Assessment of the strength of cathode ray tubes by time-averaged holographic interferometry

The application of time-averaged holography to the non-destructive testing of CRT faceplates is described. The antinodes revealed by the fringe patterns have been found to correspond to the weakest points on the faceplates. The relative strengths of different CRTs can be deduced from the number of fringes obtained when they are vibrated over a range of amplitudes. The information can aid designers of new types of CRT.     

Picosecond three-color holographic digital interferometry

The possibilities of multicolor digital holographic interferometry are experimentally studied upon measuring displacements of a surface in radiation of a picosecond Nd:YAG laser with the radiation frequency conversion into harmonics (λ₁ = 1.06 μm, λ₂ = 0.53 μm, and λ₃ = 0.35 μm). It is shown that three-color digital holographic interferometry makes it possible to increase the measurement accuracy of displacements. The peculiarities of multicolor digital holographic interferometry by picosecond pulses are discussed.     

Sensitivity and optimization in the determination of deformations by holographic interferometry

The use of a fringe-counting holographic interferometric technique for measuring small superficial displacements can present difficulties in determining the measurement inaccuracy. A useful general method is given here for the analysis of errors in displacement measurements. The optimum number and type of observation directions are determined and correspond to counting in the four semi-diagonals of the holographic plate. The technique, as applied to skull experiments, makes it possible to measure directly displacements of up to 20 μm, in the object plane parallel to the holographic plate. The associated errors range from 0.1–0.3 μm, for a counting inaccuracy of of a fringe and increments of traction of 100 g to 1 kg of force.     

Shape measurement by multi-illumination method in digital holographic interferometry

In this paper, temporal phase unwrapping and short time Fourier transform (STFT) are proposed for shape measurement of an object with height steps by digital holographic interferometry (DHI). A series of digital holograms is recorded with a high-speed CCD camera using a multi-illumination method. Each pixel is processed along the time axis independently of other pixels. Two novel methods are proposed to process the wrapped phase maps: the first method is based on complex phasor (CP) and temporal phase unwrapping, and the second method is based on CP and STFT. In the STFT method, continuous phase maps are obtained by integration of the exacted instantaneous frequency. The continuous phase map can characterize the profile of the object with height steps. An experiment is conducted to verify the validity of the proposed methods.     

Instantaneous velocity measurement of dynamic deformation by digital holographic interferometry

A novel method for the instantaneous velocity measurement of dynamic deformation by digital holographic interferometry is proposed. During dynamic deformation, a series of digital holograms is recorded by a high-speed camera. At each pixel of the phase difference maps, phase and amplitude information are combined as complex phasor (CP). Each pixel can be then considered as an independent sensor and a sequence of complex phasors of such a sensor is analyzed by short time Fourier transform (STFT) along the time axis. A fast iterative algorithm is developed for the computation of instantaneous velocity. The displacement of each pixel can also be obtained by integration of the instantaneous velocity over time and phase unwrapping process is thus avoided. The performance of the proposed CP method is compared experimentally with the commonly used digital phase subtraction method.     

Quantitative vibration amplitude measurement with time-averaged digital speckle pattern interferometry

A new method of using time-averaged digital speckle pattern interferometry for the quantitative measurement of vibration amplitude was developed. Signal processing techniques especially the Hilbert transformation for quantitative evaluation of the Bessel fringes obtained in time-averaged digital speckle pattern interferometry were explored. The quadrature signal after Hilbert transformation is equivalent to a 90° phase-shifted interferogram for a monotonically increasing or decreasing phase function. An algorithm was developed for Bessel fringe contrast enhancement and phase extraction. The techniques were tested numerically and experimentally. Sub-fringe quantification of the time-averaged vibration fringes is realised with the proposed method. Compared with the commonly used phase shift method which requires a minimum of two images for image processing, this method requires only one fringe pattern for data extraction.     

