Measuring ultra-precision freeform surfaces using a hybrid fitting and matching method

Nowadays, freeform optical components are being used in various applications such as photonics, telecommunications, automotive lighting, etc. However, there is still lack of international standards for characterizing the form error of a freeform surface due to its geometrical complexity. At present, there are generally two main approaches to inspecting the form deviation, the fitting and matching approaches, respectively. However, the fitting approach is inevitably affected by the 'error-eroded' measured data while the matching approach is susceptible to the 'swelling/squeezing' data caused by the measurement instrument. This paper presents a hybrid fitting and matching method (HFMM) which takes the advantages and addresses the shortcomings of both fitting and matching approaches. The HFMM has been verified by a series of measurement experiments. A sample of the freeform surface was measured and characterized by different techniques and the results were compared. It is found that the proposed method performs better than the fitting or matching approach. The successful development of the hybrid fitting and matching method significantly contributes to a potential solution for the standardization of the freeform surface characterization method.