A dual-scanning white-light interferometer for exact thickness measurement of a large-thickness glass plate

Conventional scanning white-light interferometers (SWLIs) cannot measure the exact shape of a thick glass plate because of the large dispersion effect. In this paper, a novel dual SWLI is proposed where the thicknesses of both an object and a compensation glass plate can be measured exactly in four-step measurements by decreasing the dispersion effect in both arms of the dual SWLI. The compensation glass plate is made of the same material as the object and has a similar thickness. The peak positions of the interference signals in the SWLI can be exactly measured using a piezoelectric transducer stage and an additional interference signal which detects the time-varying optical path difference in the SWLI. Experimental results show that the measurement error is less than 60 nm for a glass plate of thickness 1 mm. (paper)