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

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 field.

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

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

Real-Time Colour Holographic Interferometry (from Holographic Plate to Digital Hologram) 7

Spectrum 70 ionized gas laser (mixed argon and krypton) that produces approximately 10 visible lines with a total power of 4.7 W. The three wavelengths retained are 647 nm for the red line of krypton and 514 nm and 476 nm for the green and blue lines of argon. All three exhibit a TEM00 mode because thee laser is equipped with a Fabry-Perot etalon to increase the coherence length of the three lines selected. The etalon is treated on both faces to get 66% transmittance for the blue line, 63% for the green line and 61% for the red line. This treatment increases the blue line's coherence length 2 or 3 centimetres at a range of several tens of centimetres. This is sufficient because, in our study, the reference and measurement paths can be roughly equalized and the optical path variations to be measured are no greater than a few microns. In the other hand, a large light energy is needed to record the studied phenomena at ultra-high speed (35,000 f/s with an exposure time of 750 10-9 s).

Since Russian panchromatic plates came on the market twenty years ago, progress has been made in true colour holograms (Hubel, 1991; Bjelkhagen & Vukicevic, 1991; Bjelkhagen et al., 1996). The various chemical treatments applied to these plates are explained in detail by several authors (Bjelkhagen, 1993; Sasomov, 1999). The plates used are silver-coated singlefilm PFG 03C plates from the Slavich company in Moscow. Their chemical treatment first includes a hardening of the gelatine, development, bleaching, a series of rinses in alcohol and slow drying. The hologram's spectral characteristics were analyzed by taking a double exposure holographic interferogram and placing a small mirror near the object to be analyzed, in order to make a spectral analysis of the reconstructed light waves. The spectrograms of the reconstructed waves in our very first validation tests showed that the three peaks corresponding to the reconstructed colours are slightly shifted by a few

These differences are reduced practically to zero in wind tunnel tests when the hologram is

Fig. 3 shows the feasibility setup implemented in the laboratory. The Innova Spectrum laser emits eleven lines in the visible simultaneously. The red, green, and blue lines we want are diffracted by an acousto-optic cell in which are generated three frequencies f1, f2 and f3 appropriate to the three wavelengths 1, 2 and 3. A beamsplitter cube splits the reference beams and three object beams. The three reference beams are collimated onto the holographic plate by an achromatic lens located a focal length from the pinhole (diameter 25 m) of a spatial filter having a microscope objective lens (x60). The three object beams are collimated the same way to form three parallel light beams between two large achromatic

The hologram is thus illuminated on the same side by the three parallel reference beams and the three convergent measurement waves. A diaphragm is placed in the focal plane just in front of the camera in order to be able to filter out any parasitic interference. The hologram is first illuminated in the absence of flow and is then developed and placed back in exactly its original position. When the hologram is illuminated with the reference beam, nine diffraction images are seen. They are clearly visible in Fig. 4. Of the nine, three coincide and

placed normal to the bisector of the angle formed by the object and reference beams.

nanometres, which corresponds to the contraction of the gelatine thickness:

**4.1.2 Panchromatic holographic plates** 

 471 nm for the blue, or – 5 nm, 511 nm for the green, or – 3 nm, 640 nm for the red, or – 7 nm.

lenses and illuminate the test section.

**4.1.3 Laboratory results** 

Fig. 2. Formation of colour interference fringes

transmitted ROC, GOC and BOC. The profiles of the ROD and ROC, GOD and GOC waves, and the BOD and BOC waves are strictly identical to each other if no change has occurred between the two exposures and if the hologram gelatine has not contracted during development. So there will be three simultaneous interferences among the object waves constructed by the hologram and the live object waves. In this case, a flat uniform colour can then be observed behind the hologram. If a change in optical path is created in the test section of wind tunnel, the three live waves will deform and adopt the profiles 'ROC, 'GOC and 'BOC while the waves reconstructed in the hologram, ROD, GOD and BOD, remain unchanged. Any colour variations representing optical path variations will thus be visualized in real time behind the hologram (Step 4, Fig. 2).
