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Since the advent of the laser, the rapid development of new interferometric techniques such as holography and shearography has brought precision measurement and non- destructive testing to a new dimension. Many novel variations of these techniques are developed and used, but in general, their working principles are based on the comparison between two optical gratings - the known reference grating and the object grating that is distorted by the test surface. Comparison of the two gratings will result in the formation of a fringe pattern that depicts lines of equal spatial coordinates and surface displacements (both in-plane and out-of-plane), and lines of equal surface slopes and surface displacement gradients. Conventionally, optical interferometry is developed using high-resolution films but with the rapid advancement of computer and image- processing technology, these film-based techniques have given way to digital techniques. While the theories of both film-based and digital techniques are now well developed, students, however, often find it difficult to understand how the fringe pattern is formed and reconstructed, especially when the shift from the film based to the digital techniques. In this paper, the formation of the optical gratings (that is, the object grating and the reference grating) is explained in the light of the well-known Young's interference fringes. The formation and reconstruction of the visible fringe pattern is explained in the light of the well-known moire phenomenon caused by the interference of the two optical gratings. In the film-based techniques, the visible fringe pattern is explained with reference to image- addition(that is, addition of two optical gratings) and subsequent image-multiplication (that is, multiplication of two optical gratings). In the digital techniques, the fringe pattern is reconstructed using image-addition and subsequent image-subtraction.
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