An integrated form characterization method for measuring ultra-precision freeform surfaces

Ultra-precision freeform surfaces are complex surfaces that possess non-rotational symmetry. Currently, there is a lack of definitive international standards and techniques for the characterization of form accuracy and surface quality of ultra-precision freeform surfaces. This paper presents an integrated form characterization method (IFCM) to measure the form accuracy of ultra-precision freeform surfaces. The IFCM is developed based on the five-point pre-fixture, dual cubic B-spline and an iterative precision adjustment algorithm with coordinate transfer. To verify the capability of the proposed method, a series of computer simulation and measurement experiments were undertaken. The results indicate that the IFCM can realize the precise alignment of measured and normal surfaces with accuracy in nanometer range which provides adequate base for good form characterization of ultra-precision freeform surfaces with form accuracy down to below sub-micrometer range.