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Surface contour measurement using an optical scanner

The use of aspheric and unconventional optical components has rapidly increased over the past few years. But the fabrication techniques of such components is limited by lack of suitable methods for assessment of surface shape of large aspheric optical components, especially for large convex surface. In this paper we discuss a 3D pencil beam profiler which is suitable for measuring optical aspheric surfaces, having different parameters and a large radius with interferometric accuracy without the need of standard surface an null lens. The principle of the profiler is based on a pencil beam optical scanner. The tested workpiece is put on a turnable table with a standard plane plate which is used to give the information of the tilt of the turbable table when the table is rotated for 3D contour measurement. The information will be applied for removing the error introduced during the table turning from the final map of the surface. The measurements are made along the meridian of the tested surface. The input slope data of the meridian are converted to Taylor series by least square fitting and to get the height map of the surface. After repeating above procedure on several lines on the tested surface, Zernick polynomials are employed to build up 3D map of the tested surface according to the data from the Taylor series and azimuth information of the turnable table. Examples of using this profiler to test optical components are given in the paper. The outcome is compared with the result given by the interferometer and the difference between two results is less than 0.05 wavelength.