Pulsed holographic interferometry vs Schlieren photography.

Abstract

A hologram, sometimes called a holographic interferogram, provides a fringe pattern revealing changes in optical path length that occur between exposures of a scene. The method is thus obviously applicable to gas flow analysis. Appearing to offer approximately the same sensitivity as schlieren techniques, it requires no tedious alignment. To afford a direct comparison of results obtainable with the two methods, the experiments described herein were conducted. Holographic as well as schlieren techniques were employed in measuring the density distribution of different twodimensional air flow patterns: one from a supersonic jet impinging on an inclined flat plate; the other from an ejector. Figure 1 diagrams the experimental setup used for making the interferograms. A ruby laser operating in a single axial and transverse mode served as the light source. Beamsplitter BS split the laser beam into a reference beam and an illumination beam that provided scene backlighting through diffusing glass D. Path length matching was not required since the laser operated in a single axial mode and had a coherence length greater than a meter. The test scene occupied an approximate 4X 4-in. area, whereas hologram plate dimensions were 4 X 5 in. The holographic interferogram of the supersonic jet (see photo, Fig. 2a) was made by first firing the laser under conditions of no air flow, then opening the flow control valve for the second laser shot. The holographic interferogram of the gas ejector, photographed in Fig. 3a, was obtained by taking the first laser shot with flow and the second with no flow. Because the fringe patterns of the interferogram are produced by air density changes that vary the optical path length in the test section between exposures, the effects of dirt, optically nonuniform windows, and other stationary distortions of optical path length tend to cancel out. Figure 3 shows the effect of dirt on test chamber windows. The dirt did not noticeably affect the hologram (Fig. 3a) but caused air flow boundaries in the schlieren photograph (Fig. 3b) to be obscured. Since the double-exposure technique is sensitive only to optical path length changes, there is no necessity