FILM THICKNESS MEASURING METHOD AND SHEET MANUFACTURING METHOD

摘要

PROBLEM TO BE SOLVED: To provide a film thickness measuring method by optical interference that can execute measurement with high accuracy even when a measured object has double refraction and an interference spectral spectrum is distorted. SOLUTION: An optical spectrum at a measuring point on a sheet is measured and a wavelength that gives an extreme value of this optical spectrum is then determined. An estimated film thickness is determined and an interference order of the wavelength that gives the extreme value of the optical spectrum is then determined. A film thickness of a sheet is calculated based on the wavelength that gives the extreme value and the interference order.