Introduction to secondary optical design of LED street light lens

1 background

The development of LED (Light Emitting Diode) technology has opened up a new era of lighting technology revolution. LED has many advantages such as small size, long life, high electro-optic efficiency, environmental protection and energy saving. LED street lighting technology has been rapidly developed in recent years. At present, the actual single LED light source on the market can already achieve 100 lumens per watt. The street lamp with the traditional 250 watt sodium lamp as the light source can only produce the same brightness after only replacing more than 60 LEDs with the LED light source. Greatly saves energy consumption.

Since the radiation angle distribution of most LED light sources is Lambertian distribution of 110 degrees to 120 degrees, if there is no light distribution design, the light pattern on the ground will be a large circular spot. 50% of the light is scattered outside the road and is not used, and it will cause glare to distant vehicles or pedestrians, which is inconsistent with the requirements of road lighting. The Urban Road Lighting Design Standard requires that the light distribution of street lamps be a rectangular spot, and almost all the light is distributed on the road surface [1]. For the main road, a light-cut or half-cut light distribution design is also needed, which can improve the utilization efficiency of light on the one hand and avoid glare on the other hand.

2 LED street light distribution design

There are many kinds of light distribution designs for LED street lamps. The most common ones are as follows:

The first type is an LED street light arranged in a curved shape. The single LED module uses an axisymmetric total reflection lens or a reflector to distribute light. The radiation angle of the lens is sufficient to cover the width of the road. Then the LED modules are arranged on a curved surface, and the curvature of the curved surface is adjusted. The direction of the road produces a rectangular light distribution. Figure 1 shows the design of an LED street light in a curved arrangement. The street light uses 60 high-power OSRAM Lambert distributed GoldenDragonLEDs. The output luminous flux of a single LED is 80 lumens per watt. The lens design uses an axisymmetric transmission--total reflection combination structure, as shown in Figure 2. The middle portion of the lens is a plano-convex aspherical lens. The plano-convex aspherical lens uniformly distributes light within an angle of ±64° from the angle of the optical axis from the LED within ±30°. After the remaining 64° to 90° part of the light passes through the cylindrical surface of the inner side surface of the lens, it is totally reflected by the curved surface of the outer side, and the reflected light of this part is also transmitted within the range of ±30° after passing through the tapered surface of the upper surface. distributed. The light beam of the transmissive portion and the total reflection portion of the lens is superimposed to finally form a relatively uniform beam distribution (uniformity greater than 60°) within a range of ±30°. The ray tracing of the lens and the far field angular distribution of the light intensity are shown in Fig. 3. The far field angle distribution of the light intensity is butterfly shape.

Figure 1 LED street lights in a curved arrangement and design

The LED street lamp is constructed by arranging the LED lens modules on a curved surface, and by adjusting the curvature of the curved surface, the lamp head forms a light distribution of about ±60° in the arc direction, so that the lamp head can be installed at a height of 10 meters. A square light pattern of about 35 meters in length is formed on the road surface, covering a square of 25 lanes having a width of about 10 meters.

The design and processing of the secondary optical components (lenses or reflectors) of such LED street lamps is relatively simple, and the introduction of a total reflection lens can maximize the utilization efficiency of light, and the theoretical calculation efficiency exceeds 98%. However, since the transmittance of the lens material itself is about 92%, the lens efficiency of the actual injection is about 90%. The lens needs to have a certain angular distribution to cover the required road width at the desired height position, while the road direction light distribution is adjusted by the arc of the LED arrangement. The arc-shaped LED street lamps are relatively beautiful, and the unfavorable factor is that the arc-shaped arrangement makes the design of the heat-dissipating plate of the high-power LED and the structural design of the lamp cap more troublesome.

The second type is a flat LED street light. The LED street lamp is designed with a free-formed optical element (lens or reflector) with asymmetrical rectangular light distribution in the XY direction. The rectangular light distribution is directly performed on a single LED optical component. The overall street lamp only needs to have a rectangular light distribution LED. The modules are simply arranged on a flat plate. The LED street lights are relatively simple in terms of mechanical structure, heat dissipation and power control. Road lighting of different grades of roads and different pole heights only needs to add different numbers of LED modules. . Since the light distribution is a rectangular asymmetric distribution, a simple axisymmetric total reflection lens cannot be realized, and an asymmetric free-form surface lens is required, and the lens design and processing process are complicated. The design of this freeform lens will be highlighted here.

