Double-exposure speckle photography for the measurement of small displacements

The method of double-exposure speckle photography (DESP) has been illustrated in experiments to determine the small displacements of a solid object. The object was illuminated by laser light and the speckle pattern was photographed on a stationary photographic film twice, once before and once after the object was displaced. The doubly exposed film, after being photographically processed, was illuminated by laser light to produce equally spaced Young's interference fringes on the Fraunhofer diffraction plane. From the spacing and the orientation of these fringes, both the magnitude and the direction of the displacement were determined. We have used the DESP method to measure the small length change of a cadmium tube due to thermal expansion and determined the linear thermal expansion coefficient of cadmium.