In this technique, a Mach-Zehnder type of interferometer was used for the measurement ofthe fringes using ESPI. A coherent light beam of wavelength λ, which travels along with a distance (d) at time t1, was used. This beam then interfered with the reference beam. The process was then repeated at another time t2. Both the images which were obtained are subtracted from each other. Bright fringes were obtained when fringe order relationships were satisfied, leading to constructive interference .