DIGITAL THREE-COLOR HOLOGRAPHIC INTERFEROMETRY DEVOTED TO FLUID MECHANICS

摘要

This paper presents work and results performed with LAUM collaboration in digital three-color holographic interferometry applied to Fluid Mechanics. In this method, three different wavelengths are used as luminous light source of the interferometer and the optical setup generates three micro interferences fringes which constitute three spatial carrier frequencies. When these images are recorded with a color sensor, the resolution of reconstructed hologram depends on the pixel size and pixel number of the sensor used for recording and also, the shape and the overlapping of three filters of color sensor influence strongly the three reconstructed images. This problem can be directly visualized in 2D Fourier planes on red, green and blue channels. To better understand this problem and to avoid parasitic images generated at the reconstruction, three different sensors have been tested : a CCD sensor equipped with a Bayer filter, a Foveon sensor and a 3CCD sensor. The best results have been obtained with the last one. In the recording principle, interference micro fringes produced by the superimposition of three reference waves and three measurement waves can be simultaneously recorded on the three spectral bands (red, green, and blue). Phase and amplitude images are computed using 2D Fourier transform in delayed time. Spectral filtering is applied on each Fourier plane in order to eliminate the parasitic diffraction orders. Then, phase differences are obtained by subtracting the reference phase to the probe phase. Several optical setups were tested and the best configuration allows the visualization of field about 70mm and increases the sensitivity since the measurement wave crosses twice the test section. Interferences induced by the wake flow have been recorded and intensities have been computed from the phase differences. Finally, one shows that fringes obtained with this process are those found with real-time color holographic interferometry using classical holographic plates.