ABSTRACT
The speckle phenomenon came into prominence after the advent of laser. When an optically rough surface is illuminated with a coherent beam, a high-contrast granular structure, known as speckle pattern, is formed in space. Speckle pattern is a three-dimensional (3D) interference pattern formed by the interference of secondary, dephased wavelets scattered by an optical rough surface or transmitted through a scattered medium that imposes random phases of the wave. The speckle pattern formed in the space is known as the objective speckle pattern. It can also be observed at the image plane of a lens and it is then referred as subjective speckle pattern. It is assumed that the scattering regions are statistically independent and uniformly distributed between −π and π. The pioneering contributions of Leendertz [1] and Archbold et al. [2] in 1970 demonstrating that the speckles in the pattern undergo both positional and intensity changes when the object is deformed, triggered the origin of speckle work. The randomly coded pattern that carries the information about the object deformation provided to develop a wide range of methods, which can be classifi ed into three broad categories: speckle photography, speckle interferometry, and speckle shear interferometry. Speckle photography includes all those techniques where positional changes of the speckle are monitored, whereas speckle interferometry includes methods that are based on the measurement of phase changes and hence intensity changes. If instead of phase change, we measure its gradient, the technique falls into the category of speckle shear interferometry. All these methods can be performed using digital/electronic detection using a charge-coupled device (CCD) and image processing system. The attraction to speckle methods relies their applicability for optical metrology: (1) static
and dynamic deformations and their gradient measurements, (2) shape of the 3D objects, (3) surface roughness, and (4) nondestructive evaluation (NDE) of engineering and biomedical specimens. The range of the objects that can be evaluated using the speckle methods is of the order of few hundred microns such as micro-electro-mechanical systems (MEMS) to few meters such as space craft engineering structures. The prominent advantages of the speckle methods with the real-time operation systems (photorefractive crystals, CCD, and image processing systems) are (1) noncontact measurement, (2) real-time whole fi eld operation, (3) data storage and retrieval for analysis, (4) quantitative evaluation using phase shifting, (5) variable measuring sensitivity, and also (6) possibility to reach remote areas.
