Daniel Allano

Digital in-line holography: influence of the shadow density on particle field extraction

We have used a digital in-line holography system with numerical reconstruction for 3D particle field extraction. In this system the diffraction patterns (holograms) are directly recorded on a charge-coupled device (CCD) camera. The numerical reconstruction is based on the wavelet transformation method. A sample volume is reconstructed by computing the wavelet components for different scale parameters. These parameters are related to the axial distance between a particle and the CCD camera. The particle images are identified and localized by analyzing the maximum of the wavelet transform modulus and the equivalent diameter of the particle image. The general process for the 3D particle location and data processing method are presented. As in classical holography we found that the signal to noise ratio depends only on the shadow density. Nevertheless, we show that both the volume depth and the shadow density affect the percentage of extracted particles.

Near-field Lorenz-Mie theory and its application to microholography

The full near-field Lorenz-Mie theory is described and applied to Gabor microholography to predict the intensities that are recorded on a microholographic plate. The influence of light polarization and the complex refractive index are discussed. Then, the full near-field Lorenz-Mie theory is compared with the Fraunhofer, the Fresnel, and the far-field Lorenz-Mie theories. Criteria of validity of the simplified theories are deduced.

Digital in-line holography for three-dimensional?two-components particle tracking velocimetry

We have used a digital in-line holography system with numerical reconstruction to determine 2D velocity fields in several slices of a sample volume. This system records directly on a charge-coupled device (CCD) camera the diffraction patterns of small particles illuminated by a double-pulse laser diode. The numerical reconstruction is based on the wavelet transformation method. A slice is reconstructed by computing the wavelet components for different scale parameters. These parameters are related to the axial distance between a particle and the CCD camera. The particle images are identified and localized by analysing the maximum of the wavelet transform modulus and the width of the particle image (L50). Afterwards, a point-matching algorithm is applied to the set pairs containing the particles. This step is followed by velocity vector extraction. The details of the velocity extraction and the data processing method are presented and the simulations and experimental results are discussed.

Application of multiple exposure digital in-line holography to particle tracking in a Bénard–von Kármán vortex flow

Digital in-line holography is applied to studying the trajectories of individual water droplets in airflow. In order to track the particles, multiple exposure holography is performed using a modulated laser diode emitting at the wavelength of 635 nm and a lens-less CCD camera. This method leads to an accuracy better than 100 µm on the axial location. A study of the signal-to-noise ratio of such holograms shows that the number of exposures must be limited. Preliminary tests of this method are carried out in a Benard–von Karman street first characterized by laser Doppler velocimetry and hot wire anemometry. An example of a trajectory of a water droplet obtained in this flow at Reynolds number Re = 63 and Strouhal number St = 0.13 shows that digital holography is a promising method to extract the trajectories of droplets in laminar or turbulent flows.

Formulation of in-line holography process by a linear shift invariant system: application to the measurement of fiber diameter

Abstract In-line holograms of glass fibers are digitally recorded during the manufacturing fiberization process. The numerical reconstruction is realized by a wavelet based method. We show that the recording–reconstruction process can be interpreted as a linear shift invariant system with a Gaussian point-spread function. A demonstration is proposed and the result is illustrated by numerical simulations. This new interpretation is of a great interest because a reconstructed image can be viewed as a convolution operation of the object function with a predefined point-spread function which is not dependent on the recording axial distance. Experimental results are provided for the diameter measurement of glass fibers.

Size measurement of bubbles in a cavitation tunnel by digital in-line holography

Digital in-line holography (DIH) with a divergent beam is used to measure size and concentration of cavitation bubbles (6-100 μm) in hydrodynamic facilities. A sampling probe is directly inserted in the cavitation tunnel, and the holograms of the bubbles are recorded through a transparent test section specially designed for DIH measurements. The recording beam coming from a fiber-coupled laser diode illuminates the sample volume, and holograms are recorded by a CMOS camera. From each hologram, the sampling volume can be reconstructed slice by slice by applying a wavelet-based reconstruction method. Because of the geometry of the recording beam, a magnification ratio must be introduced for recovering the 3D location and size of each bubble. The method used for processing holograms recorded in such a configuration is presented. Then, statistical results obtained from 5000 holograms recorded under different pressures in the cavitation tunnel are compared and discussed.

Three-dimensional velocity near-wall measurements by digital in-line holography: calibration and results

Velocity measurements in the vicinity of an obstacle remain very complicated even when optical diagnostics based on displacement of micrometric tracers are considered. In the present paper, digital in-line holography with a divergent beam is proposed to measure the three-dimensional (3D) velocity vector fields in a turbulent boundary layer and, in particular, on the near wall region of a wind tunnel. The seeding droplets (1-5 μm) transported by a turbulent airflow are illuminated by a couple of laser pulses coming from a fiber coupled laser diode. These double exposure holograms are then recorded through a transparent glass reticle specially designed for this application with an accurate surface positioning combined with a particularly attractive in situ calibration method of the investigation volume (less than 10 mm(3)). The method used for processing holograms recorded in such a configuration is detailed. Our original calibration procedure and the assessment of its accuracy are presented. Our holographic probe has been tested in a wind tunnel for a large range of different velocities. Then 3D velocity vector fields extracted from more than 13000 holograms are analyzed. Statistical results show the capability of our approach to access in a turbulent boundary layer. In particular, it leads to relevant measurements for fluid mechanics such as velocity fluctuation and the shear stress in the very close vicinity of a wall.

Processing of glass cylinder diffraction patterns scanned with a photodiode array: influence of the optical transfer function of diodes on dimensional measurements

Far-field diffraction patterns of a glass cylinder are spatially sampled with a photodiode array. Three photometric signal processing methods are discussed. The first one is based on the analysis of fringes in the central lobe. It can be used for diameter monitoring, but not for a wide range of diameter measurements. The second method consists of best-fitting between the theoretical model of the irradiance distribution in the central fringe and the corresponding experimental data. The accuracy of this method (±1 μm, in the range 10 to 50 μm) is improved by introducing the optical transfer function (OTF) of the photosensitive area into the calculation. The third method uses a reliable estimation of the intensity at the center and the half-width of the central fringe in order to resolve the inverse problem by Newtons method. This method can be used for realtime measurements but must be improved by statistical approaches.

Long time exposure digital in-line holography for 3-D particle trajectography

One advantage of digital in-line holography is the ability for a user to know the 3-D location of a moving particle recorded at a given time. When the time exposure is much larger than the time required for grabbing the particle image at a given location, the diffraction pattern is spread along the trajectory of this particle. This can be seen as a convolution between the diffraction pattern and a blurring function resulting from the motion of the particle during the camera exposure. This article shows that the reconstruction of holograms recorded under such conditions exhibit traces that could be processed for extracting 3D trajectories.

Dual-wavelength digital holography for 3D particle image velocimetry: experimental validation

A multi-exposure digital in-line hologram of a particle field is recorded by two successive pulses of different wavelengths. During the reconstruction step, each recording can be independently analyzed by selecting a given wavelength. This procedure enables avoiding the superimposition of particle images that may be close to each other.