**5.1 Description of optical bench**

The optical setup could be named "Denisyuk" because it uses a holographic plate in a classical Lippmann-Denisyuk in-line experiment. To obtain a very simple setup, all the optical pieces are located on the same side of the wind tunnel, except the flat mirror which reflects the light rays back into the test section. Due to these considerations, the optical setup based on real-time colour reflection holographic interferometry has been designed. It is presented in Fig. 7.

Here, the light source used behind the interferometer is constituted with three different lasers. An argon-krypton laser delivers the red line (1 = 647 nm), and a green line (2 = 532 nm) and a blue line (3 = 457 nm) are given by two diode pumped solid state lasers. A spatial filter SP is just located at the focal point of the large achromatic lens AL3 (see photograph, Fig. 7) which is set in the front of the test section TS so that the object under

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

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

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

The optical setup could be named "Denisyuk" because it uses a holographic plate in a classical Lippmann-Denisyuk in-line experiment. To obtain a very simple setup, all the optical pieces are located on the same side of the wind tunnel, except the flat mirror which reflects the light rays back into the test section. Due to these considerations, the optical setup based on real-time colour reflection holographic interferometry has been designed. It is

Here, the light source used behind the interferometer is constituted with three different lasers. An argon-krypton laser delivers the red line (1 = 647 nm), and a green line (2 = 532 nm) and a blue line (3 = 457 nm) are given by two diode pumped solid state lasers. A spatial filter SP is just located at the focal point of the large achromatic lens AL3 (see photograph, Fig. 7) which is set in the front of the test section TS so that the object under

section.

gas density in the field.

**5.1 Description of optical bench** 

described.

presented in Fig. 7.

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

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) daylight reversible colour films are suitable.

### **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 reflection holographic interferometer.

Real-Time Colour Holographic Interferometry (from Holographic Plate to Digital Hologram) 15

three wavelengths which can be adjusted with the acousto-optic cell. A main inconvenience resides in the fact that it is not possible to adjust the diffraction efficiency of the holographic plate. It is only fixed by the chemical treatment and it is a function of gelatine thickness. The unique solution to solve this problem will consist in a specific treatment of the surface of the flat mirror and this operation implies prior knowledge of the diffraction efficiency of the

Finally, the three interference fringe patterns will exist and can be recorded if the coherence length of the three wavelengths is more than twice the distance between the holographic plate and the flat mirror located just behind the test section. Compared to the setup of transmission holographic interferometry, here it is not possible to adjust the length between the reference and measurement rays. In this experiment, two types of holographic plates have been tested: Russian plates (PFG03C) from Slavich and French plates (Ultimate 08) from Gentet, typically 10 m thick. For information, energies at each wavelength applied at

**PFG03c (Slavich ) Ultimate 08 (Gentet)**  1.0 10-3 J/cm² @ 457nm 0.8 10-3 J/cm² @ 457nm 1.3 10-3 J/cm² @ 532nm 0.8 10-3 J/cm² @ 532nm 1.0 10-3 J/cm² @ 647nm 0.8 10-3 J/cm² @ 647nm These values can be compared with tests and results found in Petrova et al., 2000 who determine the holographic characteristics of panchromatic light sensitive material for

The problem of gelatine shrinkage is described in detail in Desse, 2006. In Fig. 9, one can see how the interference fringes are inscribed into the gelatine when the holographic image is recorded by transmission or reflection. In transmission, the interference fringes are perpendicular to the plate and a small variation in the gelatine thickness caused by the chemical treatment of the hologram does not modify the three inter fringe distances. On the other hand, in reflection, interference fringes are recorded parallel to the plate surface and the inter fringe distance is very sensitive to a small variation of the gelatine thickness. Fig. 9 presents the effects of the gelatine contraction when a reflection hologram is recorded with a green wavelength (514 nm). At reconstruction, a white light source (xenon source) illuminates three different holograms at the incidence angle that the reference wave had at recording. One can see that if the gelatine thickness is kept constant (e=0), the hologram only diffracts the recording wavelength, i.e. for the green hologram, the green wavelength contained in the xenon spectrum. If the gelatine thickness has decreased by 5%, (e = - 0.5 m), the fringe spacing will be proportionally reduced and the diffracted wavelength

> *e e*

where *e* is the gelatine layer thickness (about 10 m). The hologram will diffract a wavelength equal to 488.3 nm corresponding to a blue line and, if the gelatine thickness increases by 10% (e>0), the hologram illuminated in white light will diffract a wavelength

(4)

the first exposure are given in Table 1 for Slavich and Gentet plates.

hologram.

reflective 3D display.

