Holographic particle image velocimetry: analysis using a conjugate reconstruction geometry

Holographic recording techniques have recently been studied as a means to extend two-component, planar particle image velocimetry (PIV) techniques for three-component, whole-field velocity measurements. In a similar manner to two-component PIV, three-component, holographic PIV (HPIV) uses correlation-based techniques to extract particle displacement fields from double-exposure holograms. Since a holographic image contains information concerning both the phase and the amplitude of the scattered field it is possible to correlate either the intensity or the complex amplitude. In previous work we have shown that optical methods to compute the autocorrelation of the complex amplitude are inherently more tolerant to aberrations introduced in the reconstruction process, Coupland, Halliwell, Proc. Roy. Soc. 453 (1960) (1997) 1066. In this paper we introduce a new method of holographic recording and reconstruction that allows a constant image shift to be introduced to the particle image displacement. The technique, which we call conjugate reconstruction, resolves directional ambiguity and extends the dynamic range of HPIV. The theory of this method is examined in detail and a relationship between the image and object displacement is derived. Experimental verification of the theory is presented.     

Some characteristics of relief formation on photothermoplastic carriers used in double-exposure interferometry

An analysis is made of the main trends in the development of a surface relief on photothermoplastic carriers of optical information used to record double-exposure holographic interferograms. It is shown that high-quality interferograms require recording conditions (taking into account the composition of the material and the heating temperature of the thermoplastic carrier layer, as well as the parameters of the sensitizing corona discharge) where the surface of the photothermoplastic carrier can be treated in the equipotential approximation. Zh. Tekh. Fiz. 67, 76–78 (August 1997)     

Calculating and matching the spatial frequency spectrum for recording quasi-fourier holograms on photothermoplastic media

We propose a modification of the optics for recording Fourier holograms. We calculate the spatial frequency spectra of the interference field. We have studied the effect of the parameters of the objectives on the spectral matching, determined by the recording optics and the resonance type of transfer characteristic for the photothermoplastic recording medium. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 1, pp. 124–128, January–February, 2006.     

Image recording on photothermoplastic media with different thicknesses of the thermoplastic layer

The quality of images recorded on photothermoplastic media (PTPMs) with various thicknesses of the thermoplastic layer is investigated. It is found that the potential of a corona-forming electrode initiating deformations on a PTPM surface and the diffraction efficiency, corresponding to the additional maxima in its dependence on the spatial frequency, increase with decreasing thickness of the thermoplastic layer, in contrast to the diffraction efficiency at the resonance frequency. It is shown that the frequencies corresponding to additional maxima in the dependence of the diffraction efficiency on the spatial frequency are preferable for application as the carrier frequency for raster-scan recording and as the center carrier frequency for holographic image recording on PTPMs in order to reach a higher diffraction efficiency, image contrast, and signal-to-noise ratio than those attainable at the resonance frequency. An anomalous relation between the resolution and sensitivity of PTPMs is found under the suggested recording conditions and carrier frequencies. It is shown that both the resolution and the sensitivity of PTPMs increase with decreasing thickness of the thermoplastic layer.     

Diffraction efficiency and light-scattering power of photothermoplastic holographic gratings

The diffraction efficiency and light-scattering power of holographic gratings formed on the surface of a two-layer photothermoplastic carrier with a pit structure of deformation are studied. The influence of the height and regularity of the pit deformation on the diffraction efficiency and light-scattering power is studied. It is shown that the pronounced resonance character of the dependence of the diffraction efficiency on the spatial frequency in the case of pit deformation is related to the distribution of the deformation depth over spatial frequencies; however, the dominant factor is the mutual arrangement of pits within the interference fringe. It is found that the light-scattering power is determined for the most part by the size and density of the deformation. A new method is proposed for determining the resolution of photothermoplastic carriers with pit deformation, which apparently can be used for photothermoplastic carriers with any irregular structure of deformation. This method is based on the analysis of the dependence of the light-scattering power of holographic gratings on their spatial frequency. It is shown that the resolution depends inversely proportionally on the thickness of the thermoplastic layer of the optical carrier.     

