**2. Advantage of using one source to multiple wavelengths**

In monochromatic interferometry (for instance, =647nm), it is well known that the classical interference pattern is represented by a succession of dark and bright red fringes. For two successive fringes, the optical path difference is equal to the wavelength of the laser source (Fig.1a). Unfortunately, the zero order of interferences fringes can never be identified and it

Fig. 1. Spectra and interference fringes given by three different light sources

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.

are far from having the spatial resolution of holographic plates.

> I Blue 476 Green

b)

c)

a)

400 500 600 700 800

**(nm)**

<sup>514</sup> Red 647

400 500 600 700 800

**(nm)**

Fig. 1. Spectra and interference fringes given by three different light sources

(nm)

400 500 600 700 800

Red 647 nm

(nm)

**(nm)**

(nm)

150 W Xenon

I

I

**2. Advantage of using one source to multiple wavelengths** 

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

In monochromatic interferometry (for instance, =647nm), it is well known that the classical interference pattern is represented by a succession of dark and bright red fringes. For two successive fringes, the optical path difference is equal to the wavelength of the laser source (Fig.1a). Unfortunately, the zero order of interferences fringes can never be identified and it

> 0 400 800

> > 0 400 800

0 400 800

> -1 0 1 2 3 4 5 6 **(m)**



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 diagram is unique.
