**1. Introduction**

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

<sup>1</sup> Colour interferometry has the property to exhibit a unique white fringe visualizing the zero order of interferences

Real-Time Colour Holographic Interferometry (from Holographic Plate to Digital Hologram) 5

is one of the major difficulties with interferences fringes in monochromatic light. Sometimes, it is not possible to follow the displacement of the fringes through a shock wave, for example, or to count the fringe number in a complex flow. When the light source is a continuous source (500 Watt xenon, see Fig. 1b), the interference pattern is a coloured fringe pattern in a sequence approximately matching Newton's colour scale. This fringes diagram exhibits a unique white fringe, visualizing the zero order of interference and it allows one to measure very small path differences, because six or seven different colours define the interval 0-0.8 microns. But, when the path difference is greater than three or four microns, the colours can no longer be separated and the larger path differences cannot be correctly measured (Desse, 1997b). Fig. 1c shows the fringes obtained with a laser that emits three different wavelengths (one blue line, one green line and one red line). One can see that the disadvantages of the two others sources have disappeared. The zero order is always identifiable and the colours always remain distinguishable for the small and the large path differences. The interference pattern also presents the following peculiarity: while the white fringe is not visible on the interferogram, the sequence of three successive colours in the

The various holographic interferometry methods – double exposure, time-averaged, or realtime holography – are the main scientific applications of holography. Until recent years, experiments in holographic interferometry were performed with a single laser, i.e., they were monochromatic. Most experiments found in the literature relate to transmission holograms (Rastogi, 1994) and few experiments have been performed to date using holographic interferometry with reflected white light (Smigielski et al., 1976; Vikram, 1992). It should be said that, in monochromatic mode, experiments in reflected white-light holography have little advantage over holographic interferometry in transmitted light. Some publications mention the use of three-wavelength differential interferometry (Desse, 1997b) and holographic interferometry by reflection (Harthong, 1997; Jeong, 1997) and all show that the essential advantage of colour is that the achromatic fringe can be located in the observed

Real-time true colour holographic interferometry uses three primary wavelengths (red, green, blue) to record the interference between the three object beams and the three reference beams simultaneously on a single reference hologram. Under no-flow conditions, the undisturbed object waves RO, GO and BO are recorded in the hologram by virtue of their interference with the three reference waves RR, GR and BR. As can be seen in Fig. 2, step 1, at recording and for a plate recorded in transmission, the three reference waves and the three object waves arrive on the same side of the plate while in reflection, they come from opposite sides of the holographic plate. After treatment of the plate and resetting in the optical bench, the three reference waves RR, GR and BR are diffracted by transmission or by reflection according to the recording mode used to form the three diffracted object

Then the hologram is illuminated simultaneously by the three reference beams and three object beams, from which we get the three object beams ROD, GOD and BOD reconstructed by the holographic plate simultaneously with the three live object waves

**3. Principle of real-time colour holographic interferometry** 

waves ROD, GOD and BOD (Step 3, Fig. 2).

diagram is unique.

field.

phenomena (i.e., of transparent objects) and for real-time analysis of mechanical deformations (diffusive objects). The usual double-exposure method consists in recording the holograms of a transparent or diffusive object in two different states in succession, on the same photographic plate. This proven technique has yielded good results for several years, but it does have the disadvantage of not allowing the interferogram of the phenomenon being studied to be observed immediately and without interruption. Real-time colour holographic interferometry, on the other hand, allows direct observation through a reference hologram and makes it possible to take an ultra-high speed movie of the interferogram of a changing phenomenon (Surget, 1973). This was the method used.

After presenting the advantages associated with the use of a polychromatic light source rather than monochromatic, the principles of three-wavelength holographic interferometry in real-time are detailed. The feasibility of the method is shown when silver-halide panchromatic holographic plates are used either in transmission or reflection. The advantages and disadvantages of these techniques for recording and reconstruction, though substantially different, are presented through an application that examines the unsteady wake flow downstream of a cylinder at a subsonic Mach number. To conclude this chapter, colour digital holographic interferometry is presented as a method preferable to holographic techniques using holographic plates even if the new generation of CMOS or CCD sensors are far from having the spatial resolution of holographic plates.
