Speckle Methods in Experimental Mechanics

Speckle techniques are a class of whole-field methods that offer sensitivities ranging between those obtainable by moire methods and those of hologram interferometry. Additional advantages accrue, and hence these techniques have found a number of applications in experimental mechanics. A diffuse object illuminated by a coherent beam of radiation produces a grainy structure in space. This grainy structure is called the speckle pattern. The grains (speckles) in the pattern undergo changes in both position and irradiance when the generating surface undergoes changes. Therefore speckle techniques can be basically grouped under two heads: (1) those based on the positional shift of the speckles, and (2) those based on the irradiance changes of the speckles [1]. Speckle photography includes all those methods where positional changes of speckles are monitored, Whereas speckle interferometry includes methods that are based on the measurement of phase changes and hence irradiance changes. If instead of phase change we measure its gradient, the technique is one of speckle shear interferometry. Both speckle interferometry and speckle shear interferometry can be performed using electronic detection, and the technique is broadly known as electronic speckle pattern interferometry (ESPI) [2].