**1. Introduction**

In‐line and off‐axis digital holographic interferometry is now became an optical metrological tool more and more used in the domain of fluid mechanics [1]. For instance, it is widely developed in macro‐ or microscopy for measuring in the flow the location or size of particles [2, 3] or for measuring the temperature or the thermal exchanges in the flames [4, 5]. Other authors have developed digital colour holographic interferometry by using three different wavelengths (one red, one green and one blue) as a luminous source. Qualitative results have

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

been obtained for visualizing convective flows induced by the thermal dissipation in a tank filled with oil [6]. Quantitatively, the feasibility of three‐wavelength digital holographic interferometry has been demonstrated for analysing the variations in the refractive index induced by a candle flame [7] and the technique has been applied in wind tunnel on two‐ dimensional unsteady flows where the time evolution of the gas density field has been determined on the subsonic near wake flow downstream a circular cylinder [8]. But, when the flow regime reaches the transonic or supersonic domain, problems appear because refractive index gradients become very strong and a shadow effect is generated by the shock waves, for instance, superimposes to the micro‐fringes of interferences. Phase shifts appear and limit the interferogram analysis.In orderto solve these different problems, the authors propose to study three different cases of flows presenting high‐density gradients using specific optical techni‐ ques based on digital holography. The first one concerns a small supersonic jet analysed by Michelson colour digital interferometry, colour holographic interferometry using Wollaston prisms and monochromatic digital holography without reference wave. The second case is to compare Michelson and Mach‐Zehnder interferometers for analysing the unsteady wake flow around a circular cylinder at transonic Mach number. And finally, digital and image holo‐ graphic methods are presented to visualize and measure the refractive index variations, convection currents or thermal gradients occurring inside a transparent and strongly refract‐ ing object. In the case of image holographic interferometry, a comparison with transmission and reflection holograms is provided.