Photothermoplastics for spectral holography

For spectral holography, the opportunity of using real-time information recording in situ using photothermoplastic (PTP) materials is considered. Applying the physical model of free charge carrier photogeneration for photothermoplastics (PTPs), based on charge transfer complexes, the photosensitivity is investigated considering its dependence on exposure time (exciting laser pulses duration). For sequential PTP processes, it is shown that the experimental values of the photosensitivity over a spectral range of 300–1000 nm is maintained down to picosecond exposures. This is in good agreement with theoretical calculations. It is concluded that, for the achieved values of photosensitivity and taking into account other characteristics of PTPs—such as real-time recording in situ, high values of space resolution (to 1000 line pairs mm⁻¹), diffraction efficiency (to 20%) and cyclability (to 1000 cycles)—PTPs are useful materials for laboratory investigations in the field of spectral holography.     

Visualization of blast waves created by exploding bridge wires

Blast waves created by small exploding bridge wires are used as a test bed for the development of a particle image velocimetry (PIV) technique that uses polymers, seeded with scattering particles, as dynamic witness plates. Combined with pulsed, incoherent schlieren photography, the PIV method permits visualization of the instantaneous velocity vector field in a plane cutting through the blast wave.     

Study of diffraction gratings for various profiles of the pit relief of a deformation

The influence of parameters of pit deformation on the diffraction efficiency η of relief phase diffraction gratings resulting from recording interference gratings in a simultaneous photothermoplastic process is studied. The depth of a pit, the height of a “breastwork,” and the height of the relief are considered among the parameters of the pit deformation. The quantity $\sqrt \eta The influence of parameters of pit deformation on the diffraction efficiency η of relief phase diffraction gratings resulting from recording interference gratings in a simultaneous photothermoplastic process is studied. The depth of a pit, the height of a “breastwork,” and the height of the relief are considered among the parameters of the pit deformation. The quantity is shown to be a linear function of the depth of a pit, the height of a breastwork, and the height of the relief in the actually (most often) used range of pit depths. The height of the relief produces the smallest effect on (the proportionality factor is 1), and the greatest effect is produced by the height of a breastwork (the proportionality factor is 2.5). The distribution of the height of the relief as well as the depth of a pit over spatial frequencies in the near-resonance region is found to be Gaussian. __________ Translated from Optika i Spektroskopiya, Vol. 90, No. 4, 2001, pp. 675–677. Original Russian Text Copyright ? 2001 by Panasyuk, Nastas.     

Particle image velocimetry in aerodynamics: Technology and applications in wind tunnels

Particle image velocimetry (PIV) is increasingly used for aerodynamic research and development. The PIV technique allows the recording of a complete flow velocity field in a plane of the flow within a few microseconds. Thus, it provides information about unsteady flow fields, which is difficult to obtain with other experimental techniques. The short acquisition time and fast availability of data reduce the operational time, and hence cost, in large scale test facilities. Technical progress made in the last years allowed DLR to develop a reliable, modular PIV system for use in industrial wind tunnels. The features of this system are summarized and results of recent PIV applications are presented.     

Experimental and numerical investigations of resonant vibration characteristics for piezoceramic plates.

Electronic speckle pattern interferometry (ESPI) is a full field, non-contact technique for measuring the surface displacement of a structure subjected to static loading or, especially, to dynamic vibration. In this article we employ an optical system called the amplitude-fluctuation ESPI with out-of-plane and in-plane measurements to investigate the vibration characteristics of piezoceramic plates. Two different configurations of piezoceramic plates, namely the rectangular and the circular plates, are discussed in detail. As compared with the film recording and optical reconstruction procedures used for holographic interferometry, the interferometric fringes of AF-ESPI are produced instantly by a video recording system. Because the clear fringe patterns will be shown only at resonant frequencies, both the resonant frequencies and the corresponding mode shapes are obtained experimentally at the same time by the proposed AF-ESPI method. Excellent quality of the interferometric fringe patterns for both the in-plane and out-of-plane vibration mode shapes is demonstrated. The resonant frequencies of the piezoceramic plates are also measured by the conventional impedance analysis. From experimental results, we find that the out-of-plane vibration modes (type A) with lower resonant frequencies cannot be measured by the impedance analysis and only the in-plane vibration modes (type B) will be shown. However, both the out-of-plane (bending) and in-plane (extensional) vibration modes of piezoceramic plates are obtained by the AF-ESPI method. Finally, the numerical finite element calculations are also performed, and the results are compared with the experimental measurements. It is shown that the numerical calculations and the experimental results agree fairly well for both the resonant frequencies and the mode shapes.     

