DIGITAL HOLOGRAPHIC-INTERFEROMETRY IN FLOW RESEARCH

A plane-wave holographic interferometric set-up for the quantitative determination of the density distribution existing in the flow field around arbitrarily shaped objects using digital image-processing is presented. By utilizing two reference/reconstruction beams, generated by a slightly misaligned Twyman-Green (T-G) interferometer in the reference/reconstruction beam path, the wavelength used to reconstruct the holographic plate is allowed to differ from the recording wavelength. To obtain again an infinite fringe pattern after such wavelength change, one of the two mirrors constituting the T-G interferometer is slightly rotated. Phase-stepped interferograms are generated successively by piezo-electrically translating the other mirror of the T-G interferometer. Overlapping cross reconstructions, which are inherently induced in such a set-up, are eliminated by a spatial filter in the focus generated by the imaging lens. By applying plane traversing waves instead of diffuse light, automatic distinction between the foreground pixels (i.e., the pixels not lying in the shadow of the object) and the background pixels is made, starting from the modulation intensity as a discriminating parameter. The above procedure is demonstrated by measuring the density distribution in an axisymmetric supersonic jet of air. © 1990.