Gauge block parameter measurement through static interferogram using virtual grating moire technique

Abstract

Gauge block parameters are usually measured using Koester interferometer or Twyman interferometer. Conventionally the fraction part of central length of the gauge block within one wavelength may be obtained from static interferogram by fringe analysis. Advanced algorithms should be applied to locate the fringe centers precisely. With phase shifting interferometry (PSI), wavefront reflected from the surfaces of gauge block and its base can be retrieved from several interferograms with a definite phase shift between the adjacent interferograms. Additional hardware, such as PZT phase shifter, should be used to introduce phase shift. In this paper, the gauge block parameters are obtained from the static interferogram with a certain spatial frequency introduced by tilting the reference mirror of the interferometer. The spatial frequency of the actual interferogram can be calculated by FFT algorithm. Then a virtual sinusoidal grating with this spatial frequency is generated in the computer and overlapped on the interferogram to get a moire pattern. By moving the grating 1/4 of the grating period, 4 moire patterns with π/2 phase shift between adjacent patterns are obtained. After removing the high frequency fringes in the above four moire patterns with low pass filtering, four interferograms with accurate π/2 phase shift between the adjacent ones are remained. Then the wavefront is retrieved with 4-bucket algorithm. In this method, the static interferogram is analyzed with PSI algorithm but there is no need to have phase shifter. Besides, the phase shift is introduced mathematically to avoid the associated error. The actual interferograms are processed and results are obtained.