**5.3 Problem of gelatine shrinkage** 

will be shifted by a quantity equal to

close to yellow (565.4 nm).

Fig. 8. Formation of colour interference fringes in optical setup

First, the holographic plate HP is simultaneously illuminated with the three wavelengths (Fig. 8a). The panchromatic hologram records simultaneously the three sets of interference fringes produced by the three incident waves and the three waves reflected by the flat mirror LFM (first exposure). Then the hologram is developed and it is reset in the optical bench at the same location. At the second exposure, if the diffraction efficiency of the holographic plate is near to 50% for the three lines, 50% of the light is reflected by the hologram (dashed lines) and 50% crosses the holographic plate (solid lines). If a mask is inserted in the front of the test section, one can observe on the screen the three images diffracted by the plate (Fig. 8b). This operation allows for verifying the quality of the holograms diffracted. When the mask is moved, 50% of the light crosses the test section twice and interferes in real-time with the three references waves (solid lines). Interference fringes are not localized because they can be observed from the holographic plate to the camera. If no disturbances exist in the test section, a uniform background colour is obtained in the camera (Fig. 8c). If variation in refractive index exists in the test section, colour fringes will be seen on the screen. As the luminous intensities of reference and measurement waves are basically equal, the contrast of colour interferences fringes will be maximum (Fig. 8d).

This optical setup is very simple but it presents some advantage and some inconvenience. The advantage lies in the small number of optical pieces which are used. The reference beams and the measurement beams are co-linear and there is just a flat mirror behind the test section. The contrast of colour interferences fringes depends on the diffraction efficiency of the holographic plate and the colours saturation depends on the luminous intensity of the three wavelengths which can be adjusted with the acousto-optic cell. A main inconvenience resides in the fact that it is not possible to adjust the diffraction efficiency of the holographic plate. It is only fixed by the chemical treatment and it is a function of gelatine thickness. The unique solution to solve this problem will consist in a specific treatment of the surface of the flat mirror and this operation implies prior knowledge of the diffraction efficiency of the hologram.

Finally, the three interference fringe patterns will exist and can be recorded if the coherence length of the three wavelengths is more than twice the distance between the holographic plate and the flat mirror located just behind the test section. Compared to the setup of transmission holographic interferometry, here it is not possible to adjust the length between the reference and measurement rays. In this experiment, two types of holographic plates have been tested: Russian plates (PFG03C) from Slavich and French plates (Ultimate 08) from Gentet, typically 10 m thick. For information, energies at each wavelength applied at the first exposure are given in Table 1 for Slavich and Gentet plates.


These values can be compared with tests and results found in Petrova et al., 2000 who determine the holographic characteristics of panchromatic light sensitive material for reflective 3D display.

### **5.3 Problem of gelatine shrinkage**

14 Advanced Holography – Metrology and Imaging




First, the holographic plate HP is simultaneously illuminated with the three wavelengths (Fig. 8a). The panchromatic hologram records simultaneously the three sets of interference fringes produced by the three incident waves and the three waves reflected by the flat mirror LFM (first exposure). Then the hologram is developed and it is reset in the optical bench at the same location. At the second exposure, if the diffraction efficiency of the holographic plate is near to 50% for the three lines, 50% of the light is reflected by the hologram (dashed lines) and 50% crosses the holographic plate (solid lines). If a mask is inserted in the front of the test section, one can observe on the screen the three images diffracted by the plate (Fig. 8b). This operation allows for verifying the quality of the holograms diffracted. When the mask is moved, 50% of the light crosses the test section twice and interferes in real-time with the three references waves (solid lines). Interference fringes are not localized because they can be observed from the holographic plate to the camera. If no disturbances exist in the test section, a uniform background colour is obtained in the camera (Fig. 8c). If variation in refractive index exists in the test section, colour fringes will be seen on the screen. As the luminous intensities of reference and measurement waves are basically equal, the contrast of colour interferences fringes will be maximum (Fig. 8d). This optical setup is very simple but it presents some advantage and some inconvenience. The advantage lies in the small number of optical pieces which are used. The reference beams and the measurement beams are co-linear and there is just a flat mirror behind the test section. The contrast of colour interferences fringes depends on the diffraction efficiency of the holographic plate and the colours saturation depends on the luminous intensity of the

