Shape measurement of a thin glass plate through analyzing dispersion effects in a white-light scanning interferometer

•Phase position Zp of π nearest to a maximum amplitude position Za in a complex-valued interference signal (CVIS) is analyzed for measuring thickness T of thin glass plates by simulations.•Coefficients that relate the Za and Zp values to the physical thickness T are obtained.•Periodical position jumps occurred in Zp are made clear, and the number of occurrences of the jump is calculated from the Za and Zp values.•In experiments, a shape of 20 μm-thickness glass plate is measured exactly with an accuracy of 6 nm from the Za and Zp values.