Simulation design of high-power white LED street lighting system

　Abstract: With high-power white light emitting diodes (LEDs) as light sources, a street lighting system that meets specific power and illumination requirements is simulated and designed using optical software Trace Pro, and its feasibility is verified through experiments. This simulation system has guiding significance for actual development.

　　1 Introduction

　　Streetlights are an important part of urban outdoor lighting. At present, streetlight sources generally use high-pressure sodium lamps or metal halide lamps, which have a power of several hundred watts or more, consume a lot of power, have a short lifespan, and require a lot of maintenance. Light-emitting diodes (LEDs) are semiconductor light-emitting solid devices that have the advantages of low operating voltage, small size, high reliability, long lifespan, high conversion efficiency, and environmental protection. As a "green light source", its use in streetlight lighting systems will change the various shortcomings of traditional streetlights and is the trend of future lighting development. Nowadays, there are few examples of using white light LEDs in outdoor lighting such as streetlight lighting systems. Therefore, the development of an energy-saving high-power LED streetlight system is of great market value. In order to avoid the blindness of actual development and reduce development costs, it is necessary to conduct model simulation design, which can provide a guiding theoretical basis for the actual development of high-power LED streetlights.

　　2 Design requirements and ideas

　　In combination with the actual situation, the following design requirements are proposed for the design of high-power LED street lamp system: As shown in Figure 1, the total system power is less than 150 W, the street lamp installation height is h=8 m, the maximum illumination vertical to the road surface (the area near point O) is more than 15 lx, the illumination at the lateral distance s=15 m from the road surface where the street lamp is installed (the small area at point P) is more than 3 lx, and the illumination at the longitudinal distance r=10 m from the road surface where the street lamp is installed (the small area at point Q) is more than 3 lx.

Figure 1 Street light structure diagram

　　When LED is used as a light source in the field of lighting, multiple LEDs are generally arranged in an array of a certain shape. The light from each LED reaching the target plane can be superimposed to obtain the illuminance distribution of the target plane.

　　The number of array LEDs required is determined by the total power limit of the entire system, the illumination requirements of the target surface, and the spatial light intensity distribution requirements of the illumination optical system. Considering the cost and energy saving, it is hoped that the number of LEDs can be reduced as much as possible while meeting the lighting requirements. This involves the reasonable arrangement of the LED array, including the selection of the LED spacing size and the determination of the LED light output angle.

　　3 System Simulation Design

　　3.1 Single LED Modeling

　　A 1 W high-power SMD LED was selected as the light source, and its simulation model was constructed at a 1:1 ratio. The model consists of three parts: LED light-emitting chip, external epoxy resin package, and base. One side of the LED light-emitting chip is set as the light-emitting surface with angular distribution of light intensity, the total luminous flux is set to 55 lm, and the refractive index of the external epoxy resin is 1.55. Experiments have shown that the position of the LED light-emitting chip determines the angular distribution characteristics of the model's outgoing light. After multiple adjustments, the angular distribution parameters of the sample LED were basically achieved, and the final model is shown in Figure 2.

Figure 2 Single LED simulation model (left) and its light simulation diagram (right)

　　3.2 Light uniformity simulation

　　The spacing between LEDs greatly affects the uniformity of road illumination and the lighting effect of the lighting system. Reasonable selection of LED spacing can avoid blindness in design. The model is shown in Figure 3.

　　Taking the combination of two LEDs as an example, the illumination distribution under different spacings is simulated and the results show that: as the LED spacing increases, the illumination at the center of the reference plane slowly decreases. When the spacing d is 90 mm, the illumination distribution curve becomes flat (as shown in Figure 4), and the illumination uniformity in the area between the two LEDs is good. When designing the array distribution of multiple LEDs, for the sake of illumination uniformity, try to keep the spacing between LEDs within 90 mm.

