Current status of LED street lamp research and development

As the light source of street lamps, LED has many advantages over traditional street lamps. First, LED is a semiconductor diode with a very long life. When the luminous flux decays to 80%, its life reaches 25,000 hours. The life of metal halide lamps is 6,000 to 12,000 hours, and the life of high-pressure sodium lamps is 12,000 hours. Second, the basic structure of LED is a piece of electroluminescent semiconductor material, placed on a lead frame, and then sealed with epoxy resin around to protect the internal core wire, so LED has good shock resistance. Third, the light color of white light LED is better than that of high-pressure sodium lamps. At the intermediate visual level, the human eye can more easily distinguish things in a high color temperature environment than in a low color temperature environment. The color rendering of white light LED is also much better than that of high-pressure sodium lamps. The color rendering index of high-pressure sodium lamps is only about 20, while that of white light LED can reach 65 to 80. Fourth, LED can achieve a more perfect dimming function. Since the working range of LED is large, its light output is proportional to the working current, so the current can be reduced to dim the light. In addition, since frequent switching of LED does not cause much damage to it, the dimming of LED can also be achieved by pulse width modulation. By adjusting the duty cycle of the voltage and the working frequency, the luminous intensity of the LED can be effectively adjusted. Fifth, in the optical system of the lamp, the luminous flux loss of LED light source is minimal. Unlike traditional light sources, LED is a light source that emits light in half space. High-pressure sodium lamps or metal halide lamps are light sources that emit light in the whole space. It is necessary to change the direction of the emitted light in one half space by 180° and project it into the other half space. When we rely on reflectors to complete it, the absorption of light by the reflector and the light blocking of the light source itself are inevitable. When using LED as a light source, there will be no loss in this regard, and the utilization rate of light is higher than that of traditional light sources. Finally, LED light sources do not contain harmful metal mercury, unlike high-pressure sodium lamps or metal halide lamps that cause harm to the environment when scrapped. China Lighting Network Technical Papers · LED Technology

When night falls, you drive on the road, and the street lights accompany you faithfully, allowing you to return home safely. The visual comfort brought by street lights first comes from the relatively uniform brightness of the road surface, and then people or objects on both sides of the road can also be seen, so that you can understand possible emergencies. When the lighting of street lights meets the above requirements, its light distribution must be reasonable. Therefore, if you use LED street lights, although it has advantages such as better light color, long life, and dimming function, its light distribution level is still crucial. At present, LED street lights have just started and need to be continuously improved. Whether it is the optical structure design of street lights or heat dissipation technology, they are still being improved.

The optical design method of LED street lights is different from that of street lights using traditional light sources. Traditional light source street lights use reflectors to evenly distribute the luminous flux of a light source to the illuminated road surface. The light source of LED street lights is composed of a large number of LEDs. By designing the projection direction of each LED, the illuminated road surface obtains uniform illumination.

At present, the application prospect of LED street lights on secondary trunk roads and branch roads is very good. Secondary roads are roads that are combined with main roads in the city to form a road network and serve as a traffic distribution network. The illumination requirements for secondary roads are 15lx, the illumination uniformity is 0.4, the average brightness is 1.0cd/m2, the total brightness uniformity is 0.4, the longitudinal brightness uniformity is 0.5, and the threshold increment is ≤10. For LED street lights that reach energy-saving levels, while the lighting quality meets the above requirements, the lighting power density should be less than the national standard. When the number of lanes is ≥4, the lighting power density is ≤0.70, and when the number of lanes is <4, the lighting power density is ≤0.85. In order to meet the above requirements, the light distribution shape of LED street lights should have strict requirements. In the longitudinal direction of the road, the light beam should be projected to a farther place, so that the spacing between lamps is increased. Below the street lamp, the light intensity should be the smallest, and as the elevation angle γ increases, the light intensity I′ increases. When I′ and γ meet a certain functional relationship, the road surface can obtain uniform illumination. This functional relationship is as follows:

Of course, due to the complexity of optical design, it is difficult for the light distribution shape to completely conform to this functional relationship. The projection range of the γ angle can be reduced and the distance between lamps can be reduced to obtain uniform illumination. In the longitudinal direction of the road, the projection direction of the maximum light intensity should form a certain angle c with the longitudinal line of the road. The size of the angle c depends on the width of the road, as shown in Figure 2. In the direction perpendicular to the road, the shape of the light distribution curve of the street lamp can also be designed according to this functional relationship. The range of the γ angle is determined by the width of the road (c=90º) and the position of the sidewalk (c=270º), as shown in Figure 3.

At present, there are also many examples of good light distribution in LED street lamps designed by many factories. For example, Figure 4 is the light distribution curve of a 120WLED street lamp. Its maximum light intensity passes through the c angle of 15° and the γ angle of 57.5°. The shape of the light distribution curve of c=0°, 180, and 15° is close to the functional relationship of Formula 1. This lamp is used on a secondary trunk road with four lanes and is arranged symmetrically on both sides. When the lamp pole height is 8 meters and the lamp spacing is 30 meters, the lighting quality meets the requirements of the national road lighting standards. The results are shown in Table 1. Its lighting power density (LPD) is 0.70, which also meets the lighting power density requirements for high-pressure sodium lamp street lamps.

