Chapter 11
Speckle Methods
IntroductionSpeckle methods are high-sensitivity non-contact optical methods for measuring displacements. They are based on the speckle effect. Speckles are granular dots that result from the illumination of a diffusively reflecting rough surface with coherent light. The reflected or scattered wavelets interfere to create a random speckle pattern with statistical properties. The laser speckle is due to the coherence of the light and the roughness of the surface of the order of the wavelength of light.Even though the earliest observations about the speckle effect were made by Newton towards the end of the seventeenth century the speckle effect first came to prominence with the invention of the laser. It was first regarded purely as a nuisance. Images of objects illuminated by laser were covered with a grainy structure. It was later realized that the speckles can be studied for their own sake and became the basis of a series of optical methods in experimental mechanics grouped under the name "speckle methods".Speckles can be used as "fingerprints" to impose a texture on a surface. For displacement measurement, two exposures, one for the specimen undeformed and a second for the specimen deformed are superimposed. The method of speckle photography is based on this principle. The sensitivity and range of speckle photography can be increased by adding a reference beam resulting in the method known as speckle interferometry. With the implementation of video and digital techniques a method known as electronic speckle pattern interferometry(ESPI) was developed. For the measurement of first derivatives of surface deformations a full-field, non-contact high-sensitivity interferometric method named shearography was developed. The interference is generated by two identical laterally sheared object beams, and no reference beam is required.In the following, we present the speckle effect and the methods of speckle photography, speckle interferometry, shearography, and electronic speckle pattern interferometry (ESPI) for the measurement of displacements.
