ESPI with Spatial Phase Shifting
ESPI with Spatial Phase Shifting
Electronic speckle pattern interferometry (ESPI) in combination with the phase shifting technique has become a practical interferometric deformation measurement method. The widespread "Temporal Phase Shifting" (TPS) has the disadvantage that the phase-shifted interferograms have to be recorded one after the other. If the measurement setup is disturbed thermally or mechanically during this period or if the measurement object changes, the calculated object deformation can deviate considerably from the actual value.
One way to record the phase-shifted interferograms simultaneously and thus exclude time-dependent interference is to use "Spatial Phase Shifting" (SPS). This requires a modification of the conventional speckle interferometer. A lateral offset of the reference wave relative to the object wave in one direction leads to a linear relative phase shift of the wave fronts at the location of the sensor. In each column of the sensor, the phase of the reference wave is therefore shifted relative to the neighbouring column by an amount dependent on the shift. The phase position of the object wave in the centre pixel can be calculated from the image information of at least three neighbouring pixels. 
The image shows two phase distributions with a time interval of 20 ms, which appear interlaced due to the interlaced operation of a CCD camera. The measurement was carried out on a fluctuating phase object with a spatially phase-shifting Michelson interferometer. The use of the TPS would have required at least three recordings at intervals of 20 ms each. With the existing phase fluctuations, it would therefore not have been possible to obtain any useful results with this technique. For further information see publications.