Development Status and Technical Analysis of LED Street Light Applications

Editor's note: LEDs are the source of streetlights, and they have many advantages over traditional streetlight sources. First, the LED is a semiconductor diode that has a very long lifetime. When the luminous flux decays to 80%, its lifetime reaches 25,000 hours. The life of metal halide lamps is 6000~12000 hours, and the life of high pressure sodium lamps is 12000 hours. Second, the basic structure of the LED is an electroluminescent semiconductor material, placed on a leaded shelf, and then sealed with epoxy resin to protect the internal core wire, so the LED has good seismic performance. Third, the light color of white LEDs is better than that of high pressure sodium lamps. At the intermediate visual level, the human eye is more likely to distinguish things in a high color temperature environment than in a low color temperature environment. The color rendering of white LEDs is also much better than that of high pressure sodium lamps. The color rendering index of high pressure sodium lamps is only 20 right and left, while white LEDs can reach 65 to 80. Fourth, the LED can achieve a perfect dimming function. Since the working range of the LED is large, its light output is proportional to the operating current, so the current can be reduced by dimming. In addition, since the LED is not frequently damaged by frequent switching, the dimming of the LED can also be obtained by the pulse width adjustment method, and the luminous intensity of the LED can be effectively adjusted by adjusting the duty ratio of the voltage and the operating frequency. Fifth, in the optical system of the luminaire, the luminous flux loss of the LED light source is minimal. Unlike traditional light sources, LEDs are half-space light sources. A high-pressure sodium lamp or a metal halide lamp is a light source that emits light in a full space, and it is necessary to change the outgoing light of one half space into the other half space by changing the direction of 180°. When we rely on the reflector to complete, the absorption of light by the reflector and the blocking of the light source itself are unavoidable. With LEDs as the light source, there is no such loss, and the utilization of light is higher than that of conventional light sources. Finally, LED light sources do not contain harmful metallic mercury, unlike high-pressure sodium lamps or metal halide lamps that are hazardous to the environment at the time of disposal.

When the night comes, you drive on the road, and the street lamp will accompany you faithfully, so that you can go home safely. The visual comfort that the street light brings to you first comes from the uniform brightness of the road surface, and then people or things on both sides of the road can also be seen, so that you can understand the situation that may be sudden. When the illumination of the street lamp meets the above requirements, its light distribution must be reasonable. Therefore, if you use LED street light, although it has the advantages of good light color, long life, dimming function, etc., its light distribution level is still crucial. At present, LED street lights have just started and need to be constantly improved. Whether it is the optical structure design of street lamps or heat dissipation technology , it is still being improved.

The LED street light is different from the optical design using conventional light source street lights. Conventional light source street lamps use a reflector to evenly distribute the luminous flux of a light source onto an illuminated road surface. The LED street light source consists of a very large number of LEDs. By designing the projection direction of each LED, the illuminated road surface is evenly illuminated.

At present, the application prospects of LED street lamps on secondary trunk roads and branch roads are very good. The secondary trunk road is a road that combines with the main road to form a road network and acts as a gathering and distributing traffic. The illuminance requirement of the secondary trunk road is 15lx, the illumination uniformity is 0.4, the average brightness requirement is 1.0cd/m2, the total brightness uniformity is 0.4, the brightness longitudinal uniformity is 0.5, and the threshold increment is ≤10. LED street lights that meet the energy-saving level, the lighting power density should be less than the national standard when the lighting quality meets the above requirements. 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 ≤ 0.85. In order to meet the above requirements, the light distribution shape of LED street lamps should have strict requirements. In the longitudinal direction of the road, the beam should be projected farther away, increasing the spacing of the luminaires. Below the street light, the light intensity should be minimal, and as the elevation angle γ increases, the light intensity I' increases. When I' and γ satisfy a certain functional relationship, the road surface can obtain uniform illumination. The relationship of this function is as follows:

Of course, due to the complexity of the optical design, the light distribution shape is difficult to fully conform to this functional relationship, and the projection range of the gamma angle can be reduced, and the lamp spacing can be reduced to obtain uniform illumination. In the longitudinal direction of the road, the projection direction of the maximum light intensity should be at an angle c to the longitudinal line of the road. The angle c is determined by the width of the road, as shown in Fig. 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 function 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 Fig. 3.

At present, there are many examples of light distribution in LED street lamps designed in many factories. For example, Figure 4 is the light distribution curve of a 120W LED street light. Its maximum light intensity is 15° through the c angle and 57.5° from the γ angle. The shape of the light distribution curve of c=0°, 180, and 15° is close to the functional relationship of Formula 1. The luminaire is used on the four-lane secondary trunk road, and the two sides are symmetrically arranged. When the height of the pole is 8 meters and the distance between the lamps is 30 meters, the lighting quality meets the requirements of the national road lighting standard. The results are shown in Table 1. Its illumination 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 illumination of the LED light source is used. The LED light source is used in the luminaire, and the luminous flux loss of the light source is less than that of the high-pressure sodium lamp or the metal halide lamp. For the time being, the luminous efficacy of the LED light source does not reach the level of the high-pressure sodium lamp or the metal halide lamp, that is, the light source of the same power, and the luminous flux is not as high as that of the high-intensity gas discharge lamp. However, due to the half-space illumination of the LED light source, the luminous flux output of the LED luminaire can be close to that of the high-intensity discharge lamp. When the street light illuminates the road, there is only half the space in the direction of the light. The illuminating space of the high-intensity discharge lamp occupies the entire space, as shown in Fig. 5(a). If there is no luminaire, the light in the upper half of the light source is completely wasted. Using a luminaire, the reflector can reflect the light in the upper half space into the lower half space, but the absorption of the light by the reflector, and the occlusion of the light source itself is converted into thermal energy, or the reflector reflects the light twice. Reabsorbed, a portion of the light from the source is lost.

