Moire fringes are obtained when two gratings of about same frequency are superposed on each other. Normally the two gratings are of special geometrical curves. For the present particular application, one grating is assumed to be a grating of constant but unknown frequency. The other grating which is the measuring grating consists of different types of curves. The theory of the resulting pattern and how it is used to measure the frequency is presented.
M. V. R K. Murty,
R. P. Shukla,
"Estimation and Measurement of the Frequency of Coarse Gratings by Moire Fringe Techniques," Optical Engineering 11(1), 110120 (1 February 1972). https://doi.org/10.1117/12.7971594