(color online) Different methods to operate the interferometer, with (a) for recording full fringes and (b) for performing measurements at the midfringe. In the conventional method, the phase is scanned step by step, while in the FLM, the phase is modulated by with respect to an appropriate center, denoted as φ⁰ here. The dash lines are given here to guide the eyes. The two thick green lines are the tangent lines of the midfringe, which show good approximation for the fringe near the center.