Multiwavelength Digital Holography for Polishing Tool Shape Measurement

Classical mechano-chemical polishing is still a valuable technique, which gives unbeatable results for some types of optical surfaces. For example, optics for high power lasers requires minimized subsurface damage, very high cosmetic quality, and low mid spatial frequency error. One can hardly achieve this with use of subaperture polishing. The shape of the polishing tool plays a crucial role in achieving the required form of the optical surface. Often the shape of the polishing tool or pad is not known precisely enough during the manufacturing process. The tool shape is usually premachined and later is changed during the polishing procedure. An experienced worker could estimate the shape of the tool indirectly from the shape of the polished element, and that is why he can achieve the required shape in few reasonably long iterative steps. Therefore the lack of the exact tool shape knowledge is tolerated. Sometimes, this indirect method is not feasible even if small parts are considered. Moreover, if processes on machines like planetary (continuous) polishers are considered, the incorrect shape of the polishing pad could extend the polishing times extremely. Every iteration step takes hours. Even worse, polished piece could be wasted if the pad has a poor shape. The ability of the tool shape determination would be very valuable in those types of lengthy processes. It was our primary motivation to develop a contactless measurement method for large diffusive surfaces and demonstrate its usability. The proposed method is based on application of multiwavelength digital holographic interferometry with phase shift.