MILLISECOND PHASE ACQUISITION AT VIDEO RATES

We can measure the phase of a static wave front with great accuracy and resolution by using standard phase-shifting interferometry techniques. Since wave-front changes during or between the acquisition of the individual interferograms cause measurement error, the measurement of transient phenomena still requires an essentially static wave front during acquisition. The 2 + 1 phase-shifting algorithm, originally developed to freeze out vibration when testing large optics, permits fast acquisition by requiring only two time-critical interferograms. The third interferogram provides a dc level or null and is only required once. The current 2 + 1 implementation is a low-cost interferometric system that is capable of phase acquisition in < 2 ms at video rates. The repetition rate is limited by the readout time of the detector. The 2 + 1 has produced map movies of turbulent air acquired at the 30-Hz frame rate of a CCD camera.