Freeform surface lens design for uniform?illumination

A method of designing refractive freeform surface lenses for uniform illumination is described. Using Snell's law, the differential equations are given to denote the relationship between the directions of the normal of the freeform surface and the input/output ray vectors; the Runge–Kutta formulae are used to calculate it. Moreover, two optical models for uniform illumination are discussed. By this process, uniform, narrow divergence half-angle, high-usage illumination over the given area is achieved. Some practical examples are also included in this paper.