(color online) Different methods to operate the interferometer: (a) recording full fringes, and (b) performing measurements at mid-fringe. In the fringe-scanning method, the phase is scanned step by step, while in the fringe-locking method, the phase is modulated by ±π/2 with respect to an appropriate center denoted as φ⁰ here. The dashed lines are here to guide the eyes.