The development of automobile headlamp has experienced the development of incandescent lamp, halogen lamp, xenon headlamp and LED[
Infrared and Laser Engineering, Volume. 49, Issue 3, 0314003(2020)
Design and fabrication of a Fresnel lens for laser lamps
Compared with LED lamp, laser lamp has the advantages of small energy consumption, small volume and high brightness. It will become a new trend of vehicle lamp. A design method of laser lamp is proposed. The light source is laser diode, which is converted into white light by phosphors film. The optical test platform is built to study the optical characteristics of the source, and the machine vision method is used to study the change of the shape and size of the light spot before and after the phosphors, which provides a theoretical basis for subsequent optical design. Fresnel lens is designed by non-imaging optical design method to achieve laser convergence and shaping. The Fresnel lens is machined using ultra-precision turning. The laser lamp prototype is assembled and tested, and the feasibility of the design method is verified by optical simulation and optical experiment.
0 Introduction
The development of automobile headlamp has experienced the development of incandescent lamp, halogen lamp, xenon headlamp and LED[
In 2013, BMW launched a laser headlamp for automobiles, which consists of a semiconductor laser, a phosphor, and mirrors. The laser is a blue-light semiconductor laser. Three laser beams converge into a packaged tank with phosphor through three mirrors, and the phosphor emits long-wavelength light. Then blue laser and long-wavelength light mix together into white light. The white light is reflected through the light distribution mirror and directed to the road ahead[
Automobile illumination system belongs to the field of non-imaging optics, which needs high energy efficiency and illumination uniformity. At present, a lot of researches have been done on LED lamp[
Non-imaging optical design can be carried out only under the known characteristics of the light source. Such as for the LED lamp design, the light distribution curve, luminous intensity, spectral distribution of the LED light source must be known. Blue laser diodes are often used to excite the yellow phosphor, which can generate white light for automobile headlamps. But there are few studies on the characteristics of laser-driven white source[
The laser lamps designed in this paper are used for off-road vehicles, engineering vehicles, emergency vehicle, etc., which need to be used for night or long-distance inspections, construction lighting and other lighting needs in workplaces such as night work or in the wilderness. The spotlights are mounted on the top of the automobiles. Compared with traditional lens, Fresnel lens has the advantages of light weight and small volume.
In this paper, a method for detecting the characteristics of laser-driven white source is proposed. An optical model of the laser-driven white source is established to design the laser lamp. The lamp uses Fresnel lens to achieve converging and shaping of laser. According to the characteristics of laser-driven white source, this paper completes the optical design and processing of Fresnel lens, system assembling, testing of the laser lamp prototype.
1 Design method
1.1 Test and analysis of light source
The laser source uses a 450 nm laser diode. The light emitted from the laser diode is concentrated by the first lens, then incidents to the phosphor. The light emits after absorption, scattering and refraction of phosphor. Therefore, the laser lamp consists of two parts: laser-driven white source system and optical system. The phosphor is the bridge between these two parts. For the optical design and simulation, it is necessary to test the characteristics of the light source and build a test platform, which studies the light efficiency of laser source, the distribution of spot energy and so on.
Firstly, the power conversion efficiency of laser-driven white source is tested, and the test platform is shown in
The power conversion efficiency is the ratio of optical power to the electric power. Using optical power probe, the light power is measured with different excitation electric powers, and the power conversion efficiency of the source is 26%, as shown in
Figure 1.Laser source structure and test platform
Figure 2.Optical power of laser-driven white source under different power supplies
Because of the complexity of phosphor luminescence mechanism, it is difficult to accurately establish simulation model in optical simulation software. In this paper, the machine vision method is used to study the change of the shape and size of the light spot before and after the phosphors, which provides a theoretical basis for subsequent optical design. As shown in
Figure 3.Spot detection before and after phosphor
Figure 4.Spot image incident on the phosphor of different convergent lenses
1.2 Principle of optical design
Fresnel lenses are commonly used in concentrated photovoltaic (CPV) systems to collect solar light[
The optical structure of the laser lamp is shown in
Figure 5.Schematic diagram of laser light structure
The design requirement is that the diameter of the spot is less than 3 m at a distance of 25 m from the light source, and the irradiance of the receiving is not less than 150 lx.
Two approximations are made to the computational model: the light source is a point source and the exit light is collimated.
The design of Fresnel lens is based on two dimension geometric construction method (2DGCM)[
Figure 6.Design principle of Fresnel lens
The whole calculation flow chart is shown in
Figure 7.Whole calculation flow chart
The vector of the incident ray is
If the left surface normal vector of the lens is
Similarly, the vector
According to the maximum incident angle γmax, the exit light vector
The calculation flow chart of discrete point
Figure 8.Calculation flow chart of discrete point of Fresnel lens
Firstly, the first prism unit of Fresnel lens is calculated, and the edge points of the incident plane is A1,1. The incident ray
The edge point of working face is B1,1, B1,1 = A1,1,
Where,
Then, ray vector
Where,
According to the same calculation process, the discrete points Bi,j and the normal vector
Figure 9.Calculation and modeling of discrete points of Fresnel lens
1.3 Optical simulation
The established model is imported into optical software for ray tracing under the point source, as shown in
Figure 10.Ray tracing results of point source
Figure 11.Illumination distribution on receiving surface. (a) distance 40 mm from the light source, (b) distance 25 m from the light source
2 Fabrication and optical experiment
The Fresnel lens is machined by ultra-precision single-point diamond lathe[
The design surface accuracy of Fresnel lens is less than 50 μm, and its machining accuracy is 5-10 μm. The Fresnel lens can be produced for mass production by injection molding, and the surface accuracy is less than 30 μm, can meet the design requirements.
The prototype of Fresnel lens laser lamp is shown in
Figure 12.Ultra-precision machining of Fresnel lens
Figure 13.Laser Fresnel lamp module
The optical experiment of the prototype is carried out, as shown in
Figure 14.Lighting experiment of laser lamp
If the application is popularized, aspheric lens is made of glass die-casting and Fresnel lens is made of injection molding. The cost is low and it has a certain competitive advantage over the traditional lamp.
3 Conclusions
In this paper, a design method of laser lamp is proposed. The light source is laser diode, which is converted into white light by phosphors film. The mapping relationship of the spot size between before and after phosphors, which guides the optical design. The laser lamp uses Fresnel lens to shape the beam, and the design method of Fresnel lens is 2DGCM. The correctness of the research process is verified by optical simulation and optical experiment. But the overall size of the design is too large, and the illumination value can be gradually increased, which will be improved in future research.
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Ying Cheng, Zechao Wang, Xiaobing Xie, Yongbin Lu, Wenshuang Chang. Design and fabrication of a Fresnel lens for laser lamps[J]. Infrared and Laser Engineering, 2020, 49(3): 0314003
Received: Nov. 18, 2019
Accepted: --
Published Online: Apr. 22, 2020
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