Digital color holography applied to acoustics and mechanics

Digital holography became properly available since its confirmation was established in 1994. In few years, a lot of spectacular applications have been demonstrated such as microscopic imaging and phase-contrast digital holographic microscopy, three-dimensional object recognition and securing information, vibrations analysis with pulsed lasers and time averaging, also multidimensional dynamic investigations and digital color holography. This paper presents recent developments in digital color holography and especially focuses on its applications in the field of acoustics and mechanics. After a brief review on how the simultaneous recording of digital color holograms is made possible, we will discuss about the different opportunities to reconstruct these holograms. In order to compensate for the wavelength dependence of the pixel pitch in the Fresnel transform and to save the physical horizon of the object, we present different reconstruction strategies based on the Fresnel transform, spectrum scanning and convolution with adjustable magnification. The close relation that exists between the optical phases encoded in holograms and the displacement at the surface of any structure will be discussed, leading to the design of a simultaneous and full field three-color sensing probe having the capability of measuring three dimensional displacement fields. The principle of the three-chromatic holographic sensing is then adapted to structural mechanics and acoustics. Illustrations of the proposed method concern applications in flow measurements at Mach 0.45, 3D surface acoustic wave visualization and measurement, and crack investigation in composite materials. Since such new scientific instrument uses several laser wavelengths, attention must be paid to the error-free holographic reconstructions. In this context, we will discuss about origin of chromatism in digital color holography and propose a robust and simple method to compensate for such aberrations.