根据非成像光学原理，提出了基于经纬划分和切面迭代的自由曲面求解方法。先在三维坐标系中建立自由曲面与目标面的相对位置模型，并对自由曲面沿经纬方向角度均分；再根据自由曲面各经纬带上能量与目标面各微带上能量对应相等，实现目标面网格划分并求得与自由曲面各网格对应的目标节点坐标；选取自由曲面初始网格节点，利用Snell方程和切面迭代法，沿经纬方向分别求解，得自由曲面各节点坐标，即可拟合成所求自由曲面。对用该方法设计的LED透镜进行模拟，结果表明对于目标面为轴对称区域的均匀照明，该方法可快速精确地获得对应的光学系统，使得LED光源能够广泛应用于道路照明系统中。

According to non-imaging optical principle, a method to obtain freeform based on longitude-latitude division and tangent-plane iteration was proposed. Firstly, a relative position model was constructed in three dimensional coordinate system, and the freeform was divided along its longitude and latitude direction with uniform angel. Secondly, because the energy of both each longitude and latitude freeform micro-belt and each corresponding micro-belt on the target plane was equal, the target plane was divided into grids and each coordinates of corresponding points on the target plane were obtained accurately. Finally, by choosing an initial grid node on the freeform, and using Snell equation and tangent-plane iterative method, the coordinates of all grid nodes on the freeform were obtained, which could be synthesized for the freeform surface. The simulation using the designed LED lens shows that for the axial symmetry target plane with uniform illumination, the corresponding optical system can be obtained quickly and accurately with this method, which enables LED light source to be applied in road lighting system.