Optical alignment using Fourier imaging phenomenon and Moire technique

Optical alignment operations and Fourier imaging phenomena have the common property that both occur in space along a straight line. The self-imaging property of a circular grating is applied to centring-error detection and to alignment in space. Moiré fringes due to the interaction of Fourier (or Fresnel) images with a detecting grating provide an indicator of the alignment state. The merit of this approach is that it is easy and comfortable to make visual observations. Various applications of the method depending on the choice of the reference points used are discussed and some experimental data are presented.