A quantitative knife-edge testing method for local deformation evaluation in optical aspheric fabrication

This paper presents a shadow measuring method applicable to quantitatively assessing the local deformation of aspherical mirrors. The method calls for placing an image capture setup behind the traditional knife-edge testing device to record the resultant shadow maps. Local deformation of the surface of the work-piece under testing are then extracted from illumination level changes of the shadow maps via Fourier transformation. The shape of the work-piece can be constructed from the extracted deformation data. Experimental studies showed that performance achieved with the present method agrees well with that using Zygo interferometer for a polished aspherical surface on low frequency deformation. The proposed approach serves to expand the usefulness of the knife-edge method into quantitative testing, enabling its applications to the manufacturing of aspherical mirrors.