PBC QWP Hologram TS LFM

: 50%



Mask

: 50%

: 50%

: 50%


100%

100%

: 50%

100%

: 50%

Fig. 8. Formation of colour interference fringes in optical setup

a) Reference hologram recording

b) Hologram resetting with mask

c) 2nd exposure with undisturbed object waves

d) 2nd exposure with disturbed object waves

d)

The problem of gelatine shrinkage is described in detail in Desse, 2006. In Fig. 9, one can see how the interference fringes are inscribed into the gelatine when the holographic image is recorded by transmission or reflection. In transmission, the interference fringes are perpendicular to the plate and a small variation in the gelatine thickness caused by the chemical treatment of the hologram does not modify the three inter fringe distances. On the other hand, in reflection, interference fringes are recorded parallel to the plate surface and the inter fringe distance is very sensitive to a small variation of the gelatine thickness. Fig. 9 presents the effects of the gelatine contraction when a reflection hologram is recorded with a green wavelength (514 nm). At reconstruction, a white light source (xenon source) illuminates three different holograms at the incidence angle that the reference wave had at recording. One can see that if the gelatine thickness is kept constant (e=0), the hologram only diffracts the recording wavelength, i.e. for the green hologram, the green wavelength contained in the xenon spectrum. If the gelatine thickness has decreased by 5%, (e = - 0.5 m), the fringe spacing will be proportionally reduced and the diffracted wavelength will be shifted by a quantity equal to

$$
\Delta \mathcal{X} = \frac{\mathcal{X}}{e} \Delta e
\tag{4}
$$

where *e* is the gelatine layer thickness (about 10 m). The hologram will diffract a wavelength equal to 488.3 nm corresponding to a blue line and, if the gelatine thickness increases by 10% (e>0), the hologram illuminated in white light will diffract a wavelength close to yellow (565.4 nm).

Real-Time Colour Holographic Interferometry (from Holographic Plate to Digital Hologram) 17

normalized intensity of 0.4, the width of the curve is near to 16 nm and 27 nm for the Slavich and Gentet holograms respectively. It is of 10 nm and 17 nm for the blue line. If a small variation of the gelatine thickness is allowed then the wider spectral response of Gentet holograms is advantageous. A bad adjustment of the Fabry-Perot etalon in the argonkrypton laser cavity, meaning that the coherence length was reduced, explains the weakness

0

400 450 500 550 600 650 700 **(nm)**

Hologram Slavich

= 457 nm 2= 514 nm 3= 647 nm

0.2

0.4

0.6

**Normalized Intensity**

When no shrinkage exists, the curves are basically centred on the three different lines of the lasers and the best diffraction efficiency of hologram is reached. The three different gratings inscribed in the hologram gelatine diffract very well the three different wavelengths (blue, green and red) of the laser sources. In fact, the response of the hologram is the best one because the bell curves are centred on the three laser wavelengths. In these conditions, there is basically no difference in the gelatine thickness before and after the chemical treatment of the plates. Moreover, if the power of the three reference and measurement wavelengths are

The diffraction efficiency DE of the holographic plates can be evaluated when Russian or French plates are illuminated in white light. For example, Fig. 11 shows how the diffraction efficiency of Gentet holographic plate is determined from the spectrum of the xenon light source transmitted by the plates. It very easy to see the three hollows corresponding to the part of the white light diffracted by the Gentet holographic plate. For each line, the bandwidth can be determined when the diffraction efficiency is more than 35%. In fact, as regards luminous intensity, when the diffraction efficiency of the holographic plate is equal to or more than 35%, the visibility coefficient of interference fringes ((Imax-Imin)/(Imax+Imin)) is near to 0.21, that means the colour interference pattern will be of very high? contrast. For the blue line, the acceptable wavelength shift is 8 nm (1.75%), 9 nm (1.75%) for the green line and 13 nm (2%) for the red line. This particularity is very interesting because the theoretical constraints on the variation of the gelatine thickness (near 0.2%) become larger (about

Fig. 10. Measurement of the gelatine shrinkage – Gentet and Slavich holograms

the same, it is possible to obtain very bright, high contrast fringes.

