Shape Measurement Of The Optical Freeform Surface With Wavefront Sensing Technology

Optical freeform surface provides a new design concept which can simplify thesystem structure and improve the optical performance. Therefore, it has been moreand more widely used in the aerospace, communications and other fields. However,the increasing ultra-precision requirements of the freeform surface are found difficultto manufacture, which also makes developing a measuring method of the morphologyand surface error of the freeform becomes a core issue. Most of the existing methodsare contact mode for shape measurement of optical freeform surface. Even though itcan get a high accuracy, the measurement efficiency is low and causes some damageto the work-piece. The optical non-contact measurement method shows moreadvantages in these aspects, which adapts to the development of the ultra-precisionmeasurement in the future. Compared with the interferometer, the wavefront sensorhas a simpler structure, a smaller size, a lower cost and stronger real-timecharacteristics. In this article, an optical freeform surface measurement system basedon the Shack-Hartmann wavefront sensor (SHWS) was designed and developed. Themain research works are as follows:1. The dynamic range of the SHWS is limited by the Micro-lens Array (MLA)，the lens size, the mesh and the spot position. According to the traditional method ofcentroid extraction of averaging the pixel intensity, a centroid calculation method withthe help of connected domain was proposed based on the image processingtechnology to meet the measurement requirements of surface with larger curvature.The centroid coordinates of any spot can be obtained accurately and the dynamicrange of the measurement can be improved for4times or above through rearrangingthe spots.2. To meet the surface measurement requirement, the surface information shouldbe deduced from the reflected wavefront. A mapping modal has been established fromthe reflected wavefront data to the surface shape information according to the raytracing and the optical path difference principle, which can ensure the accuracy of themeasurement system2-3times better than the traditional one.3. As an indispensable component in the reflected surface measurement system,the beam splitting prism can bring distortion to the wavefront and even the system. Therefore, an error model has been built through the light propagation theory. Theproposed compensation scheme has been validated to be effective through simulation.At the same time, the physical limits of the system have been analyzed anddetermined through simulation and actual experiments, the wavefront reconstructionprogram has been completed, and the alignment of the optical system and other keyissues have been discussed in detail. Finally, an experimental system combined withthe simulation platform was built to measure the surface of the cubic phase plate,which is a typical optical freeform surface. The experimental results compared withthe data achieved by Veeco white light interferometer have verified the correctness ofthe system.