Since intensity varies as , the corresponding phase difference for a fringe shift is . The equivalent optical path difference is . The interference fringes are ordered i.e. the fringe counted for example from the left will represent a phase difference of with respect to the reference wave. In distance measurements an integer number of fringe shifts is used and hence the resolution in distance measurement is .

In a given problem, individual portions of the light beam traverse distinct optical paths and the phase difference is a spatially distributed variable. In an interferometric measurement, this beam combined with one that has a constant phase. In regions where the phase difference between the two is the intensity of the combined beam is zero and we get destructive interference. When the phase difference is we get constructive interference. The corresponding path differences are and respectively. The phase field in the form of a fringe pattern can be recorded, say by a camera, to extract information about the primary variables of the problem being studied. In applications, quantitative measurements are possible if lines of constant phase exist so that fringes form, and a fringe shift can be identified from one fringe to another.