1.75%) due to the spectral broadening of the transmission curve.

0.8

1

of the green response in the Slavich graph.

= 457 nm 2= 514 nm 3= 647 nm

Hologram Gentet

400 450 500 550 600 650 700 **(nm)**

**5.5 Diffraction efficiency measurement** 

0

0.2

0.4

0.6

**Normalized Intensity**

0.8

1

Fig. 9. Effect of the gelatine contraction on the different waves

On the other hand, it is well known that the chromatic perceptibility of eye *l* varies with the wavelength. It is defined as being the variation *l* between two different wavelengths perceived by the eye at constant luminosity. It is about 1 nm in the green and yellow colours and 6 nm in the blue and red colours, which corresponds to relative variations of 0.2% and 1.5% respectively. For the diffracted colour change not to be detected by a human eye, it is mandatory that *l/*has to be less than that which implies that the variation in gelatine thickness should be less than 0.2%. This means changes in thickness of more than 20 nm are not acceptable. As the optical technique is based on the knowledge of the true colours diffracted by the hologram, variations of the gelatine thickness are a cause of large errors in the data analysis. It is for this reason that the gelatine shrinkage problem has to be perfectly mastered.

### **5.4 Gelatine contraction control**

In this experiment, two types of holograms have been tested: Russian plates from Slavich and French plates from Gentet. Concerning the first ones, a specific treatment proposed by Kim, 2002, has allowed obtaining a gelatine contraction smaller than 20 nm by mixing 2 ml of glycerol in the last bath of ethanol (100% ethanol drying). One can mention that the treatment applied to the Russian plates includes about ten steps and it is very sensitive to the temperature and the PH of the solutions. About the French plates, following Gentet's recommendations, we have also obtained basically no variation in the gelatine thickness diffracted by Gentet and Slavich plates when they are illuminated in white light.

In these graphs of Fig. 10, the dashed lines represent the spectrum of white light diffracted by the holograms with no treatment of the plates. We can observe a shifting of about 1.8 % with Gentet holograms and about 1.1% with Slavich holograms which is not acceptable. It can be also seen that the spectrum diffracted by Gentet holograms is wider than the spectrum diffracted by Slavich holograms. For example, if one looks at the red line at a

Fig. 9. Effect of the gelatine contraction on the different waves

has to be less than

wavelength. It is defined as being the variation

mandatory that

mastered.

*l/*

**5.4 Gelatine contraction control** 

On the other hand, it is well known that the chromatic perceptibility of eye

perceived by the eye at constant luminosity. It is about 1 nm in the green and yellow colours and 6 nm in the blue and red colours, which corresponds to relative variations of 0.2% and 1.5% respectively. For the diffracted colour change not to be detected by a human eye, it is

thickness should be less than 0.2%. This means changes in thickness of more than 20 nm are not acceptable. As the optical technique is based on the knowledge of the true colours diffracted by the hologram, variations of the gelatine thickness are a cause of large errors in the data analysis. It is for this reason that the gelatine shrinkage problem has to be perfectly

In this experiment, two types of holograms have been tested: Russian plates from Slavich and French plates from Gentet. Concerning the first ones, a specific treatment proposed by Kim, 2002, has allowed obtaining a gelatine contraction smaller than 20 nm by mixing 2 ml of glycerol in the last bath of ethanol (100% ethanol drying). One can mention that the treatment applied to the Russian plates includes about ten steps and it is very sensitive to the temperature and the PH of the solutions. About the French plates, following Gentet's recommendations, we have also obtained basically no variation in the gelatine thickness

In these graphs of Fig. 10, the dashed lines represent the spectrum of white light diffracted by the holograms with no treatment of the plates. We can observe a shifting of about 1.8 % with Gentet holograms and about 1.1% with Slavich holograms which is not acceptable. It can be also seen that the spectrum diffracted by Gentet holograms is wider than the spectrum diffracted by Slavich holograms. For example, if one looks at the red line at a

diffracted by Gentet and Slavich plates when they are illuminated in white light.

