Measurement of three-dimensional displacements by scanning a double-exposure hologram.

Measurement of displacements and strains is performed by extracting with a new scanning method the information from a double-exposure hologram. A real image is formed by a conjugate reference beam that illuminates a small part of the plate defining an observation direction for each point of the object under test. Sweeping the beam across the plate, which corresponds to a variation of the viewing direction, produces a movement of the fringes on the surface of the observed object. The fringes are counted during each scanning by a detector placed at the observed point. A minimum of three scannings provides all the information required to determine the three-dimensional displacement of the point itself.