Geometric parameters of moiré fringes.

The incoherent illumination moiré phenomenon between a periodic and a quasiperiodic structure is examined by the use of the spatial frequency analysis of the resultant transmittance. This approach has an advantage in that it completely explains all the fringe parameters without considering the averaging effect of the detecting system. We obtain the basic equation of the original moiré pattern due to the beat phenomenon exclusively, without any external factors like, for example, the influence of the observing system. From the basic equation, we derive the fringe profile equation representing the moiré fringe shape and the fringe equation expressing the spacing, the orientation, and the local displacement of the moiré fringes. The fringe profile equation is experimentally verified. The sensitivity enhancement by the fringe multiplication method is readily interpreted by the use of the derived fringe equation.