**4. Fast phase-shifting interferometry**

Phase shifting is an attractive and very robust technique for the analysis of fringe patterns. Since PS takes multiple images over a finite time period, it is sensitive to the time-dependent phase shifts due to vibrations. These vibrations are difficult to correct since the optimum algorithm depends on the frequency and the phase of the vibration. For a given vibration amplitude, the phase error is a function of the ratio of the vibration frequency relative to the frame capture rate. Several methods have been used to try to get around the vibration problem in phase-shifting interferometry (PSI). One of those methods is speeding up the data collection process by capturing all frames simultaneously to be in single shot. Single-shot algorithm is reliable, fast, and less sensitive to vibration and turbulence in surface micro-topography measurement. In this section, we review one technique for fast phase-shifting-based singleshot parallel four step combined with Fizeau interferometer.
