Speckle patterns for measuring displacements described as diffraction gratings

When an object is illuminated with a laser a speckle pattern is produced. If the speckle pattern is photographed and then the model loaded and the resulting speckle pattern superimposed on the first photograph, the double exposed photograph contains information about the displacement of the model under load. This information can be seen by shinning a laser beam through a small area of the negative and observing an interference pattern of a screen. An explanation for this effort is presented in this paper. The explanation is reasonably simple and gives a basis for analysis as well as limitations of the technique.