**Acknowledgements**

**Figure 29** shows a comparison between results obtained with digital holography and those obtained with image holography. **Figure 29a** shows the image obtained with the amplitude and phase change measured by digital holography, after calculating the intensity = (1 + cos()), where is the phase change and *A* the amplitude image. **Figure 29b** shows the interference fringes obtained with the set‐up of **Figures 28a** and **29c** shows those obtained with the set‐up of **Figure 28b**. A very good agreement can be observed. Furthermore, the image quality given by each method can be appreciated. Image holography provides the best spatial resolution: the strand of the lamp can be clearly seen in **Figure 29b** and **c**. However, digital holography is more flexible since no chemical processing is required and a phase image can

**Figure 29.** Comparison between intensity of fringes (a) fringes calculated with digital holography, (b) fringes obtained

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

with transmission holography and (c) fringes obtained with reflection holography.

be obtained.

314 Holographic Materials and Optical Systems

**6. Conclusion**

good quality and, basically, no phase shift.

The authors thank the French National Agency for Research (ANR) for funding this work under grant agreement no. ANR‐14‐ASTR‐0005‐01.
