Measuring micro-displacements of specular surfaces using speckle interferometry

The displacement field of an object surface can be measured by using speckle interferometry. This technique is based on the phenomenon of laser speckle and consists in correlating speckle interferograms taken after and before the deformation of the surface. The main requirement is that the surface under study must be optically rough to generate the speckle patterns to be correlated. In this paper, we present a very simple and intuitive method based on speckle interferometry to measure out-of-plane displacements on specular reflecting surfaces that generate no speckle patterns. The method consists in a modified digital speckle pattern interferometer which requires no special equipment other than that used in conventional speckle interferometers. The proposed method could find useful application in the measurement of thermal deformation of mirrors.