**6. Conclusion**

This chapter has shown several possibilities of digital holographic interferometry for analysing high‐density gradients encountered in transonic and supersonic flows. Concerning the analysis of a small supersonic jet, a comparison is given between three different techniques, two techniques use reference waves: Michelson holographic interferometry and digital holography using Wollaston prisms; the last one uses a specific diffraction grating to obtain several different diffractions of measurement waves and to avoid having the reference wave.

For analysing transonic flows in wind tunnel, two types of interferometer have been devel‐ oped. The first one is very simple to implement because it is a Michelson interferometer with double crossing of the test section for increasing the sensitivity and the second one is a Mach‐ Zehnder interferometer, more difficult to adjust, with a single crossing of the test section. These two interferometers are equipped with a pulsed laser and interferograms obtained have a very good quality and, basically, no phase shift.

Finally, a digital holographic method is proposed to visualize and measure refractive index variations, convection currents, or thermal gradients, occurring inside transparent but a strongly refracting object. The principle of this technique is provided through the visualization of refractive index variation inside a lighting ampoule. Comparisons with image transmission and reflection holographic interferometry demonstrate the high image and phase quality that can be extracted from the stochastic digital holographic set‐up.

Currently, digital holographic interferometry is developed by ONERA for studying 3D flows from multi directional tomographic interferograms recorded in several directions. The aim is first to compare this method with other techniques yielding the gas density field as differential interferometry, back‐oriented schlieren (BOS) and colour BOS; and secondly, to find the best compromise between the number of sight of view, the computation time and the results accuracy.
