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

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