Pulsed digital holographic interferometry by using a flexible fiber endoscope

A system based on digital holographic interferometry in combination with a flexible fiber endoscope is described. A Q-switched pulsed laser is used. Two digital holograms of the test object, corresponding to the two laser pulses, are captured at separate video frames of a CCD-camera, transferred in a frame grabber and further processed in a PC. If the object undergoes a deformation during the interval between the two laser pulses (usually in the range of 5–600 μs), a fringe pattern will result from the difference between the two holograms. This fringe pattern has the information needed to evaluate quantitatively the amount of the deformation. A compact system has been developed to be used for various applications, both mechanical and biological, where measurements need to be performed at “hidden” surfaces or inside more or less closed objects. The quality of the results obtained by using mechanical objects is usually better than for biological objects. This can be explained easily by the fact that a biological surface is much more complex, in particular some parts of the surface may reflect the light well whereas some other parts may absorb it. Experimental results are presented.     

Determination of displacement derivative in digital holographic interferometry

Digital holography (DH) and digital shearography (DS) both play an important role in non-destructive evaluation. In this paper, a novel method based on digital holographic interferometry (DHI) and complex phasor (CP) is proposed to determine displacement derivative. An algorithm is employed to filter the imaginary and real parts of complex values without the need of direct phase manipulation. Two-dimensional short time Fourier transform (STFT) is employed subsequently to process wrapped phase maps. An experiment is conducted to demonstrate the validity of the proposed method.     

基于数字全息干涉的实时微振动检测

基于数字全息理论的方法,构建了一套微振动实时测量的实验系统。通过运用基于全息干涉图像的振动快速解算算法,直接从记录的全息图中提取振动信息,实现对振动物体振幅值进行实时测量。该系统的接收光为散射光,使得测量角度不受被测物体结构的影响。通过测量由标准正弦波驱动的被测物微振动情况,对该实验系统进行了实验验证,实验结果表明,该系统能够实时测量微振动的振幅值（算法处理时间约为0.11 ms）,在550～2700 Hz频段内被测物的振幅较为稳定,系统稳定性误差为1.66%,真实地反映了其振动情况。     

A digital phase-measurement system for real-time holographic interferometry

While holographic interferometry gives a good picture of the stress distribution over a complex structure, its use in stress-analysis has been limited by the fact that quantitative information on the deformation of the surface is directly available only at points on the fringe maxima or minima, and interpolation between these points is slow and not very accurate. An improved phase-measurement system for real-time holographic interferometry is described. This uses a diode array television camera to view the interference pattern, and digital electronics to calculate and store the phase difference at a 100 × 100 grid of points. This permits measurements of the phase with an accuracy estimated at ±2°.     

Dynamic measurement by digital holographic interferometry based on complex phasor method

In this paper, complex phasor (CP) method is employed in digital holographic interferometry. Unlike commonly used digital phase subtraction (DPS), the proposed technique processes a CP instead of phase. It is shown that the results obtained by directly filtering the phase produce large errors. It is demonstrated that the phase is not a signal but rather a property of a signal. In addition, the results obtained by the CP method are also compared with those obtained by conventional sine/cosine transformation method.     

Measurement of time dependent elliptical high-beta plasma deformations by means of holographic HF laser interferometry

Multi-wavelength holographic HF laser interferometry was developed and applied for the first time to measure plasma density profiles. An ellipticity of the plasma cross-section in the ISAR T1 theta-pinch-like high-beta stellarator was determined. Oscillations of the minor elliptical semi-axis were observed.     

Advances in real-time holographic interferometry for the measurement of vibrations and deformations

We describe the latest improvements in holographic interferometry that enable the real-time measurement of vibrational modes and static deformations in surfaces using low-power laser illumination and a photorefractive Bi₁₂TiO₂₀ crystal as the recording medium. An efficient setup has been developed where the most critical elements have been optimized: target illumination and backscattered light collection, distribution of light between the object and reference beams, and stabilization system operation. Experimental results for vibration and deformatiom measurement are reported.     

