Real-Time Colour Holographic Interferometry (from Holographic Plate to Digital Hologram)

In the area of Fluids Mechanics, detailed analysis and characterization of complex, unsteady flows require non-invasive optical methods to measure smaller and smaller quantities over space or time, or even both at once. Therefore, many researchers have spent considerable time over the last fifty years to develop metrology tools adapted to quantitative flow visualization. Some of these methods such as shadowgraph or schlieren method are based on measuring the light deviation through the test section (Merzkirch, 1974), other methods such as interferometry or holography are based on optical interferences and on measurement of the optical path difference or the signal phase (Vest, 1979). When qualitative measurements of the flow are sought, the former techniques can be used. The concepts and the many applications of shadowgraph or schlieren techniques can be found in (Settles, 2001). If quantitative data are required, Mach-Zehnder or Michelson interferometers have been developed, but these instruments are very sensitive to external vibrations, especially when the two arms of the interferometer have unequal length (Merzkirch, 1974). To avoid this problem, differential interferometry or Wollaston-prism shearing interferometry using a polarized white light source can be implemented (Philbert, 1958; Merzkirch, 1965; Smeets, 1975), but these techniques visualize the first derivative of the refractive index in the test section. The same optical technique equipped with high speed camera can be also used to analyze high speed flows (Desse, 1990, 2006). In this case, a sequence of colour interferograms is recorded at a high framing rate from which the derivative of the gas density can be extracted. The interferograms are analyzed nearly automatically by an image processing software specially designed for modelling the light intensity of the interference fringes as the path difference varies (Desse, 1997a). As the method gives a differential measurement, integration is necessary to get the full gas density field, whence a certain imprecision arises in the measurements. To avoid such imprecision related to integration and to maintain the advantage of colour interferograms1, real-time colour holographic interferometry has been developed. One of the two variants of colour holographic interferometry is perfectly suitable for analyzing unsteady aerodynamic