on a large scale in street lighting projects, it will inevitably cause maintenance difficulties and waste of equipment." Therefore, he emphasized that the manufacturers' road lighting accessories should be universal or relatively unified, and leave room for upgrading. 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 improvements or update requirements in stages, so as to form a unified standard when the technology matures. "For some LED road lighting that lack interchangeability and are difficult to repair on site, it is necessary to limit the current scope of use." Xie Junyan said.