Algorithm for Measuring the Geometric Parameters of Optical Freeform Surfaces Using a Coordinate Measuring Machine

Abstract

Nowadays use of freeform surfaces in optical systems for various purposes – from mobile displays to space optics – becomes increasingly popular. This design solution makes it possible to create a system with a minimum number of components, low weight, and high image quality. However, as the surfaces become more complex, problems arise with their control. Aim of the study was to develop an algorithm for inspecting such surfaces using a coordinate measuring machine and to formulate technical requirements for the measurement process. An analysis of the advantages of freeform optical surfaces is presented. It is noted that due to lack of symmetry and variable profile height, classical measurement methods are not applicable for such parts. The inspection algorithm using a coordinate measuring machine involves two main approaches: high-performance "fast scanning" to assess overall deviations from the nominal value, and more accurate but time-consuming sectional scanning which allows obtaining detailed quantitative data on the surface profile. The paper is mainly focused to the measurement algorithm and process requirements: preliminary temperature adaptation of equipment, mandatory calibration of probes using a ceramic reference ball, and software-controlled limitation of tool movements to protect polished surfaces. The proposed algorithm, based on the use of CAD-models, makes it possible to achieve the required accuracy of monitoring complex surfaces on standard shop‑floor equipment, despite significant time costs compared to aspherical surfaces’ measuring.