In this schlieren system the probing laser beam is attenuated by a neutral density wedge (instead of the usual knife edge) so that the intensity is proportional to the deflection angle, which in turn is proportional to the density gradient. Time integration of the attenuated probe beam intensity yields the absolute refractive index variation across moving objects. The device is tested by measuring the density jump across a Mach 9 shock wave.
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Ahlborn et al. (1976) studied this question.
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