The contrast of speckle images using viscous fluid flows

Abstract

We have obtained a novel formula for the contrast of speckle images considering the Voigt distribution using Fraunhofer diffraction and convolution operations. An experiment on speckle formation using fluid flow illuminated with a coherent laser sheet is made. We imaged a speckle of fluid flow in a cylindrical tube using a CCD camera. The obtained speckle images are resized to dimensions 1024 ×1024 pixels. These speckle images are produced from the randomness of fluid flow convoluted with the Point Spread Function (PSF) of the Gaussian beam. We investigated the average speckle contrast for different viscous fluids such as Glycerin, Shake powder in water, and vegetable oils. Line plots at different heights show various irregularities corresponding to the movement of the fluids. In addition, the effect of a circular aperture placed in the fluid plane is studied. Finally, we can compute the fluid velocity from the average speckle contrast related to the speckle field autocorrelation.