Research On Design Of Freeform Surface Lens For LED Lighting System

Compared with traditional light source, LED has many advantages, such as longworking life, small size, high luminous efficiency, low-voltage DC drive, etc, so it hasbeen widely used in lighting system for projection display and so on. For thedevelopment of LED technology, the lighting design for LED becomes more and moreimportant. But because LED has an analogous Lambertian radiation distribution whichwill result in its non-uniform light intensity distribution, LED couldn’t be applieddirectly in certain circumstances, such as achieving a uniform illumination area in themicro display projection system. In order to solve the problem, the secondary opticaldesign which aims at redistributing the light emitted by the LED light source should bedone to achieve the specific illumination requirements.The traditional lighting system employs optical rod or fly eyes lens. With thedevelopment of freeform surface technology, freeform surface lens receives increasedattention. Because freeform surface has various design freedom and can control the lightirradiation freely, and freeform surface lens has many advantages like less loss ofluminous energy, small system size, more compact structure compared with traditionallighting system, it is playing a more important role in non-imaging optics.In order to achieve the secondary light distribution of the LED light source, afreeform surface lens is designed to obtain uniform illumination of circular area andsquare area on the target plane. In this paper the freeform surface of lens is establishedby the partial differential equation method which achieves the points of freeform surfaceby strict mathematical reasoning and numerical calculation with computer, the methodcan fit different lighting occasion and it has high computation speed, so this method isused most in secondary optical design. The process of the partial differential equationmethod can be summarized as: first the mapping relationship between the light sourceand the target plane is achieved by the law of conservation of energy, and the partialdifferential equations of freeform surface are established by the vector form of Snell’slaw, then the discrete points of freeform surface are obtained by numerical calculationwith the software, and the freeform surface is achieved by NURBS with modelingsoftware. During the progress of designing the lens, in order to solve the problem of surface discontinuity, the process of solving the differential equations is described indetail and the integrability condition is introduced to modify and adjust the points’position of freeform surface, thus reducing the errors accumulating during the processof numerical calculation and improving the continuity of the freeform surface. Bysoftware simulation, the continuity of the freeform surface and the uniformity ofillumination improve a lot, so it is validated that the results meet the requirements andthe design method using integrability condition can improve the continuity of thesurface and uniform illumination.In addition, this paper presents a design on freeform lens with cyclic annularstructure. By combining several lenses which can respectively refract different angles oflight to achieve a same area with uniform illumination independently, every point on thetarget plane receive more light with different incident angles, so that the followingoptical system can take full advantage of the light received by the target plane. Thisresearch has practical significance in illumination application.