Development of measurement techniques on the rough surfaces by optical profilometry using laser interferometer with wide field of view

Abstract

Aspheric lens molds require high shape accuracy. However, shape error of the aspheric lens mold in an ultraprecision cutting process affects the total shape accuracy of the mold in subsequent ultraprecision polishing process. To overcome this problem, we need to measure the shape errors of the molds just after the ultraprecision cutting process by three-dimensional profilometry, and feed them back to the ultraprecision polishing process. In a previous study, a scanning laser interferometer with wide field of view was developed by introducing a reference plate between a specimen surface and the laser microscope with wide field of view. Using this laser interferometer, we could acquire interference fringes of concave specimens that simulated the lens molds. And using the interference fringes as contour lines, the three-dimensional shapes of the concave specimens could be determined. However, when this method was applied to rough surfaces, it was very difficult to measure the surface shapes because a number of interference fringes appeared very densely with such rough surfaces. Thus, roughness of such surfaces should be reduced with the valleys of rough surfaces filled up with something soft. In this study, transparent film coated with a thin layer of metal and painting with markers were tried. We verified whether interference fringes on rough surfaces were acquired. As a result, the interference fringes could be observed with the both methods.