This paper discusses on holographic imaging based on six lateral phase derivatives. Six lateral derivatives are generated
by a high-resolution reflection mode holographic grating that is designed in a “kite” configuration. The integration of the
derivative yields the phase and the optical thickness. Demonstration of the proposed approach is carried out for the case
of the analysis of the supersonic flow of a small vertical jet, 5.56mm in diameter.
This paper presents a digital holographic interferometer based on polarization holography. The setup was developed to analyze high density gradient flows. The set-up is less sensitive than a classical Michelson configuration since the test section is crossed only one time. Furthermore, the shadow effect limiting the other techniques is widely reduced. By using an astigmatic configuration, spatial carrier frequencies can be adjusted, so that a single shot and real time capability is obtained. Experimental results for a small supersonic jet when the injection pressure varies demonstrate the proposed approach.
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.
We present a simple set-up for digital color holography in which the reference beam has a unique way and the recording
uses a stacked photodiode sensor. A dedicated algorithm allows the color object to be reconstructed along each channel.
Experimental results confirm the proposed approach.
The feasibility of Real-time Colour Holographic Interferometry (RCHI) has been shown for several years at ONERA to analyze high speed two-dimensional flows. The light source of the interferometer is made of three different wavelengths (one red, one green and one blue) and panchromatic holograms are recorded by transmission. The method has been successfully applied in the ONERA wind tunnel to analyze the two dimensional wake flow around a circular cylinder. High speed interferograms of the unsteady wake flow have been obtained at a high framing rate of 35,000 frames per second. The authors propose to extend this method for analyzing three dimensional flows. In order make that, Real-time Colour Denisyuk Holographic Interferometry (RCDHI) has been developed. The feasibility of the optical setup is shown in one direction sight, the aim being to reproduce the same optical setup along several sight directions, each shifted by a given angle. Contrary to the optical setup developed for the analysis of the 2D flows, in the one proposed for 3D flows, reflection holograms are used. In the case of reflection holograms, the diffraction efficiency is strongly influenced by the variations in the gelatine thickness produced during the holograms treatment. Solutions are proposed to control the gelatine shrinkage and the first results obtained in wind tunnel are shown in one sight of view. High speed interferograms of unsteady wake flow around a circular cylinder have been obtained in narrowed fringes and uniform background at Mach 0.45.
A new optical technique based on real time holographic interferometry in trues colors has been implemented around the transonic wind tunnel of the ONERA-Lille center to analyze 2D unsteady wake flows. Tests realized in color interferometry, real time and double exposure, use simultaneously three wavelengths of a continuous waves laser (argon and krypton mixed) and holograms are recorded on silver-halide single-layer panchromatic Slavich PFGO3c plates. The very principle of real-time true color holographic interferometry uses three primary wavelengths (red, green, blue) to record, under no-flow conditions, the interference among the three measurement beams and the three reference beams simultaneously on a single reference hologram. After the holographic plate is developed, it is placed on the test setup again in the position it occupied durng exposure and the hologram is illuminated again by the three reference beams and three measurement beams. A flat, uniform color can then be observed behind the hologram. So a horizontal, vertical, or even circular fringe pattern can be formed and the achromatic central white fringe can be made out very clearly. This single color is used to determine the path difference zero on the interferograms. The flow studied was the unsteady flow downstream of a cylinder placed crosswise in the test section. A sequence of hundred interferograms was recorded on the flow around the cylinder at Mach 0.37. The vortex formation and dissipation phases can a be seen very clearly, along with the fringe beat to either side of the cylinder.
Some experiments concerning color holography and color holographic interferometry have been conducted at ISL. In a first stage, static holograms have been recorded with c.w. lasers in PFG-03C silver-halide panchromatic holographic plates. Color holographic interferometry is at present experimented and developed. Quantitative measurements by double-exposure holography are not possible. An example, on a cantilever plate, showing the achromatic fringe, is visualized on the reconstructed image. Quantitative deformations of the plate are presented. An experimental set-up designed for the real-time holographic interferometry of phase objects is also presented. A video film, recorded at 25 frames per second and relating laboratory experiments on some fluid mechanics flows, is shown. New difficulties occur when these experiments are carried out by using pulsed lasers. Our first tests on this technique are presented. Three pulsed lasers have been used: a ruby laser, a frequency doubled YAG laser and a frequency doubled titanium-sapphire laser. Ruby and YAG lasers, delivering red and green waves, are commercially available. A special frequency doubled titanium-sapphire laser, built as ISL produces the blue line. Preliminary results are presented and discussed.
A new optical technique based on color holographic interferometry is presented in this paper. Experiments on color interferometry in real time and by double exposure are carried out using three coherent wavelengths (blue, green and red) produced by c.w. lasers. Holograms are recorded on single-layer panchromatic silver-halide Slavich PFG 03C plates. As an example, a photograph of a colored Lippmann- Denisyuk interferometric hologram is presented, showing the mechanical deformations of a metallic sample. A video film on real-time trichromatic interferometry is presented. The video film, recorded at 25 frames per second, shows colored interference fringes produced by phase objects.
In this paper we relate our preliminary experiments on real- time trichromatic holographic interferometry. For this purpose a CW `white' laser (argon and krypton of Coherent- Radiation, Spectrum model 70) is used. This laser produces about 10 wavelengths. A system consisting of birefringent plates and polarizers allows to select a trichromatic TEM00 triplet: blue line ((lambda) equals 476 nm, 100 mW), green line ((lambda) equals 514 nm, 100 mW) and red line ((lambda) equals 647 nm, 100 mW). In a first stage we recorded a trichromatic reflection hologram with a separate reference beam on a single-layer silver-halide panchromatic plate (PFG 03C). After processing, the hologram is put back into the original recording set-up, as in classical experiments on real-time monochromatic holographic interferometry. So we observe interference fringes between the 3 reconstructed waves and the 3 actual waves. The interference fringes of the phenomenon are observed on a screen and recorded by a video camera at 25 frames per second. A color video film of about 3 minutes of duration is presented. Some examples related to phase objects are presented (hot airflow from a candle, airflow from a hand). The actual results show the possibility of using this technique to study, in real time, aerodynamic wakes and mechanical deformation.
The visualization technique presented herein is based on white light differential interferometry using large Wollaston
biprisms. The particularity of our set-up is that it yields two instantaneous interferograms, taken at precisely the same time,
on which the interference fringes are differently oriented. Thus, the instantaneous fields of two different partial derivatives of
density are simultaneously recorded and it is possible, through the use of both, to obtain the value of the density in domains
where this cannot be done using a single interferogram. This method has been successfully tested in the two-dimensional
unsteady flow past a cylinder.