Automatic analysis of speckle photographs with extended range and improved accuracy

Limits and possibilities of laser speckle and white-light image-correlation methods: theory and experiments

A laser-speckle method and a white-light image-correlation method are used for strain mapping. A schematic model of the correlation function of two speckle patterns is proposed for investigation of strain influence on displacement-measurement accuracy. A specific software has been developed to calculate by direct correlation the displacement values between two pictures with a grainy pattern at any point on an object's surface. Its efficiency is demonstrated in several tests. Moreover, theoretical results are checked through experimental measurements. The limitations and performances of both optical techniques are discussed.

Iteration of phase window correlation and least-squares fit for Young's fringe pattern processing

Correlation by means of phase windows that artificially create phase shifts is combined with the least-squares fit to provide a processing algorithm. A Young's pattern is correlated by means of the phase windows to generate four patterns with phase shifts of 0 degrees, 90 degrees, 180 degrees, and 270 degrees. The phase of the Young's pattern is derived from the four patterns. Then the derived phase is least-squares fitted to a phase plane. The above steps are repeated once to improve accuracy. Speckle noise is suppressed by the correlation operations, as well as a loop including phase calculation, fringe reconstruction, and smoothing. The magnitude and direction of the displacement associated with Young's pattern are determined from the phase plane. The algorithm takes less calculation time than the fast Fourier transform method and does so with improved accuracy. Software has been developed and is used in the experiment.

Real-time velocity measurement by the use of a speckle-pattern correlation system that incorporates a ferroelectric liquid-crystal spatial light modulator

We describe a technique for noncontact velocity measurement by using double-exposure speckle-pattern techniques with optical signal processing. The two speckle patterns are recorded on a ferroelectric liquid-crystal (FLC) spatial light modulator (SLM), which is a bistable optically addressed SLM, and the composite pattern is then analyzed by an optical system similar to a joint transform correlator, in which another FLC-SLM and a position-sensitive detector are used. We show that the performance of the system can be significantly improved by adjusting the time between exposures using a real-time feedback system that is based on the position of the correlation spot in the output plane.

Removal of the diffraction halo effect in speckle photography by using a negative mask

A method to neutralize the diffraction halo effect in speckle photography is presented. A negative of the halo is used in the fringe observation plane to achieve this goal. Experimental irradiance distributions as well as Young's fringes with and without the negative mask are presented. It is shown that the apodization procedure is effective in solving the problems caused by varying the diffraction halo irradiance.

Removing the diffraction halo effect in speckle photography of sinusoidal vibration

Expressions to eliminate the diffraction halo effects in speckle photography of sinusoidal vibration are derived. Both maxima and minima positions in Young's fringes pattern tern are considereda, nd only the experimentalh alo intensity variation is assumed known.