As mentioned above, the half-space luminescence characteristics of LED light sources, when used in lamps, the loss of luminous flux of the light source is less than that of high-pressure sodium lamps or metal halide lamps. At present, the luminous efficiency of LED light sources cannot reach the level of high-pressure sodium lamps or metal halide lamps, that is, the luminous flux of light sources with the same power is not as high as that of high-intensity gas discharge lamps. However, due to the half-space luminescence characteristics of LED light sources, the luminous flux output of LED lamps can be close to the level of high-intensity gas discharge lamps. When street lamps irradiate light onto the road surface, the projection direction of the light only exists in half of the space. The luminous space of high-intensity gas discharge lamps occupies the entire space, as shown in Figure 5 (a). If there is no lamp, the light in the upper half of the light source is completely wasted. When using lamps, the reflector can reflect the light in the upper half of the space into the lower half of the space, but the reflector absorbs the light, and the light is blocked by the light source itself when it is reflected and converted into heat energy, or the reflector re-absorbs the light reflected twice, so part of the light from the light source is lost.

The lamp efficiency is a parameter that reflects the degree of light loss of the light source. From the perspective of the photometric performance of low-power high-pressure sodium lamps (70W~150W), the lamp efficiency is generally at a level of 65~75%. Taking a 100W high-pressure sodium lamp as an example, even if the luminous flux of the light source reaches 8000lm, the luminous flux output of the street lamp is 5200lm~6000lm. LED light sources have advantages in this regard. The light emission of LEDs only exists in half of the space, as shown in Figure 5 (b). Without the use of lamps, 100% of the light can be projected onto the road surface. The lamps need to be designed to change the projection direction of the light within a smaller angle range. In this process, the loss of light is minimal. At present, the luminous efficiency of LED street lamps has reached 40~50lm/W. Taking a 100WLED street lamp as an example, the luminous flux output of the lamp can reach 4000~5000lm. At present, LED street lamps have just started. I believe that by continuously strengthening the design power, the light distribution of street lamps will be further optimized and finally reach perfection.

At present, LED street lights cannot be used on trunk roads. Trunk roads are trunk roads connecting the main subdistricts of a city, and are separated by motor vehicles and non-motor vehicles. Motor vehicles travel faster on trunk roads than on secondary roads, and the lighting level is higher than on secondary roads. The illumination requirements for trunk roads are 30lx, the illumination uniformity is 0.4, the average brightness is 2.0cd/m2, the total brightness uniformity is 0.4, the longitudinal brightness uniformity is 0.7, and the threshold increment is ≤10. Street lights that reach energy-saving levels should have lighting quality that meets the above requirements and the lighting power density should be less than the national standard. When the number of lanes is ≥6, the lighting power density is ≤1.05, and when the number of lanes is <6, the lighting power density is ≤1.25. Most of the 250W or 400W high-pressure sodium lamps are used on trunk roads. The luminous flux of the 250W or 400W high-pressure sodium lamps reaches 28000lm (250W) and 48000lm (400W). The lamp efficiency of this high-power street lamp is generally between 75% and 85%, so the minimum luminous flux output of the lamp is 21000lm. Taking a 250W high-pressure sodium lamp street lamp as an example, the lamp efficiency is 83.5%, and the light distribution curve is shown in Figure 6. Its maximum light intensity passes through the c angle of 10° and the γ angle of 70.0°. The shape of the light distribution curve of c=0°, 180, and 10° is good. This street lamp can be used on four to six lanes of main roads, and the road lighting quality results are shown in Table 2. At present, the low luminous flux is the main reason why LED street lamps cannot be used on main roads. Lamps with different luminous flux grades should be used on roads of different levels, otherwise, the energy-saving effect cannot be achieved.

As a new light source, LED should be used in lighting equipment to fully utilize its advantages and improve its disadvantages. LED is used in embedded lighting fixtures. Due to the high heat generated by LED chips, it is difficult to make high-power lamps. One of the issues that companies are currently working hard to solve is "heat dissipation". Although the luminous efficiency is getting higher and higher, the heat generated by LED chips is still very large. If heat dissipation measures are taken, the temperature of the LED chip will be too high, causing the performance of the chip itself and the packaging resin to deteriorate, which will eventually lead to a decrease in luminous efficiency and a shortened life. The most important performance of LED is "long life". In order not to affect this indicator, it is necessary to find a way to dissipate the heat generated by the chip. Improving the heat generation problem of LED is the key to conquering high-power lighting fixtures.

Regarding the energy-saving status of LED lamps, different product application types should be understood differently. Compared with traditional light sources, white light diodes are not as good as monochromatic light diodes in energy saving. Initially, LEDs were used as indicator light sources for instruments and meters. Later, LEDs of various colors were widely used in traffic lights and large-area display screens, producing good economic and social benefits. Take a 12-inch red traffic light as an example. It originally used a low-efficiency 140-watt incandescent lamp as the light source, which produced 2,000 lumens of white light. After passing through the red filter, 90% of the light was lost, leaving only 200 lumens of red light.

In the newly designed traffic light, 18 red LED light sources are used, including circuit losses, with a total power consumption of 14 watts to produce the same light effect. Car signal lights are also an important field of LED light source application. In 1987, China began to install high-mounted brake lights on cars. Due to the fast response speed of LEDs (nanoseconds), drivers of trailing vehicles can be informed of the driving conditions early, reducing the occurrence of car rear-end collisions. The energy saving of white light diode lamps is mainly reflected in low-power lamps that originally used incandescent light sources, such as flashlights, downlights, etc. LED street lights have been continuously improving, and the pace is getting faster and faster. I hope that in the near future, LED street lights will appear more widely in the city with excellent performance.