We present an experiment based on speckle photography, to study the distribution of velocities in a fluid at a given instant. This technique is complementary to laser velocimetry (UDV), in which the velocity is measured at a certain point of the fluid, as function of time. A section of the fluid is lit by a laser beam widened in one direction by a cylindrical lens and an image of this section is formed on a photographic plate. When polystyrene balls are added to the fluid, the image of the section lit is speckled. The laser beam is modulated so that the section of the fluid being studied is lit by two short pulses separated by a known interval of time. The photographic plate thus records two speckles displaced from each other; this displacement is variable in the plane of the photographic plate, because of the distribution of velocities in the lighted plane. An analysis of this photographic plate permits us to construct the map of velocities of the illuminated section of the fluid.
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