**7. Conclusion**

24 Advanced Holography – Metrology and Imaging

Colour interferences fringes and gas density field are shown in Fig. 17 for the first three

Fig. 17. Evolution of colour interference fringes and gas density field – Mach 0.45

 

phase maps with following the following relationship :

by averaging the 8 maps of instantaneous gas density field.

**6.4 Comparison between holographic plate and digital holograms** 

decreases to 73% of

The intensity of the interference fringes is computed on the three channels R, G, B from the

I A (1 cos( )

The gas density measured to the cylinder nose is particular as the gas density is equal to the stagnation gas density though the position of the vortex street, that means the colour found at this point has to be the same on each interferogram. Here, the intensity of colour interferences fringes is computed by imposing the white colour ( = 0 m) on each interferogram. Note this shifting is only made possible by the use of colour in the experiments. The time evolution of the gas density fields shows that the gas density

As regards previous results obtained, silver-halide plate and digital holographic interferometry can be compared. The only possibility to compare plate and digital interferograms is to compare the interferograms displaying the interference fringes. Indeed, the technique of holographic interferometry in real time using panchromatic plates directly displays the colour density variations of the flow. It's a light intensity information that is obtained. With digital holography, the three monochromatic intensity maps are superimposed to obtain a colour map of the intensity of the interference fringes. This map

 

*io* in the vortex core. Then, the averaged field of one cycle is calculated

(10)

images of one cycle of the vortex street.

The possibilities of image and digital colour holographic interferometry have been demonstrated. Colour holographic interferometry using panchromatic plates will continue to be used due to the high resolution of holographic plates. In near future, digital threewavelength holographic interferometry seems the best candidate to characterize the future complex flows. Although CCD resolution and size are not as good as that of holographic plates, the digital approach is more accessible and versatile since the time for the hologram processing is greatly reduced and the processing is purely numerical. On the other hand, the value of using colour has been demonstrated as the zero order fringe can be easily determined and the variation in the background colour due to disturbances can be quantified. The limitations of the digital method seem to lie in the wide spectral sensitivity of the sensor which produces light diffusion in each monochromatic hologram. Work is currently in progress for removing the colour diffusion using a segmentation approach. Success in this strategy will allow increasing the spatial resolution in the reconstructed object. Future work will focus on the extension of the proposed technique for analyzing 3D

Real-Time Colour Holographic Interferometry (from Holographic Plate to Digital Hologram) 27

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unsteady wake flows. At present, a specific setup of digital holographic interferometry has been defined in a single sight direction, and the aim will be to reproduce the same optical setup along several sight directions, each shifted by a given angle. It is obvious that the optical setup can be reproduced no more than three or four times. But the lack of sight directions should be compensated by high tomographic interferogram resolution for the reconstruction of the 3D gas density field.
