Abstract
It is desirable in the moiré topographic method as well as in the ordinary moire method that the fringe patterns with narrow fringe widths can be obtained, so that a high accuracy in the measurement of interfringe-spacings is secured. In the case of the ordinary moiré method, it is known that the use of grids with different widths between bright and dark parts is an effective means for this purpose. In this paper, an approximate theory for the sharpening of fringe widths for the grid illuminating method, which is a typical moiré topographic method, is derived from that proposed previously for the ordinary moiré method. Besides, an another modified theory of that proposed by S. Yokozeki et al. is presented. In case that the grids with considerably large pitches are used, it is verified that the authors' theory may serve as a criterion for estimating the fringe widths.
