The development in optical manufacture, alignment and testing has enabled the increasing use of freeform surfaces in all kinds of optical systems. The demanding system requirements need the involvement of optical surfaces that is able to provide more degrees of freedom. For better and more efficient use of the freeform surfaces, the understanding of freeform surfaces from all perspectives is necessary. We therefore study the impact of optimization strategy and freeform location on typical optical systems for varying applications. The uses of different optimization strategies, as well as the choice of locations for placing one or two freeform surfaces are considered. Their respective impacts on the final system performance are analyzed according to different aberration constitutions. By concluding all findings, we present some general rules for using and optimizing freeform surfaces in real design work. In the end, a work flow that gives instructions on how to use freeform surfaces in system design is presented.
A hyperspectral imaging spectrometer covering the wavelength range from 420 nm to 1000 nm is designed for the purpose of monitoring Earth’s environmental change. It has an entrance slit length of 24 μm, f/# of 3, smile and keystone distortion smaller than 20% of the pixel pitch and a spectral resolution of 6.5 nm. We design and review thirteen systems including one Offner system, two Schwarzschild systems and ten TMA systems for such specifications. Freeform surface and aspheric surface are used in some of the systems to achieve the required system parameters. With all system performance being summarized and evaluated, advantages and disadvantages of three different system types are compared. We down select two systems for further fine adjustments and tolerancing analysis. Final systems with superior performance and detailed tolerancing analysis are given at the end.