An important task in the sampling moiré method is phase measurement of a grating image by using the phase of the moiré fringe from a single-shot image. In the method, by thinning out the image, one gets 𝑁 phase-shifted moiré fringes, where 𝑁 is called the thinning-out index. In the conventional treatment, they are considered as phase-shifted patterns with shifts equal to multiples of 2𝜋∕𝑁. This is not an exact supposition. It is shown in this work that the exact magnitude of the phase shift is 2𝜋∕𝑇 , where 𝑇 is the period of the grating image. According to this fact, the formalism of the technique is modified. The simulation results verify the correctness of the theoretical results.
