**5. Real-time colour reflection holographic interferometry**

Since transmission holograms are used, the diffraction efficiency of holograms just reaches between 10% and 20%, which limits the quality and the contrast of interferences fringes. On the other hand, if reflection holograms are used, the theoretical diffraction efficiency can reach 100% with a monochromatic light. The development of real-time true colour reflection holographic interferometry also offers two important advantages. The first one concerns the analysis of the three-dimensional (3D) flows and the second one lies in the comparison with digital colour holographic interferometry. In fact, ONERA is looking towards analyzing unsteady 3D flows, and the optical setup to be designed must be based on several crossings

<sup>3</sup> Modelling of Luminous Interferences and Analysis of Interferograms

Real-Time Colour Holographic Interferometry (from Holographic Plate to Digital Hologram) 13

1/4 wave plate

Holographic Plate

Camera

analysis is crossed by a parallel light beam of 200 mm in diameter. A flat mirror located just behind the test section returns the three beams on the hologram HP inserted between the quarter wave plate QWP and the large achromatic lens. The hologram is illuminated on the two sides by the three collimated reference and measurement waves which are formed by the convergent and divergent achromatic CAL2 and DAL2 lenses (not shown in scheme of Fig.7). This arrangement allows one to easily obtain before the test a uniform background colour (infinite fringes) or narrowed fringes (finite fringes). In this setup, a polarizing beam splitter cube PBC is inserted between the spatial filter and the quarter wave plate which transforms the waves polarization twice (from P parallel to circular and from circular to S parallel) so that, when the rays are returning, the beam splitter cube returns the rays towards the camera. A diaphragm is placed in the focal plane just in front of the camera in order to filter out any parasitic interference. The interferences fringes produced by the phenomenon under analysis can be directly recorded using high speed camera. Here, the camera used is a CORDIN 350 Dynafax. The size of each recording is 10x8 mm² and the pictures are taken in a staggered pattern on a 35mm film. High sensitivity (800/1600 ASA)

Acousto-Optical Cell

1 2 3

Diaphragm

Mask

Spatial Filter

Interference Fringes

Fig. 7. Real-time three-color reflection holographic interferometer

**5.2 Principle of real-time three-color in-line holographic interferometry** 

Fig. 8 details how the interferences fringes are generated in the real-time three-color

daylight reversible colour films are suitable.

reflection holographic interferometer.

Polarizing beamsplitter cube

> Flat Mirror

Object

Test Section

Achromatic lenses

of the flow along different view angles. It is very evident that classical optical setup based on monochromatic holographic interferometry defined in section 4, for instance, for analyzing two-dimensional (2D) flows, cannot be reproduced three or four times. Moreover, as the optical path differences to be measured are smaller in 3D flows than in the 2D case, it is preferable that each optical ray crosses the phenomena twice in order to increase the sensitivity. Also, to simplify the setup, all the optical pieces have to be located on the same side of the wind tunnel, except the flat mirror which reflects the light rays back into the test section.

In literature, several authors have analyzed 3D flows using multidirectional tomography (Cha & Cha, 1996; Yan & Cha, 1998). They present holographic interferometric tomography for limited data reconstruction to measure an asymmetric temperature field. Other researchers designed an optical scheme for obtaining specklegrams simultaneously in four directions (Fomin, 1998; Fomin et al., 2002) or built an interferometric tomography apparatus with six viewing directions from which multidirectional data sets were analyzed following a method of examining spatial coherence (Pellicia-Kraft & Watt, 2000, 2001). The same approach is used by researchers developing digital holographic interferometric techniques. Timmerman & Watt (1995) developed a dual-reference beam holographic interferometer providing six simultaneous views of a compressible flow. One can also note the optical tomograh using six views interferometers for the measurement of 3-dimensional distribution of temperature in an evaporating liquid (Joannes et al, 2000). All the measurement techniques yield either the derivative of the refractive index (speckle holography, differential interferometry or back oriented schlieren) or the refractive index itself (holographic interferometry) and, very often, the spatial resolution of recording camera is very low compared to that of a holographic plate. As the reconstructed field depends strongly on the measured quantity, on the number of the viewing directions and on the spatial resolution, ONERA wanted to develop a metrological tool having limited viewing directions (three or four), high spatial resolution and yielding absolute value of the gas density in the field.

In colour holographic image and panchromatic holographic materials, one can note the recent work of Bjelkhagen & Mirlis, 2008 who produce highly realistic three-dimensional images. They show that the quality of a colour hologram depends on the properties of the recording material and the demand on a panchromatic material for colour holography is described.
