OPTIMIZATION OF WAVE-FIELD INTENSITIES IN PHASE-SHIFTING SPECKLE INTERFEROMETRY

Speckle interferometry relies on the interference of either one speckle field and a smooth reference wave or two speckle fields. In both cases the background intensity and the modulation of the resulting interference pattern are different for each point of the image. Phase-shifting techniques provide bad results for pixels that either saturate or show low modulation. The effect of the wavefield intensities on the reliability of phase measurements is investigated for phase-shifting speckle interferometry using a speckle reference wave. The first-order statistics of the background intensity and the modulation of the interference pattern are calculated and then used to determine the efficiency of the measurement, defined as the fraction of points that yield an interference signal exceeding a given modulation threshold without saturating. The intensity levels optimizing the efficiency, as well as the efficiency itself are calculated and compared to the case of smooth reference beam interferometers. It appears that the optimum efficiency is achieved with intensities being relatively low compared to the saturation level.