A new family of speckle interferometers

The present paper describes in details the operating principle of a completely new family of speckle interferometers. These interferometers are capable of detecting the same components of displacement given by holographic interferometry, that is the component along the bisector of the angle identified by the illumination and the observation directions, but with no need of an external reference beam. The working principle of this family of interferometers relies on the fact that only a portion of the illuminated area undergoes a sensible deformation. The implementation can be indifferently carried out by adopting a Michelson or a Mach-Zender configuration. In the paper a double focus interferometer based on the Michelson design was used to detect the out-of-plane displacements of an artificial debond in a metallic specimen.