Figure 3 Two LED simulation models

Figure 4 Illuminance diagram when LED spacing d=90 mm

　3.3 Street Light LED Array Design

　　The elevation angle θ and cantilever length d of the street light system during installation are selected as the values ​​of θ and d of the current common street lights (θ=15° and d=1.5 m). Combined with the design scheme of the illumination requirements of the road lighting in the longitudinal direction, the array adopts a horizontally symmetrical structure, consisting of 130 LEDs, 65 on each side, and the luminous flux of each LED is 55 lm, so the total light energy on each side is 3575 lm. The focus of the design is to project as much light as possible on the effective target plane. Each side structure consists of 2 base plates and 3 reflectors. The LED spacing d=60 mm is selected, and the LED array of 7 columns × 5 is evenly arranged on the base plate 1 close to the lamp pole, and the other base plate 2 is evenly arranged with 7 columns × 5 LEDs; when θ=0°, the angles between base plates 1 and 2 and the road surface are 65° and 60° respectively. Two reflectors perpendicular to the two bottom plates are installed on both sides, and the extension length is 300 mm. A reflector 700 mm long is attached to the left side of bottom plate 2, which is 120° to the road surface when θ=0°. The appearance of the system is shown in Figure 5.

Figure 5: Appearance of the left and right side structures of the system

　　4 System Simulation Analysis

　　The light output of each LED is set to 20,000, and the illuminance distribution of the 20 m × 32 m area at a distance of 8 m is shown in Figure 6. Obviously, the illuminance requirements at points O, P and Q in Figure 2-1 in Figure 6 have all met the expected design goals. For further analysis, the area above 3 lx is outlined in Figure 7.

　　In the small area (-15 000, 10 000), which is the darkest area farthest from the lamp, the illumination is below 3 lx. This has no effect, because in actual applications, more than one lamp is working.

　　The most demanding staggered arrangement of street lamps was analyzed, and the adjacent side structures of two street lamp systems were selected for road illumination simulation. The results are shown in Figure 8.

　　Similarly, the area on the road surface with illumination > 3 lx is outlined, as shown in Figure 9, and the effect is more satisfactory.

Figure 6 Simulated illumination distribution of street lighting system in the road area

Figure 7 The area with illuminance > 3 lx in Figure 6

Figure 8 Simulated illumination distribution of the road surface with two staggered street light systems

Fig. 9 The area with illuminance > 3 lx in Fig. 8.

　　5 Summary and Analysis

　　(1) Power: The entire system consists of 130 1 W LEDs, with a total power consumption of 130 W, meeting the design requirement of less than 150 W.

　　Compared with the traditional street lamp system based on high pressure sodium lamp or metal halide lamp, the power consumption is less than 26% of the traditional street lamp, which greatly saves electricity. Moreover, the life of the street lamp system based on LED packaging can reach 100,000 hours, which is far beyond the reach of traditional street lamps, and the prospects are very optimistic.

　　(2) Illumination: The system can meet the illumination requirements at several expected points. The reference values ​​of the illumination brightness of the road surface that the system can achieve are: average illumination 8.7 lx; illumination uniformity 0.345; average brightness 0.484 cd/m2; brightness uniformity 0.345. Compared with the road lighting level standard [3], it can be seen that the simulation system can basically meet the lighting requirements second only to highways. From the system simulation analysis, it can be seen that the staggered installation of multiple street lamps on the road surface can achieve a better lighting effect.

　　(3) Experiment: We selected two columns of 10 LEDs in the LED array of the simulation system for actual production, and compared and analyzed the measured data with the corresponding simulation data (due to the length of the article, this part of the experiment and simulation data is not listed). The results show that both the specific data difference and the general trend change show that the simulation model and the actual system have achieved good consistency in all parameters. Therefore, it is concluded that the entire street light system simulation model has a practical development guidance role.

　　In summary, the street lamp lighting simulation system based on high-power LEDs has well met the various requirements put forward before the design, and its feasibility has been verified through experiments, laying the foundation for the subsequent actual development and application of street lamps in the project and playing a guiding role.