*l* between two different wavelengths

that which implies that the variation in gelatine

*l* varies with the

normalized intensity of 0.4, the width of the curve is near to 16 nm and 27 nm for the Slavich and Gentet holograms respectively. It is of 10 nm and 17 nm for the blue line. If a small variation of the gelatine thickness is allowed then the wider spectral response of Gentet holograms is advantageous. A bad adjustment of the Fabry-Perot etalon in the argonkrypton laser cavity, meaning that the coherence length was reduced, explains the weakness of the green response in the Slavich graph.

Fig. 10. Measurement of the gelatine shrinkage – Gentet and Slavich holograms

When no shrinkage exists, the curves are basically centred on the three different lines of the lasers and the best diffraction efficiency of hologram is reached. The three different gratings inscribed in the hologram gelatine diffract very well the three different wavelengths (blue, green and red) of the laser sources. In fact, the response of the hologram is the best one because the bell curves are centred on the three laser wavelengths. In these conditions, there is basically no difference in the gelatine thickness before and after the chemical treatment of the plates. Moreover, if the power of the three reference and measurement wavelengths are the same, it is possible to obtain very bright, high contrast fringes.

### **5.5 Diffraction efficiency measurement**

The diffraction efficiency DE of the holographic plates can be evaluated when Russian or French plates are illuminated in white light. For example, Fig. 11 shows how the diffraction efficiency of Gentet holographic plate is determined from the spectrum of the xenon light source transmitted by the plates. It very easy to see the three hollows corresponding to the part of the white light diffracted by the Gentet holographic plate. For each line, the bandwidth can be determined when the diffraction efficiency is more than 35%. In fact, as regards luminous intensity, when the diffraction efficiency of the holographic plate is equal to or more than 35%, the visibility coefficient of interference fringes ((Imax-Imin)/(Imax+Imin)) is near to 0.21, that means the colour interference pattern will be of very high? contrast. For the blue line, the acceptable wavelength shift is 8 nm (1.75%), 9 nm (1.75%) for the green line and 13 nm (2%) for the red line. This particularity is very interesting because the theoretical constraints on the variation of the gelatine thickness (near 0.2%) become larger (about 1.75%) due to the spectral broadening of the transmission curve.

Real-Time Colour Holographic Interferometry (from Holographic Plate to Digital Hologram) 19

**y/D**

**y/D**

**1**

**2**

**3**

gas density field.

**y/D**

the gas density field was referred to

**6. Digital colour holographic interferometry** 

**x/D**

**x/D**

01234

**x/D**

**y/D**

2

1


*<sup>0</sup>*, the stagnation gas density. One can see that the

0

**x/D**

 0.970 0.945 0.920 0.895 0.870 0.845 0.820 0.795 0.770 0.745 0.720


Fig. 12. Interferogram analysis : instantaneous and average gas density field – t = 117s

observed looking at the last vortex leaving the observed field. Finally, as with transmission holographic interferometry, each colour represents a value of the gas density. In analysis,

instantaneous gas density varies from 0.70 to 0.98. The average gas density in the field has been calculated from twelve successive interferograms. The interferogram number is not very significant, but the obtained field is already symmetrical enough and the gas density varies from 0.72 to 0.97. Finally, if the colour scale of interference pattern is very well known to the user, the image of interferograms is sufficient to correctly evaluate the evolution of the

The fast development of technology, such as high resolution sensors, various DPSS lasers with large coherence, data post processing and computation power provide now opportunities to conceive new optical methods capable of simultaneous full field measurements with high spatial and temporal resolutions and giving absolute data. Digital holography with matrix sensors appeared in the last decade with cheap high resolution CCD cameras and the increasing power of computers. Image sensors have now size and spatial resolutions compatible with the needs for digital holographic recording. For example, matrices with 16361238 pixels of size 3.93.9m² are now available (Yamaguchi & Zhang, 1997). In the literature, only few papers concern works in digital colour holographic interferometry. In 2002, Yamaguchi et al. & Kato et al. demonstrated phase shifting digital colour holography using a multi-wavelength HeCd continuous wave laser (636nm, 537.8nm, 441.6nm) and a colour CCD equipped with a Bayer mosaic4. The authors demonstrated the

