Effect of magnification change in laser speckle photography

Abstract

When double-exposure speckle photography is used for stress analysis of solids, displacement along the line of sight arises naturally as a consequence of specimen deformation such as Poisson’s effect, etc. This displacement causes magnification change of the speckle pattern recorded. In the this paper a model is proposed to analyze this effect. It is shown that, upon Fourier filtering of the double-exposed speckle pattern with small magnification change, contour fringes of equivalent in-plane displacement along the direction of filtering are obtained and that the fringe density is linearly proportional to the magnification change incurred. The phenomenon is quite analogous to the moiré fringes created when identical gratings are superimposed with a gap between them.