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