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Chapter 3:
Fringe Deciphering Techniques Applied to Analog Holographic Interferometry
Different types of interferometers can be used to produce a fringe pattern phase modulated by the physical quantity being measured. Several physical quantities can be measured with interferometric fringe techniques, such as strain and stress analysis, temperature deformation and gradient, surface deformation, and many others. The ultimate goal of fringe pattern analysis is to decipher the underlying phase profile that encodes information about the physical parameter being measured. This chapter reviews many techniques often used in fringe analysis that are also useful in holographic interferometry analysis. Interferograms can be divided into two general categories: (a) those containing a spatial carrier in the interferometric pattern, typically introduced through a tilt in the reference wavefront; and (b) those in which no spatial carrier exists, which will produce additional difficulties in automatically deciphering the interferograms. This chapter starts by explaining the frequency-domain fringe deciphering techniques that assume a spatial carrier exists. In Section 3.3, the concept of fringe orientation and direction is applied to fringe deciphering. In Section 3.4, phase demodulation using the Hilbert transform is discussed in detail, including its relation to fringe orientation and direction. In Section 3.5, the concept of fringe skeletonization and normalization is applied to fringe processing; Section 3.6 introduces fringe contrast enhancement. Finally, a brief discussion of phase unwrapping for interferogram analysis is discussed in Section 3.7.
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