Holographic interferometry: compensation for rigid body motion.

A technique producing an optical effect that is unaffected by rigid body motion is described. It is shown that the resulting interference fringe pattern is proportional to the change in model thickness, which for the case of plane stress is proportional to the sum of the principal stresses, and thus provides a full field isopachic fringe pattern. This method combines holographic interferometry and the moiré effect. Double exposure holograms are made from both sides of a model before and after deformation. The two sets of holographic interferometric fringes thus obtained are treated as random grids, and the superposition of these two grids produces a moiré effect that is free from the effects due to rigid body motion. The theory is experimentally confirmed with several examples, and limitations of the techniques are also discussed.