Sustained simultaneous high-speed imaging of scalar and velocity fields using a single laser

A high-speed technique that combines planar laser induced fluorescence (PLIF) detection of biacetyl and particle image velocimetry (PIV) for simultaneous imaging of scalar and velocity fields is demonstrated at a frame rate of 12 kHz for up to 32500 consecutive frames. A single diode-pumped, frequency-tripled Nd-YAG laser was used for excitation. Wavelength-separated recording was achieved for Mie scattering from silicone oil droplets with a CMOS camera and for the red-shifted fluorescence from biacetyl with an image-intensified CMOS camera. Interference between PIV and PLIF tracers was found to be negligible. Cross-talk between PIV and PLIF signals was low and a strategy to completely eliminate it was devised and is discussed. The signal-to-noise ratio is about 9 for single-shot scalar images. Example image sequences were recorded in an atmospheric pressure air jet at Re=2000. PACS 42.62.Fi; 33.50.Dq; 06.30.Gv; 06.60.Jn     

Improvement of the sensitivity of measurements in the formation of holographic interferograms of wedge plates

We consider the method for improving the sensitivity of measurements in the formation of aberration-free interference images of wedge plates, which do not require a priori knowledge of the position of the wedge edge. It is shown that the sensitivity in the measurement of the wedge angle in real time interferometry is doubled in the case of recording on the reference hologram of the wedge plate followed by its reconstruction by the object light wave that has passed through the wedge plate rotated through 180° relative to the normal. It turns out that recording of a pair of holograms of the wedge plate, one of which is the image of the wedge plate turned through 180° relative to the normal to the plate, makes it possible to increase the sensitivity of measurements by more than an order of magnitude by eliminating the systematic components of measuring errors due to complete compensation of aberrations. The results of experimental testing of the method are considered.     

Numerical investigation of the accuracy of particle image velocimetry technique in gas-phase detonations

We numerically investigate the accuracy of the Particle Image Velocimetry (PIV) technique for the flow characterization in high-speed, compressible regimes, in particular in gas-phase detonations. We carry out synthetic PIV reconstruction of the flow field in a two-dimensional, planar detonation propagating under atmospheric conditions and modelled using single-step Arrhenius kinetics. The flow is uniformly seeded with monodispersed Al₂O₃ particles with sizes 50 and 200 nm, along with initially co-located massless Lagrangian tracer particles. The effect of massive particles on the detonation speed and thermodynamic state of the flow is investigated and is found to be negligible. We further assess the ability of massive particles to sample the flow field and while it is found that 50 nm particles sample the flow field better than the 200 nm ones, they also exhibit significant clustering. By comparing the trajectories of massive particles with those of massless tracers, it is shown that almost all massive particles rapidly diverge from the actual flow pathlines. Finally, we quantify the accuracy of the PIV reconstruction of the velocity field in comparison with the actual velocity field in the numerical simulations. It is shown that while PIV is generally capable of capturing the bulk flow features in the streamwise direction, its accuracy is not sufficient to characterize the transverse velocity component or velocity fluctuations.     

