Since the second industrial revolution, industry has been seeking more efficient measuring and testing methods with the aim of reaching higher production quality and reliability. With the invention of the first functioning laser in 1960, a modern evolution of optical measurement methods began. Laser-based modern optical measurement methods have the advantages of extremely high sensitivity and noncontact, which make them ideal for use in nondestructive, online, and full-field testing in industrial applications. Shearography, also called speckle pattern shearing interferometry, is a measurement technique based on laser speckle pattern interferometry. It measures the gradient of the deformation. In order to provide a better understanding of the digital shearography technique, this chapter gives a brief review of basic optics principles by reviewing the properties of lasers, wave equations, and the speckle pattern phenomenon.
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