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Thickness measurement of transparent plate based on wavelength-tuning interferometry

Two methods that used to measure the thickness of transparent plate were introduced in this paper. In conventional interferometry, multiple-surface interference fringes and the nonzero incidence angle complicated the calculation. In this study, the absolute thickness was obtained from the interferogram generated by two beams of coherent light reflected from the front surface and the rear surface of the transparent plate without reference surface. The thickness was obtained by analyzing light intensity of each point in the interferogram which varies sinusoidally with the wavelength in period method. While slope method figured out the mean thickness of whole surface with phase-shifting algorithm and least-square fitting algorithm. When the beam was perpendicular to the surface, the absolute thickness can be figured out with the refractive index n and λ1, λ2 in first technique. The results of simulation experiments indicated that the error in first technique was with sub micrometer, however the error of slope method was only a few nanometers.