Cover Story：Feng Z X, Cheng D W, Wang Y T. Iterative freeform lens design for prescribed irradiance on curved target. Opto-Electron Adv 3, 200010 (2020).

Freeform optics is a revolution to the traditional spherical and aspherical optics. Sufficient design freedom of freeform optics enables very complicated tasks that are previously unimaginable. One of the most interesting topics for freeform optics design is to transfer the light distribution of a light source into an arbitrary illumination pattern on a given target. This difficult inverse problem can date back to 1941 when Komissarov and Boldyrev showed that the design of a non-symmetrical reflector for a point source could be formulated as a Monge-Ampère equation. Most of the current methods focus on producing prescribed irradiance distributions on planar targets. The design for curved targets is still a challenge. Recently, Professor Zexin Feng, Professor Dewen Cheng and Professor Yongtian Wang from School of Optics and Photonics of Beijing Institute of Technology presented a new method applicable for curved targets based on iterative wavefront tailoring (IWT). The new method can artfully dissolve the mathematical difficulties that arise from the fact that a curved target has varying z-values intertwined with the source-target ray map. In addition, the new method is developed using the stereographic coordinate system, which is applicable for light sources with large light emission angles such as LEDs. In many applications, the target cannot be considered as a perfect 2D plane but a 3D surface, for examples, road surfaces on mountain areas, sand tables, surfaces of sculptures and cultural relics, where the new method for freeform optics design will show its significance.