Time-resolved holography for the study of shock waves

Techniques are presented for obtaining time-resolved sequences of holograms and holographic interferograms of a shock wave impinging on a model in a shock tube. Several images are multiplexed on a single frame of film using a stationary, focused object beam and an angularly swept reference beam. At the reconstruction stage, the direction from which the reconstruction beam is shone determines which image is observed. In the original implementation, angular sweeping of the reference beam was achieved by the combination of a flat spinning mirror with a circular mirror that redirected the light on a fixed area of film. The holographic light source was a ruby laser, multiply Q- switched by a Pockels cell at rates of up to 20 thousand pulses a second. The method has been used to record either schlieren images or holographic interferograms of shock waves. Schlieren images of the phenomenon were holographically recorded by introducing an aperture at a focal point in the optical path of the object beam. Holographic interferograms were generated by overlaying two sequences on the same medium, one containing the phenomenon and the other the undisturbed field (reference exposure). The spinning mirror equipment is capable of recording sequences of some 10 frames at 50 µs separation, over a useful sweep angle of about 110 °. Proper timing of the sweep with respect to shock arrival was not assured with the original spinning mirro