The Design Of Gradient Fresnel Lens Used For LED Road Lighting

As a new kind of solid lighting source, light emitting diodes(LEDs) have many advantages such as environmental friendly, power consumption is far lower than the traditional high pressure sodium lamp, long work life and high electro-optical efficiency. It has become a development tendency for LED to replace traditional lamp in the lighting industry. Road lighting is an important content of urbanization, which aims at create a good visual environment for motor vehicle drivers, to ensure the traffic safety, improve the traffic efficiency of transportation, reduce the crime rate and beautify the city night scene. Because the radiation pattern of LED is very similar to a point light source, which means that the direct output of an LED cannot meet the requirements of road lighting. The secondary optical design is needed to modify the original light intensity distribution curve. It’s an important research topic of how to maximize improve the luminous efficiency of lamps and choose appropriate light intensity distribution form.Currently, LED lens used for road lighting mainly adopts the structure of the free-form surface lens at home and abroad, it can be divided into two types: the peanut shell and TIR(total internal reflection) lens. But these two kinds of lens have large thickness which can lead to poor heat dissipation problems, and this drawback cannot be overcome due to the inherent defect of the structure of the traditional lens. Compared to traditional lens, the biggest advantage of Fresnel lens is its thin thickness, small weight and high light transmittance. But as a result of its retained the original lens curvature radius, Fresnel lens can maintain the original optical path unchanged. The paper is based on the design of gradient Fresnel lens used for LED road lighting. The paper concentrates on the following contents:1) Firstly, the development of LED used for lighting and design method of free-form optical device is introduced. Moreover, the research content is clarified. Then, light distribution requirements of “the urban road lighting design standard "(CJJ45-2006) is researched. On the basis of introducing the nonimaging optics theory, the relative concepts of lighting, the design method of free-form optical device and the design process of Street lamp lens are analyzed.2) Under the premise of known optical radiation characteristics of light source and the illuminance distribution requirement of target surface. According to the law of energy conservation the light source and the energy intensity of target plane are divided into several small squares. Then list the partial differential equation according to the edge of the light theory, and Optical reflection theorem. Finally calculate the surface data of free-form surface by using Matlab.3) The obtained data is imported into Solidworks for solid modeling, through curve lofting, surface suture, mirror and other steps to fitting the lens entity. A new method of fast modeling is proposed in the process of research. This method allows directly generate model in Tracepro by using Scheme language. The advantages and disadvantages of two kinds of modeling method are analyzed comparatively.4) Build the gradient of Fresnel lens. Ten free-form lens which have the same light effect is but different sizes are built. Cutting them after stacking, make it looks like Fresnel surface type, so that the lens volume can be reduced and the heat dissipation performance can be increased.The Simulation results of Lighttools shows that the gradient Fresnel lens satisfies the requirement of road lighting optical. And lens thickness thinning solves the problem of poor heat dissipation performance of traditional free-form surface street lens.