4 Matrix structure in front of a sensor to create a colour information from a panchromatic monochrome



Fig. 11. Evaluation of diffraction efficiency of Gentet plate

### **5.6 Wind tunnel implementation of optical technique**

A real-time colour reflection holographic interferometer has been implemented in the ONERA transonic wind tunnel for analyzing the same unsteady wake flow around the cylinder. In this experiment, the infinite Mach number was fixed at 0.45 and the high speed interferograms were recorded with the rotating drum camera which is equipped with a 400 ASA colour film. The time interval between two successive frames is 117 s. The time exposure (750 ns) of each interferogram is given by a small window size inside the camera and the number of recorded interferograms is about 220. The images are 8x10mm² in size and they are digitalized with a SONY 325P video camera through a Matrox image processing board. Several movies have been recorded with uniform background colour (infinite fringes), circular and narrowed fringes (finite fringes). As the optical setup is very sensitive to external vibrations, the uniform background colour is difficult to adjust when the wind tunnel is running, but the fringe formation can be observed on the hologram surface so that it is possible to adjust the uniform background colour with the wind tunnel operating. Fig. 12 shows three of twelve interferograms covering about a period of the vortex street. They are recorded in infinite fringes. The interferogram colours are well saturated and of higher contrast than those obtained in previous experiments performed with transmission holograms (see interferograms of Fig. 6).

When the background colour is uniform, it is very easy to follow the vortices emitted from the upper and lower side. For instance, if one looks at the colours coming out in the vortex cores, one can easily see that the first vortex emitted from the upper side enters a formation phase where the gas density decreases in the vortex centre. A second phase of dissipation is

**------ Hypothetical spectrum transmitted by the hologram without the three diffraction** 

0 0.1 0.2 0.3 0.4 0.5

**1-(In/Is)**

440 450 460 470 480 **(nm)**

0 0.1 0.2 0.3 0.4 0.5

Fig. 11. Evaluation of diffraction efficiency of Gentet plate

**5.6 Wind tunnel implementation of optical technique** 

with transmission holograms (see interferograms of Fig. 6).

**1-(In/Is)**

515 525 535 545 **(nm)**

A real-time colour reflection holographic interferometer has been implemented in the ONERA transonic wind tunnel for analyzing the same unsteady wake flow around the cylinder. In this experiment, the infinite Mach number was fixed at 0.45 and the high speed interferograms were recorded with the rotating drum camera which is equipped with a 400 ASA colour film. The time interval between two successive frames is 117 s. The time exposure (750 ns) of each interferogram is given by a small window size inside the camera and the number of recorded interferograms is about 220. The images are 8x10mm² in size and they are digitalized with a SONY 325P video camera through a Matrox image processing board. Several movies have been recorded with uniform background colour (infinite fringes), circular and narrowed fringes (finite fringes). As the optical setup is very sensitive to external vibrations, the uniform background colour is difficult to adjust when the wind tunnel is running, but the fringe formation can be observed on the hologram surface so that it is possible to adjust the uniform background colour with the wind tunnel operating. Fig. 12 shows three of twelve interferograms covering about a period of the vortex street. They are recorded in infinite fringes. The interferogram colours are well saturated and of higher contrast than those obtained in previous experiments performed

When the background colour is uniform, it is very easy to follow the vortices emitted from the upper and lower side. For instance, if one looks at the colours coming out in the vortex cores, one can easily see that the first vortex emitted from the upper side enters a formation phase where the gas density decreases in the vortex centre. A second phase of dissipation is

9 nm

**Green line : DE > 35%**

0 0.1 0.2 0.3 0.4 0.5

**1-(In/Is)**

625 635 645 655 665 **(nm)**

13 nm

**Red line : DE > 35%**

400 450 500 550 600 650 700  **(nm)**

8 nm

**Blue line : DE > 35%**

0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

**Transmitted normalized intensity**

Fig. 12. Interferogram analysis : instantaneous and average gas density field – t = 117s

observed looking at the last vortex leaving the observed field. Finally, as with transmission holographic interferometry, each colour represents a value of the gas density. In analysis, the gas density field was referred to *<sup>0</sup>*, the stagnation gas density. One can see that the instantaneous gas density varies from 0.70 to 0.98. The average gas density in the field has been calculated from twelve successive interferograms. The interferogram number is not very significant, but the obtained field is already symmetrical enough and the gas density varies from 0.72 to 0.97. Finally, if the colour scale of interference pattern is very well known to the user, the image of interferograms is sufficient to correctly evaluate the evolution of the gas density field.
