Laser Speckle Photography: Preprocessing of Fringe Pattern Data to Improve Dynamic Range

ABSRACT

Laser Speckle Photography (LSP), a technique used for the measurements of in-plane displacement and surface tilt, is now well documented in the scientific literature. A two-dimensional, instantaneous displacement map of the object under examination is produced by double exposure photography under coherent illumination where the object motion of interest has occurred between exposures. The processed film is then pointwise analyzed by directing a narrow beam of laser light through the transparency. The resulting far field diffraction pattern consists of Young's fringes whose spacing and orientation are measured to find the magnitude and direction of the object displacement within the illuminated region of the transparency.

In order to optimize the dynamic range of this technique, careful consideration must be given to the method of fringe pattern processing, particularly when the fringe spacing begins to approach the total width of the diffraction pattern, i.e. when only a few fringes are visible. In this paper, a method of improving measurement accuracy at low fringe densities is presented which employs a theoretical model for the intensity distribution in the far field diffraction pattern. It is shown that, in addition to an improvement in the dynamic range of LSP, the technique also provides a direct measure of the signal-to-noise ratio (i.e. measurement accuracy) in terms of fringe visibility.