Profile evaluation of radial Fresnel lens directly machined on roller molds by rotating-tool diamond turning

In Roll-to-Roll manufacturing of high-quality optical Fresnel lens films, a high-precision roller mold with super-fine surface quality is essential to precisely transfer the functional microstructures from the periphery roller surface onto the flexible substrate. Unfortunately, direct diamond turning of deep circular grooves on the periphery surface of a roller mold was considered infeasible. Recently, the team has developed a novel 4-axis interactive tool-workpiece motion, Rotating-tool diamond turning (RDT), as a solution to overcome this challenge. Experiments were conducted to justify the capability of the proposed RDT process by directly machining a radial Fresnel lens on a brass roller mold, but without precise 3D profile evaluation of the lens on the roller surface. On-machine measurement of the machined lens structures using 3D touch probe is not applicable because the diameter of the probe is relatively large to penetrate into steep grooves of the Fresnel lens. On the other hand, off-machine measurement using stylus profilometer will introduce inevitable alignment errors during the measurement and lead to mismatched machining and measurement coordinates, making it difficult to evaluate the 3D lens profile generated by the RDT process eventually. In this study, a compensation and comparison algorithm is presented to precisely evaluate the form error between the machined and designed features in a three dimensional manner. Alignment errors generated when positioning the roller mold on the stylus profilometer are investigated and quantified through analyzing the characteristics of this unique micro structure with Fresnel lens wrapped on the roller periphery. As a conclusion, the machined lens structure is compensated and restored to compare with the designed profile, and the form error is obtained with the sources of errors analyzed. Such profile compensation and comparison method can be applied in other measurement and characterization studies on evaluation of complex optical structures patterned on roller molds for Roll-to-Roll manufacturing of advanced functional films.