LED street light distribution research

0 Preface

LED (Light Emitting Diode) is a new solid-state light source that is competitive in the 21st century. Compared with traditional light sources, it has the advantages of small size, long life, low energy consumption and flexible control. In road lighting, with the development of white LED technology, the luminous flux of single-chip LEDs continues to increase, and the replacement of traditional light sources for LED streetlights will become a trend. The light intensity distribution of the light source in all directions of the space is the light distribution. LED street light distribution is to make the light as much as possible to the various areas of the road surface to obtain the light distribution that meets the requirements of road lighting. Whether the LED street light distribution design is accurate or not is one of the key factors that can replace the traditional street lamp. Studying the LED street light distribution is of great significance to the future system design research of LED road lighting.

1 LED street light distribution features and requirements

1. 1 LED street light distribution shape characteristics

In road lighting, if there is no light distribution to the LED street light source, the light pattern on the road surface is a circular spot with a large area, and some of the light is scattered outside the road surface and is not used, as shown in Figure 1(a). Shown. In order to meet the requirements of brightness, illuminance and uniformity of the road surface, and to distribute most of the light on the road surface as much as possible to improve the utilization of the light and reduce unnecessary waste, it is usually necessary to illuminate the LED street light. The light pattern or spot formed by the light output from the street light on the road surface is preferably a rectangle, as shown in Fig. 1(b).

Figure 1 LED street light distribution diagram

1.2 Road lighting standard light distribution requirements

According to the requirements of "Urban Road Lighting Design Standards", the roads in road lighting are divided into express roads, trunk roads, secondary trunk roads and branch roads. The standard values of LED road lighting light distribution requirements should meet the requirements of Table 1.

Table 1 Road lighting standard values

The current research on LED street lamps is mainly focused on the application of secondary trunk roads and branch roads. The small luminous flux is the main reason why LED street lamps cannot be applied to the main road. The application on the trunk road has yet to be researched and developed by LED technology.

1. 3 LED street light distribution curve

The spatial distribution of luminous intensity is often referred to as the "light distribution curve." Below the street light, the light intensity should be minimal, and as the elevation angle θ increases, the light intensity Io increases. The function relationship is , that is, the distribution curve expression. The ideal and light distribution curve of a single luminaire for road lighting is shown in Figure 2. Due to the complexity of the optical design, the light distribution shape is difficult to fully conform to the functional relationship, and the projection range of the θ angle can be reduced, and the spacing of the lamps can be reduced to obtain uniform illumination. In general, it is desirable to achieve a wide-angle "batwing" shape after light distribution.

Figure 2 ideal light distribution curve

2 LED street light distribution scheme and analysis

The light distribution scheme of LED street lamps is based on the requirements of road lighting, and it is necessary to consider controlling glare and considering environmental factors. For road lighting, the light effect and the light distribution curve are two important parameters. At present, the LED street light distribution scheme mainly has the following basic forms.

2.1 LED light distribution once

In the power LED manufacturing process, the lens process can improve the light efficiency, reduce the light output loss, and change the light output characteristics. The LED package lens process has a certain relationship with the high-power LED street light illumination. With a well-packaged lens design, LEDs can achieve better light output characteristics.

The LED light distribution in the double-head lens package can transform the light intensity output curve of the single LED into a "batwing" shape, so as to further realize the "batwing" shape light distribution of the entire light intensity output curve of the entire street lamp. The double-lens primary light distribution structure and light distribution curve are shown in Fig. 3(a) and Fig. 3(b), and the lens is designed as a primary light distribution core.

Fig. 3 Schematic diagram of LED light distribution structure and light distribution curve of double lens package

2.2 Secondary light distribution of LED street lamps

The high-power LED in the LED street lamp adopts a lens or a reflector to further change the output light characteristic, that is, the secondary light distribution of the LED street lamp. The secondary light distribution of LED street lamps mainly has the following conditions.

Fig. 3 Schematic diagram of LED light distribution structure and light distribution curve of double lens package

LED secondary light distribution using a total reflection lens. The light is emitted from the relatively light-tight medium to the opposite light. When the human angle is greater than the critical angle, total reflection can occur, and the principle is used to design the axisymmetric total reflection lens. For example, changing the beam angle to within ±30° facilitates further light distribution design. The total reflection lens is shown in Figure 4.

Circle 4 total reflection lens

LED secondary light distribution using a free-form lens. In this design, a free-form optical element with asymmetric rectangular light distribution in the XY-axis direction is used. For example, a uniform distribution of light distribution of ±60° is generated on the X-axis to meet the illumination requirements of the length direction of the road, and a uniform distribution of light distribution of ±30° is generated on the Y-axis to obtain an LED "batwing" having a rectangular illumination effect. "Shaping light. In the free-form surface design, the differential light method, multi-parameter optimization method, multi-surface simultaneous design method and shearing method are used to obtain the light distribution of the light source and the light distribution of the illumination target surface. The freeform lens is shown in Figure 5.

