Real-time three-color reflection holographic interferometer.

A compact real-time three-color reflection holographic interferometer (RCRHI) was developed by Office National d'Etudes et de Recherches Aérospatiales for analyzing high-speed flows. As a classical in-line Lippmann-Denisyuk holographic setup, a reflection panchromatic silver-halide holographic plate is used to simultaneously record three reference holograms. The best results are obtained when the diffraction efficiency of the holographic plate reaches 50% for the three wavelengths used (red-green-blue). For that, problems in gelatin shrinkage due to the hologram treatment had to be solved for the two types of holographic plates used (Slavich and Gentet). This new optical setup was applied to analyze the two-dimensional unsteady wake flow around a circular cylinder at Mach 0.45. Interferograms recorded at a high framing rate exhibit very well saturated colors and high contrast, which eases the quasi-automated interferogram analysis. Finally, the evolution in time of the instantaneous gas density field has been obtained from the analysis of several interferograms covering one period of the phenomenon. In the future, the analysis of three-dimensional flows should be investigated using an optical bench based on RCRHI multidirectional tomography.