Luminaire efficiency is a parameter that reflects the extent of light loss from the source. From the photometric performance of low-power high-pressure sodium lamp street lamps (70W~150W), the lamp efficiency is generally at 65~75%. Take the 100W high-pressure sodium lamp as an example, that is to say, even if the luminous flux of the light source reaches 8000lm, the luminous output of the street lamp has 5200lm~6000lm. LED light sources are advantageous in this respect. LED illumination is only present in half of the space, see Figure 5(b), which can also project 100% of the light onto the road without the need for a luminaire. The need to design the luminaire is to change the direction of light projection over a small range of angles, during which the loss of light is minimal. At present, the luminous efficacy of LED street lamps has reached 40~50lm/W. Taking 100WLED street lamps as an example, the luminous flux output of lamps can reach 4000~5000lm. At present, LED street lights are just getting started. I believe that the design power will be continuously strengthened, the light distribution of street lamps will be more optimized, and finally perfect.

At present, LED street lights cannot be used for trunk roads. The main road is a trunk road connecting the main sections of the city, and is separated by a motor vehicle and a non-motor vehicle. The speed of the motor vehicle on the main road is faster than that of the secondary road, and the lighting level is higher than that of the secondary road. The illuminance requirement of the main road is 30lx, the illumination uniformity is 0.4, the average brightness requirement is 2.0cd/m2, the total brightness uniformity is 0.4, the brightness longitudinal uniformity is 0.7, and the threshold increment is ≤10. Street lamps that meet the energy-saving level should be lighter than the national standard when the lighting quality should meet the above requirements. When the number of lanes is ≥6, the lighting power density is ≤1.05, and the number of lanes is <6, the lighting power density ≤1.25. Most of the main roads use 250W or 400W high-pressure sodium lamp street lamps, and the light flux of 250W or 400W high-pressure sodium lamps reaches 28000lm (250W) and 48000lm (400W). The efficiency of such high-power street lamps is generally between 75 and 85%. Therefore, the luminous flux output of the lamps is as low as 21,000 lm. Take 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. The maximum light intensity is 10° c angle and 70.0° γ angle. The shape of the light distribution curve of c=0°, 180, and 10° is good. This street light can be used in the main road of four to six lanes. The road lighting quality results are shown in Table 2. At present, the small luminous flux is the main reason why LED street lights cannot be applied to the main road. Luminaires with different luminous flux grades should be used at different levels of roads. Otherwise, energy saving effects will not be achieved.

As a new light source, LED should make full use of its advantages in lighting equipment to improve its shortcomings. LEDs are used in embedded lighting fixtures. Due to the large amount of heat generated by LED chips, it is difficult to make high-power lamps. One of the issues that companies are currently working on is "heat dissipation." Although the luminous efficiency is getting higher and higher, the heat generation of the LED chip is still large. If heat dissipation measures are taken, the temperature of the LED chip will be too high, resulting in deterioration of the performance of the chip itself and the encapsulating resin, thereby eventually causing a decrease in luminous efficiency and a shortened life. The most important performance of LEDs is “long life”. In order not to affect this indicator, it is necessary to try to dissipate the heat generated on the chip. Improving the heating of LEDs is the key to capturing high-power lighting.

Table 2

Regarding the energy saving status of LED lamps, different types of product applications should be distinguished. Compared to conventional light sources, white light diodes do not have a significant contribution to energy savings as monochromatic light diodes. Originally used as an indicator light source for instrumentation, LEDs of various light colors were widely used in traffic lights and large-area displays, resulting in good economic and social benefits. Taking a 12-inch red traffic light as an example, a low-efficiency 140-watt incandescent lamp was used as the light source, which produced 2000 lumens of white light. After passing the red filter, the light loss is 90%, leaving only 200 lumens of red light.

In the newly designed traffic light, 18 red LED light sources, including circuit losses, consume 14 watts of electricity, which can produce the same light effect. Automotive signal lights are also an important area for LED light source applications. In 1987, China began to install high-position brake lights on cars. Due to the fast response speed (nanoseconds) of LEDs, drivers of trailing vehicles can be informed of driving conditions and reduce the occurrence of 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 and downlights. LED street lights have been continuously improving, and the pace is getting faster and faster. It is hoped that in the near future, LED street lights will appear more widely in the context of the city with excellent performance.

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