The Selection of Window in Spatial Phase Shift ESPI 2013-01-1420
Shearography is a laser based optical method that is similar to holographic interferometry and ESPI. It is a full-field, non-contacting and non-destructive measurement method for the surface deformation. It overcomes some of the disadvantages of holography; it does not need a reference beam, so that it obtains vibration isolation and simplifies the setup. These advantages grant shearography the ability to be a practical measurement tool and it has already gotten many industrial acceptances for non-destructive testing
The embedment of the phase shift technique improves dramatically the measuring sensitivity and accuracy of the shearography. It uses the piezoelectric as the carrier to generate a known phase gap and takes multiple images with the phase before and after the sample is loaded, so that the phase map is calculated. And for each pixel the phase is accurate. However, the disadvantage of the phase shift technique is the time consumption.
Spatial phase shift technique in shearography is a solution for the high speed dynamic analysis. It uses a spatial phase carrier to introduce a spatial frequency in the phase map. Then a Fourier Transform will be involved to separate the spectrum in the Fourier domain. And then a window serves as a band-pass filter to select the useful frequency part, and by using the Inverse Fourier Transform (IFT), the phase map can be extracted.
The spatial phase shift technique dramatically increases the speed of applying phase shift in shearography. The window applied in the Inverse Fourier Transform is particularly important. It is a filter in the Fourier domain. And after the IFT, the filter will also be transformed, and revolute to the original data. An improper window will bring unexpected signal into the resultant phase map, or include the information from other spectrum which will influence the accuracy of the measurement.
This paper discusses several kinds of windows that normally are used. In order to keep the characteristic of a high speed testing, the window should be simple in math and easy to apply. After experiment and theoretical analysis, a proper one is selected which is simple and effective. The experimental results and the theoretical analysis are both presented.