Dual plate speckle photography

A method for measuring in-plane displacement using two laser speckle photographs is described. The technique allows the user to cancel rigid body translation and rotation. This increases the measurement accuracy of relative displacements associated with load induced strains. The technique involves making separate speckle photographs of a test specimen. One is made with no load; the second is made with the specimen loaded (strained). A sandwich is constructed from the two speckle photographs, and data are recovered in a manner similar to that used with conventional speckle photography. However, the fringes generated during data recovery are not strictly Young's fringes. Therefore a curvature of various degrees is encountered. A method of interpreting the fringe pattern is described.