Fractional fringe order method using fourier analysis for absolute measurement of block gauge thickness

A fringe analysis method, which combines the Fourier transform with the fra ctional fringe order method for measuring the absolute thickness of a block gauge, is presented. An approximate integer part of the fringe order is es timated by mechanical measurements, and the fractional part is determined b y Fourier analysis of the interferometric fringe patterns. The fringe patte rns are obtained with a Michelson interferometer by illumination of several selected wavelengths, respectively. The use of the fractional fringe order method can determine accurately more than 2 pi phase jumps. The measured t hickness of a block gauge is 5999115.0 +/- 0.7 nm, which is comparable with the standard thickness of 6 mm. The influence of wavelength and approximat e integer part of fringe order on the measurement accuracy is discussed.