Anamorphic optical processing of multiple-exposure speckle photographs

Previous methods for analyzing double-exposed speckle photographs provide point-by-point velocity information by using two-dimensional Fourier analysis, or constant velocity contours by using spatial filtering techniques. A new method using an anamorphic optical system to measure one component of the velocity throughout a section of the flow is analyzed and demonstrated. A laser sheet, thin in the x direction and extended in the y direction, is used to probe a line of the photograph of which the anamorphic optical system forms a one-dimensional Fourier transform in the x direction and images the speckle pattern in the y direction for measuring the x-velocity component. This results in curved fringes, which have a local spacing inversely proportional to the x-velocity component at that point. Thus it is possible to measure a velocity component along a selected line in the flow. This differs from spatial filtering techniques that produce contours showing the points where a selected velocity occurs.