Design of hybrid reflector structure for LED street light optical system

1 Traditional street lighting

At present, the main types of light sources used for street lighting are: metal halide lamps, incandescent lamps and high-pressure sodium lamps. Among them, high-pressure sodium lamps are widely used due to their high luminous efficiency, long life, strong penetration and other advantages. Its basic optical structure consists of a light source, a reflector and a transparent lampshade. Due to the characteristic that the light radiation of the light source almost occupies the entire space, it is necessary to design the light distribution of the optical system of the street lamp to achieve the photometric parameters of national standard street lamps. Usually, the reflector of the high-pressure mercury lamp type street lamp uses a parabolic groove for light distribution. It can adjust the light to illuminate the required area. If LED is used as the light source for new street lighting, it is necessary to seek a new light distribution model to distribute light for the LED light source to achieve the photometric index of street lighting.

2 Optical design ideas for LED street lights

2.1 LED Street Light Optical System

The optical structure of LED street lights can be roughly divided into two types: projection type and reflective type. The optical structure of a projection type street light is usually composed of a rotating parabolic reflector and a lens, but this single structure cannot meet the light distribution requirements of street lighting, so a lens must be used for light distribution again, as shown in Figure 1a. This method has high requirements for the coordination between the reflector and the light distribution lens, which increases the difficulty of design, installation and maintenance. The reflective optical structure, as shown in Figure 1b, is adopted by more and more optical designers because only the reflector needs to be designed for light distribution. This article also adopts a reflective optical structure and designs a hybrid structure for the reflector.

Figure 1a Projection structure diagram

Figure 1b Reflective structure diagram

2.2 Computer-aided design of illumination optics

Lighting optical design belongs to non-imaging optical design, which mainly studies the optical system from the perspective of energy transfer law. It basically adopts non-sequential ray tracing. The commonly used method for calculating illumination is the Monte Carlo method. This paper uses Tracepm optical simulation software for simulation. The basic steps of the design are as follows:

(1) Determine the number of LED light sources, build a light source model and set its parameters;

(2) Establish a three-dimensional model of the hybrid reflector structure;

(3) Determine the arrangement of LED light sources and simulate them using optical software;

(4) Analyze the simulation results and compare the differences between the simulation results and the road lighting distribution requirements;

(5) Based on the differences in the simulation results or the parts that do not meet the requirements, the corresponding parameters are repeatedly modified until the design requirements are met.

2.3 Hybrid structure design

At present, reflective optical structures have been applied to more and more fields, and the difficulty lies in the design of the reflector structure, that is, how to design a reflective surface shape that meets the required optical lighting distribution requirements. In order to direct the reflected light to the specified lighting area, there are two possible ways, namely, the light emitted by the light source is reflected to the same side and reflected to the opposite side, as shown in Figure 2a and Figure 2b respectively.

Figure 2a Same-side reflection method

Figure 2b: Contralateral reflection mode

According to the requirements of the luminous characteristics and lighting range of the LED light source used, this paper adopts the method of reflection to the opposite side to design the reflective surface in the X direction and the Y direction respectively to meet the needs of on-site road lighting. The light source is placed at the front focus Fl of the ellipsoid, as shown in Figure 3, which is the ellipsoid reflective surface model.

Figure 3 Ellipsoid reflection surface model

The expression equation of the ellipsoidal reflection surface is:

Where a and b are the semi-major axis and semi-minor axis of the ellipsoid respectively.

In Figure 3, f1 and f2 represent the distances from the ellipsoid vertex to the two foci, F1 and F2, respectively, and their expressions are as follows:

By adjusting the sizes of the major and minor axes a and b of the ellipsoid, the semi-focal length of the ellipsoid reflection surface can be changed, thereby changing the angle of the emitted light to ensure that the emitted light can be irradiated within the desired illumination area.

2.4 Calculation method of illumination

The illumination calculation formula currently used is usually based on the calculation model shown in Figure 4, where ds represents a small area on the object plane near the on-axis point, ds' represents a small area on the image plane near the on-axis point, U represents the object aperture angle, U' represents the image aperture angle, and the object brightness is L and the image brightness is L'. The luminous flux Φ emitted by the object ds within the solid angle U can be expressed as:

Then the luminous flux φ' incident from the exit pupil to the image surface micro-area ds' is:

So the illumination E' of the image plane can be obtained as:

When the media of the object space and the image space converge, the image plane brightness L' is equal to the product of the object plane brightness L and the system transmittance τ0, that is:

Then formula (6) can be expressed as:

Figure 4 Schematic diagram of illumination calculation model

2.5 Arrangement of LED light sources

At present, the luminous efficiency of a single LED is limited. To be used in LED street lighting, multiple LEDs need to be arrayed in a certain shape. Considering the cost of the lamp, it is hoped that as few LEDs as possible will be used while meeting the lighting requirements. This requires finding a suitable arrangement. According to the luminous characteristics of LED light sources and the basic requirements of road lighting, this paper adopts a 10×6 array and deflects each column of light sources at different angles.

2.6 Simulation Results

FIG. 5 is the result obtained by multiple simulations using the optical software Tracepro.

Figure 5a is a simulation effect diagram of lighting area distribution. It can be seen from the figure that through the design of the reflector and the control of the light source arrangement angle, uniform lighting is basically formed in a given lighting area, which not only improves the utilization rate of the light source, reduces light pollution, protects human eye health, but also effectively improves the driver's recognition of road conditions. Figure 5b is an illumination distribution diagram.

The wider curve in the figure represents the illuminance distribution in the 0 degree direction (i.e., the direction of vehicle travel), and the narrower curve represents the illuminance distribution in the 90 degree direction. It can be clearly seen from Figure 5b that the illuminance distribution in the 90 degree direction is relatively uniform at around 121x, while in the 0 degree direction, the illuminance varies between 10x and 141x: only the illuminance in a small range of the edge area is lower than the average illuminance, but it still basically meets the illuminance requirement of 10x to 151x in the lighting area required by the national standard.

Figure 5 Simulation results

3 Conclusion

This paper designs a hybrid reflector structure for the optical system of LED street lamps. From the results of simulation, it can be seen that the requirements of uniform rectangular light distribution in the lighting area are basically met. The next step will be to continue to study how to improve the boundary of rectangular light distribution and the uniformity of illumination in the lighting area, and optimize the deflection angle of the reflector and the light source in combination with the luminous characteristics of the high-power LED light source itself, so as to achieve the required light distribution design, or conduct research and exploration from the perspective of new reflector structures.