Phase stepping algorithms are mostly based on three or more interferograms that can either be acquired sequentially, involving some temporal phase stepping mechanism, or in parallel. When a phase step is applied between acquisitions, object phase changes may cause phase errors when calculating phase. A new system that allows the object to change in between arbitrary temporal phase steps is proposed. It comprises a relatively simple polarization based two-channel speckle interferometer that acquires two π/2 phase stepped interferograms simultaneously with a single camera. This quadrature pair is phase stepped with a temporal phase stepping system that is also polarization based. Simulations and experimental results are presented that illustrate the improvements achieved with quadrature phase stepping compared to results obtained with the two-channel speckle interferometer operated without additional temporal phase stepping.
A polarization phase stepping method is presented based on the use of a polarization plane rotator that establishes a relative phase shift between two counter-rotating circularly polarized beams. The phase step can be made relatively accurate, since it just depends on the accuracy with which the rotator is manufactured, and not on its orientation. The phase stepping method has been implemented in a single-camera two-channel shearing speckle interferometer, with two optical channels, and a relative phase step of π/2 between them.
It is generally acknowledged that it takes at least three phase-stepped speckle patterns to obtain quantitative phase information. However, if only a phase change has to be determined, a two-bucket approach can be followed, under certain conditions. The background of the two-bucket algorithm, and the requirements with respect to the use of it, are explained. Examples of modeled and measured phase stepped speckle intensity data, showing numerical instability when conditions are not met are presented. It is also shown that two phase-stepped speckle pattern pairs, taken before and after an event can be sufficient to determine phase changes invoked by the event.
The implementation of temporal phase unwrapping within a real- time phase stepped shearing speckle interferometer is presented. Speckle phase maps obtained with a shearing speckle interferometer, representing an object before and after deformation, reveal sub-surface defects or damage, after subtracting the images. Phase information is only known modulo 2(pi) , and has to be unwrapped for a true representation of the deformation map. Results are easier to interpret during the deformation process when the images are unwrapped; phase unwrapping also facilitates automatic detection of suspected areas. It is shown that, compared to the more common spatial* phase unwrapping methods, temporal phase unwrapping is much faster, and can be implemented in a real-time system. In addition, this method offers an increased measuring range, reduces sensitivity for speckle decorrelation, handles discontinuities in the object, and is very reliable, even when used with noisy data. Processing strategies for the selective removal of unwanted image components, and for automatic defect detection are presented. Examples of results obtained for artificial defects in metallic and composite aeronautical components are shown. Measurements have been carried out with our phase stepped shearography system under laboratory and industrial conditions, showing improved performance under non- ideal conditions. It has been shown during these experiments that shearography cannot only be used for detection of defects, but also for characterization.
A simple scheme is presented that enables the measurement of phase changes by recording only two phase stepped images per time frame. Such a scheme is useful for the detection of defects or to monitor vibration characteristics in constructions. The differences between the here presented method and the four recordings per time frame method are discussed. Finally some experimental results are shown.