Digital holography is a well-established technique for tracer particle studies in a flow. Unfortunately, its great depth of focus increases the uncertainty along the Z axis. This drawback becomes very significant when the concentration of seeding particles increases. This is the case in real installations such as wind or water channel flows. In such cases, observing the whole volume, and therefore all the particles, and tracking individual particles in order to determine their motions becomes an awesome task. Getting a hologram of the whole scene is impossible with the existing photographic sensors. The solution offered by the present day technologies and academic studies is to take several holograms and synthesize the whole scene hologram. However, for getting individual particle behavior, it is necessary to make several processing steps, makes processing very complicated if the number of seeding tracer particles is relatively high, on one hand and on the other hand, the correlation between dynamic characteristics could not highly established. In such cases, the technique of one hologram is highly recommended. In order to overcome these problems, we propose here a new technique based on combining the two instantaneous orthogonal views and hologram aperture reduction (opposed to aperture synthesis). To establish the technique, we investigate a little volume with a reduced number of particles, and the development and experimental results are presented. We focus on the development and experimental application of this technique.
This paper proposes a quality assessment of focusing criteria for imaging in digital off-axis holography. In literature,
several refocus criteria have been proposed in the past to get the best refocus distance in digital holography. As a general
rule, the best focusing plane is determined by the reconstruction distance for which the criterion function presents a
maximum or a minimum. To evaluate the robustness of these criteria, a set of thirteen criteria is compared with
application on both amplitude and phase images from off-axis holographic data. Experimental results lead to define
general rule and to exhibit the most robust criteria for accurate and rapid refocusing in digital holography.
This paper proposes an approach based on two orthogonal views and two wavelengths for recording off-axis two-color holograms. The approach permits to discriminate particles aligned along the sight-view axis. The digital processing to get images from the particles is based on convolution so as to obtain images with no wavelength dependence. In order to get the images of particles in the 3D volume, a calibration process is proposed and is based on the modulation theorem to perfectly superimpose the two views in a common XYZ axis. The experimental set-up is applied to two-color hologram recording of moving non-calibrated opaque particles with average diameter at about 150μm. After processing the two-color holograms with image reconstruction and view calibration, the location of particles in the 3D volume can be obtained. Particularly, ambiguity about close particles, generating hidden particles in a single-view scheme, can be removed to determine the exact number of particles in the region of interest.
Characterizing tracer micro particles in fluids is of a great challenge for digital holographic techniques. The real
locations and the number of these particles are the main parameters in such studies. For the first parameter, holographic
techniques are very useful, unfortunately, they suffer from the large depth of focus which increases the location
uncertainty of the particles. To minimize this uncertainty, we proposed a two orthogonal views system which, from our
point of view, makes the location more precise by crossing the two views data in the reconstruction process. For the
second parameter (particle number), off-axis configuration is recognized to be more convenient for large particle
numbers than the in line configuration. In order to validate the effectiveness of the off-axis configuration in terms of
number of tracer particles, we carry out some experiments. The number of particle was increased continuously after each
recording. We have also tried to keep unchanged the experiment conditions during all the recording process. In the
present work, we describe the manner in which the experiments were conducted and the obtained results in term of
diffraction efficiency of the reconstructed holograms.
Adressable spatial light modulators with as much as possible ideal phase modulation are the precondition for their application in digital holography. An adapted driver electronics for the modulator
and a correct knowledge of the modulation behavior can lead to a dynamic phase modulating device with nearly linear characteristic curve and a maximum phase range of 2π. We show a system for recording and reconstruction of digital holograms applying a spatial light modulator for the optical reconstruction and the digital processing of the holograms. The data of a CCD-camera are taken to a PC and sent to a spatial light modulator. In that sense we realised an
analog-digital converter for recording and a digital-analog converter for the optical hologram reconstruction. We discuss the resolution of the reconstruction and their applications, especially possibilities for the manipulation with the reconstructed wave field.
Continuous Paul Wavelet is a suitable tool for direct phase distribution evaluation in the case of digital interferograms. The method is based on the correlation in the Fourier domain between the digitized ineterferogram and the optical Paul wavelet filter spectrums. This correlation can be made directly with a computer or in optical set-up using an addressable liquid crystals display. We present the technique and the obtained results on simulated interferograms.
One of the most important properties of a laser resonator is the highly collimated or spatially coherent nature of the laser output beam. The spatial beam quality of the output beam, namely beam diameter and propagation factor M2 are critical parameters in a wide range of practical laser applications. This is because the spatial beam quality determines how tightly the beam can be focused or how well the beam propagates over long distances without significant spreading. It was investigated by many authors in previous years how to define and on how to measure the laser beam quality. An ISO working Draft Committee has also been organized to set-up a standard for definitions and test methods of the laser beam quality. In this work, the quality factor is studied with different methods.
For the application of interferometric methods in industrial quality control and particularly in non-destructive material testing an extensive analysis of the obtained image data is needed. In the field of optical image processing wavelet transformation has been proved to be a capable tool in the detection of structures with definite spatial resolution. The selection of the analyzing wavelet function and the variation of parameters lead to a wide range of selective wavelet filters, which are able to perform detection and classification of structures. The presented paper demonstrates numerical simulations and their optical realization in an SLM- based correlator. To locate defect patterns of different classes in interferometric fringe patterns, several wavelet functions, which are optically realizable by a transmission distribution, were utilized. The optical experiments indicate a good accordance with the computer simulations.