近距离数字全息术记录和再现问题

讨论了记录距离小于菲涅耳衍射要求的近距离数字全息记录和再现问题。对全息记录与再现中高次相位的补偿问题进行了分析,证明了在CCD的参量和记录距离给定后,只要记录时使物体的大小、球面参考光波的位置和距离满足一定的条件,即使在记录距离小于菲涅耳衍射要求的最小距离情况下,也可将高次相位的影响补偿到足够小,使得近距离数字全息的数字再现仍可用快速傅里叶变换算法计算。推导出了满足高次相位补偿的条件和满足补偿条件时的数值再现计算公式。实验结果与理论分析的结论相吻合,并给出了一种修正实际记录的参考光和计算机模拟的理想参考光之间偏差的方法。     

数字重现无透镜傅里叶变换X射线全息的实验模拟研究

参照无透镜傅里叶变换（ＬＬＦＴ）Ｘ射线全息图的记录光路，采用可见光源模拟研究了无透镜傅里叶变换Ｘ射线全息从记录到数字重现的全过程，给出了实验结果，并对光学重现和数字重现结果作了比较，分析了该记录方式存在的一些问题，指出实际Ｘ射线全息记录中可直接使用波带板＋１级衍射而不必在物面加一滤波小孔。分析了利用数字重现消除记录过程中非线性效应的可行性。并着重指出波带板的多级衍射的互干涉对重现象的干扰，应设法消除。     

Randomly displaced phase distribution design for computer-generated binary hologram with narrow recording spots

A random phase mask is often used for Fourier transform holography to improve its recording and reconstruction characteristics. However, a conventional random phase pattern has the disadvantage of expanding the recording spot, which is caused by the high-resolution phase modulation. Previously, we proposed a random phase pattern that makes the recording spot narrower than the conventional one with maintaining a moderate reconstruction quality. In the present study, we apply the proposed random phase pattern to computer-generated binary holograms and evaluate both the hologram distribution and reconstruction quality in terms of practical holographic memory systems. The results confirm the effectiveness of the random phase in the reconstruction for an elementary data pattern.     

Nonvolatile photorefractive spectral holography

We demonstrate nonvolatile storage of femtosecond pulses in a photorefractive LiNbO(3) crystal with recording and readout of spectral holograms at wavelengths of 460 and 920 nm, respectively. No degradation was observed after 24 h of continuous readout. We also show that we can realize the time-lens effect with our system by appropriately setting the ratio of the recording and the reconstruction wavelengths and the spectral resolution of the recording and the reconstruction processes.     

Aberration limited resolution in Fraunhofer holography with collinated beams

The primary aberrations in standard in-line Fraunhofer holography with collimated beams of different recording and reconstruction wavelengths are described. Maréchal's criterion is then applied to establish the aberration limited size resolution of micro-objects. The resolution is described in terms of the recording and reconstruction wavelengths, the number of side lobes recorded, and the number of far-field distances. The particular case of a ruby laser recording with reconstruction by a HeNe laser is discussed in detail.     

Three-dimensional bubbly flow measurement using PIV

The experimental flow visualization tool, Particle Image Velocimetry (PIV), is being extended to determine the velocity fields in three-dimensional, two-phase fluid flows. In the past few years, the technique has attracted quite a lot of interest. PIV enables fluid velocities across a region of a flow to be measured at a single instant in time in the whole volume (global) of interest. This instantaneous velocity profile of a given flow field is determined by digitally recording particle (microspheres or bubbles) images within the flow over multiple successive video frames and then conducting flow pattern identification and analysis of the data. This paper presents instantaneous velocity measurements in various three-dimensional, bubbly two-phase flow situations. This information is useful for developing or improving existing computer constitutive models that simulate this type of flow field. It is also useful for understanding the detailed structure of two-phase flows.     

High resolution multigrid cross-correlation digital PIV measurements of a turbulent starting jet using half frame image shift film recording

This paper describes a novel approach of acquiring two single-exposed, non-overlapping images using half frame image shift (HFIS) recording on photographic film. This technique permits the recording of two single-exposed, non-overlapping images of seed particles in a flow plane on high spatial resolution film with any arbitrary time delay between exposures. A new multigrid CCDPIV (MCCDPIV) analysis method is used to analyse the single-exposed, non-overlapping sequential images resulting in PIV measurements with a larger velocity dynamic range, lower random error and better spatial resolution than standard CCDPIV analysis. HFIS recording followed by MCCDPIV analysis was employed to measure the spatio-temporal evolution of the in-plane velocity vector and the out-of-plane vorticity fields of a turbulent starting jet at Reynolds numbers based on the orifice diameter and piston velocity of 10,780 and 13,860.