Optimizing nonimaging free-form optics using free-form deformation

The increasing efficiency of high power LEDs has resulted in many new applications in general lighting. To take full advantage of the properties of LEDs, free-form surfaces can be utilized to create compact non imaging optical systems with high efficiencies and high degrees of freedom for optical designers. One of the commonly used methods to do optical design for this kind of systems is optimization. Appling this powerful tool allows the enhancement of given optical elements to achieve a desired performance. In this way, free form surfaces which are usually represented by NURBS, can be optimized and applied even close to an extended LED light source. However, using optimization for free-form surfaces is far from being straight-forward and requires a lot of experience mostly due to the high amount of possible optimization variables for NURBS. This comes along with high, computational effort and difficulties concerning the choice of boundary conditions and merit functions. This contribution presents a novel non-imaging optical design approach using the concept of free-form deformation (FFD) in conjunction with customized optimization algorithms to create efficient optical free-form surfaces for extended LED light sources. Within this framework, specific coordinate system transformations are used to modify the global shape of free-form surfaces. In this way, optimization techniques relying on relatively few and easily accessible variables can be applied successfully. All presented concepts are implemented in a flexible and fully automated FFD optimization software tool incorporating a commercial raytracer and numerical optimization techniques. Several examples are presented in detail and the scope of FFD based optimization is demonstrated.