The present paper describes in details the operating principle of a completely new family of speckle interferometers: the double-focusing. This type of interferometer is sensitive to the same components of displacement given by holographic interferometry, i.e. the component along the bisector of the angle identified by the illumination and the observation directions. In addition, no external reference beam is necessary, with a consequent reduction of the complexity of the experimental setup. The only requirement for the correct functioning of this family of interferometers is that only a portion of the illuminated area undergoes a sensible deformation. The implementation can be indifferently carried out by adopting the classical Michelson or Mach-Zender configurations, but also a particularly compact in-line implementation can be realized.
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