Figure 2 Design of an LED total reflection lens with a 60° divergence angle

Figure 3 Photometric analysis of a single LED module

Since general optical software (such as Zemax, CodeV, etc.) is not mature enough for the optimization design of free-form surfaces, designing an asymmetric free-form surface requires a lot of time to manually adjust and set the operating parameters repeatedly, a more complicated freedom. Surfaces often take up to a month or even months, and sometimes the optimized optical efficiency of the surface is not ideal. Here, the principle of conservation of edge ray etendue (Etendue) is used to create an accurate calculation method for the node vector of a free-form surface control grid, which can be optimized in a short time (generally several hours or even shorter). Free-form optics with excellent efficiency and precise light distribution.

Figure 4 Conservation of the spread of edge rays

The principle of conservation of the spread of edge rays is shown in Figure 4. It combines the principle of edge ray and the conservation of the source's etendue (EtendueConservation). The light source passing through the optical system to reach the target is a mathematically mapped relationship. The portion of the ray passing through the edge of the free-form surface, after mapping, also corresponds to the edge of the target, and the continuous portion of the free-form surface is mapped to form a continuous distribution in the middle of the target. If the optical system has no losses, the spread of the source and target of the optical system is conserved. The degree of expansion is the product of the area of the light source or target and the solid angle formed by the divergence angle of the light. According to this principle, the target and the free-form surface can be divided into equal-sized grids (such as the V&U grid and the Y&X grid in Fig. 4), and the mesh nodes of the target form a one-to-one correspondence with the mesh nodes of the free-form surface. According to the position of the target node and the normal vector, the node method vector of the control mesh of the free-form surface shown in Fig. 5 can be accurately calculated correspondingly, thereby generating the required free-form surface. The principle of conservation of the spread of edge rays can be expressed by the following equation:

Figure 5 Node normal vector of ribs and ribs on freeform surfaces

This design method is used to design a free-form lens for LED street lights. The street lamp is installed at a height of 12 meters, the street lights are separated by 40 meters, and the road width is 12 meters (3 lanes). That is, the street lamps need to be 40 meters long and 12 meters on the road surface. Wide square spot. According to this requirement, it is necessary to design a free-form lens which produces a light distribution uniformly distributed within ±60° in the X direction and a square spot of a light distribution uniformly distributed within ±30° in the Y direction. Fig. 6 is a free-form surface lens designed by the method of conservation of edge ray expansion.

Figure 6 lens shape and light distribution principle

The node normal of the control grid of the lens, according to the principle of conservation of the edge ray expansion, and the Snell's law of refraction, have the relationship of the following formula (2), where N is the normal vector and A is the incident ray. Vector, A' is the outgoing ray vector.

Figure 7 The normal of the lens control grid and the incident exit ray

The lens surface and the target spot are divided into equal meshes, and the node vectors of the mesh are correspondingly matched according to the Snell equation of the incident light and the outgoing light. The control mesh of the entire curved surface is calculated by a computer iterative method, and finally the control mesh is calculated. The surface of the skin is refilled to form a lens entity. The light effect simulation of the overall street light is ray tracing by the light tracing software LightTools, as shown in Figure 8. The simulation results are shown in Figures 9 to 11. When the street lamp height is 12 meters, the street lamp can produce a very uniform light distribution on the road surface of 40 meters x 12 meters. The far field angle distribution of the street lamp is batwing shape, the half of the peak intensity of the radiation intensity X direction is about ±60°, and the half of the peak intensity of the radiation intensity Y direction is about ±30°. The light type of the street light is shown in Figure 11, and the physical photo is shown in Figure 12. The road light shape test result passes the national urban road lighting design standard CJJ45-2006.

Figure 8 ray tracing of free-form surface lens LED street light

Figure 9 Illumination distribution at a distance of 12 meters

Figure 10 The far field angle distribution of the LED road light

Figure 11 LED road light pattern

Figure 12: Physical picture of LED street light with free-form lens

3 Conclusion

This paper mainly introduces the light distribution design of two kinds of LED street lamps and focuses on the optical design of a free-form surface lens with asymmetric light distribution. The first type of LED street lamp adopts an axisymmetric total reflection lens or a reflector, and the LED lens module is arranged on a curved surface to generate a rectangular light distribution. The introduction of a total reflection lens can greatly improve the utilization efficiency of light, but the arrangement of the curved LED modules makes the heat sink design and mechanical structure of the high power LED more troublesome. The second type of LED street light uses an asymmetric free-form lens that allows the rectangular light distribution to be directly made by a single LED optic. The whole lamp head only needs to simply arrange the LED modules with rectangular light distribution on a flat plate. The LED street lamps are relatively simple in terms of mechanical structure, heat dissipation, and power control, and roads of different grades and different pole heights. Lighting only needs to add a different number of LED modules. The design of the free-form surface uses the principle of conservation of edge ray etendue (Etendue) to create an accurate calculation method for the node vector of the free-form surface control grid. The lens is caused to have a light distribution uniformly distributed within ±60° in the X direction, and a square spot of light distribution uniformly distributed within ±30° in the Y direction.

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