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Chapter 3:
A Brief Review of Digital Shearography
As illustrated in Chapter 2, photographic shearography utilizes photographic film to record shearograms and can only record the sum of two intensities before and after loading (IþI0) using the double-exposure method. However, the term (IþI0) is usually not directly visible to human eyes. A complicated Fourier filtering procedure and a fringe readout procedure are needed to make the fringes visible, as was illustrated in Chapter 2. This situation has been greatly improved due to introduction of the digital CCD camera to shearography metrology. The second generation of shearography, digital shearography, uses a CCD camera to record the intensity of shearograms. The recorded intensity is digitized and stored in a computer. It is easy to calculate the intensity subtraction before and after loading |I0 – I|. As opposed to the double-exposure method used in photographic shearography (it calculates the sum of intensity before and after loading), digital/electronic shearography calculates the intensity subtraction before and after loading |I0 – I|. By calculating the term |I0 – I|, a highly visible fringe pattern can be directly generated and can even be observed in real time from a computer screen. Thus, since the invention of digital shearography, the complicated Fourier filtering and fringe readout procedures have no longer been needed or used. The upgrading of digital shearography from photographic shearography brings a significant improvement in the measurement speed of shearographic testing.
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