Figure 5 freeform lens

LED secondary light distribution with external lens and reflector. Select the appropriate lens and parabolic reflector to make the outgoing light meet certain requirements. The combination of the external lens and the reflector is shown in Figure 6.

Figure 6 LED diagram of the external lens and reflector

2.3 Three-time light distribution of LED street lamps

The three-time light distribution of the LED street lamp is realized by superimposing and spatially arranging a plurality of LEDs (unit modules) on the basis of the completion of the first and second light distribution of the high-power LED to meet the average brightness, average illumination and uniformity of the road illumination. Requirements. There are several ways to perform three times of light distribution.

(1) Flat light distribution

The LED street lamp is designed with a free-form optical element (lens or reflector) with asymmetric XY-direction rectangular light distribution. Since the rectangular light distribution is completed on a single LED optical component, the entire LED street lamp only needs to be arranged by the LED module. Just on the tablet. Fig. 7(a) is a schematic diagram of a planar light distribution of an LED street lamp.

(2) curved surface light distribution

A plurality of LEDs are arranged to form an LED module, and the LEDs on the LED module are light-distributed by using an axisymmetric total reflection lens or a reflector, and the radiation angular width of the lens or the reflector is sufficient to cover the width of the road. The LED modules are arranged on the curved surface, as shown in Fig. 7(b), by adjusting the curved surface, a nearly rectangular light pattern can be produced in the direction of the road.

Figure 7 Three-dimensional light distribution diagram of LED street light

(3) Multi-fold surface light distribution

LED light has good directionality. In order to obtain better road light distribution, each group of LED technology is designed to illuminate the respective areas. The simpler way is to use V-shaped surface method. In the multi-folding light distribution design, the high-power LEDs of each group in the street lamp are respectively installed on different planes, and the light output characteristics and approximate rectangular illumination effects of the street lamps are obtained by adjusting the relative angles of the respective groups to meet the road lighting standards. Claim. Figure 7 (c) is a schematic view of a multi-folded LED street light.

(4) Reflective cup light distribution

The output characteristics of the street light are obtained by the design of the LED reflector. Designing an XY-axis asymmetrical reflector for a single LED, this solution is similar to a planar light distribution, except that a free-form surface reflector is used to achieve a light output close to the "batwing" shape, designing multiple reflectors And the arrangement can also get better road lighting effects.

2.4 LED street light distribution scheme analysis

The technical means or scheme adopted by the meta-thesis, in terms of LED street light distribution, its light output characteristics meet the requirements of night road lighting standards, and the technology or scheme has good use value. The above light distribution has its own advantages and disadvantages, as shown in Table 2. An LED street light distribution scheme often requires multiple combinations of light distribution and repeated design to achieve road lighting standards. For example, the combination of LED primary light distribution (lens package) and secondary light distribution of the lamp, the combination of the secondary light distribution unit of the LED and the light distribution of the lamp, and the secondary light distribution of the reflector of the LED street lamp.

Table 2 Comparison of light distribution schemes

At present, power-type self-lighting LEDs are moving toward a single-chip high-power direction. Due to chip thermal bottleneck constraints, the heat dissipation of a single-chip high-power LED packaged in a multi-chip package is relatively difficult and the light efficiency is relatively low. Therefore, considering the factors of packaging, heat dissipation, light efficiency, color development, etc., a product with a single LED power of 1 watt to several watts and a luminous efficiency of 90-1001 m/W is usually selected, and the overall power is achieved by multi-array hybrid mode. Requirement, in the LED street light distribution, the LEDs with better primary light distribution are arranged reasonably on the same plane, and the free-form optical components (lenses or reflectors) are applied to the planar LEDs to perform secondary light distribution to achieve the road. Lighting light distribution requirements. The scheme reduces the total number of light distributions of the LED street lamps, that is, reduces the loss of the LED output light, improves the efficiency of the lamps, simplifies the relative design and processing difficulty of the lamps, ensures the uniformity of the road surface, the illumination, and the light type; design. Therefore, the program has good research prospects.

3 Summary

LED street light distribution design is one of the keys to LED road lighting applications. This paper mainly analyzes the light distribution scheme of LED street lamps, compares the advantages and disadvantages of each scheme, and believes that the light distribution design scheme based on free-form optical components has a good development prospect. The light distribution of LED street lights is of great significance in LED road lighting. With the continuous development of LED technology, the prospect of LED street lamps applied to road lighting will be even broader.

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