Deformation measurement by holographic interferometry. Analysis on curved surfaces. General aspects

Holographic Interferometry is briefly outlined in the case of large deformations. Modifications at the reconstruction are necessary to recover the previously invisible fringes. The spacing and contrast are characterized by the fringe and visibility vectors. The relevant first derivative of the path difference involves the polar decomposition of the deformation gradient and some affine connections. Owing to the modification, one must consider the aberration together with changes of geodesic curvatures and of surface curvatures. This leads to similar aspects for hypersurfaces with remarks concerning the general problem of the gravitational lens.     

Laser-ablation-induced refractive index fields studied using pulsed digital holographic interferometry

Pulsed digital holographic interferometry has been used to investigate the plume and the shock wave generated in the ablation process of a Q-switched Nd-YAG (λ=1064 nm and pulse duration=12 ns) laser pulse on a polycrystalline boron nitride (PCBN) target under atmospheric air pressure. A special setup based on two synchronised wavelengths from the same laser for simultaneous processing and measurement has been used. Digital holograms were recorded for different time delays using collimated laser light (λ=532 nm) passed through the volume along the target. Numerical data of the integrated refractive index field were calculated and presented as phase maps showing the propagation of the shock wave and the plume generated by the process. Radon inversion has been used to estimate the 3D refractive index fields measured from the projections assuming rotational symmetry. The shock wave density has been calculated using the point explosion model and the shock wave condition equation and its behaviour with time at different power densities ranging from 1.4 to 9.1 GW/cm² is presented. Shock front densities have been calculated from the reconstructed refractive index fields using the Gladstone–Dale equation. A comparison of the shock front density calculated from the reconstructed data and that calculated using the point explosion model at different time delays has been done. The comparison shows quite good agreement between the model and the experimental data. Finally the reconstructed refractive index field has been used to estimate the electron number density distribution within the laser-induced plasma. The electron number density behaviour with distance from the target at different power densities and its behaviour with time are shown. The electron number densities are found to be in the order of 10¹⁸ cm^?3 and decay at a rate of 3×10¹⁵ electrons/cm³ ns.     

Lensless Fourier holography for digital holographic interferometry on biological samples

Digital holography as a tool for highly sensitive, interferometric non-destructive testing has several advantages compared to holographic measurements based on conventional storage media like an all-digital processing and a direct access to the phase of the object wave. Experimental results of interferometric investigations of heart valve bio-prostheses with a setup for lensless Fourier holography are presented which demonstrate that this technique is applicable to such biological samples with their wet and unstable surfaces. Limitations on size and resolution of the reconstructed object caused by the properties of the CCD sensor are discussed.     

实时全息干涉视频图像自动化处理及应用研究

研究了实时全息干涉条纹视频图像计算机自动处理技术。采用Delphi可视化软件为开发工具,在Win dows环境下播放实时全息干涉图像,并同时获取在播放窗口中任意指定的灰度值、自动描绘灰度 时间以及位移 时间曲线。对测试原理和图像去噪、曲线平滑等关键处理技术进行了介绍。通过对微电子产品热变形研究的实例分析,显示了该自动处理软件的操作便捷、实用和先进性。     

XeCl discharge diagnostic by holographic interferometry

A direct measurement of the electron-density time evolution in the active medium of a self-sustained XeCl laser discharge has been performed by means of the holographic interferometry technique. The time and space resolved behaviour of the discharge seems to confirm that the halogen depletion instability and the hot spots developing in the cathode region are the basic mechanisms which determine the premature termination of the output laser pulse.     

用全息干涉法测量杨氏模量

通过对悬臂梁受力前后双曝光全息图进行再现，测量出干涉条纹序数与相对应的位置坐标，再利用最小二乘法求杨氏模量．     

Temporal phase retrieval from a complex field in digital holographic interferometry

We describe a novel method of processing complex phasors in digital holographic interferometry (DHI). Unlike the commonly used digital phase subtraction method that operates on the phase itself, the proposed method operates on the complex phasor instead. Two temporal phase retrieval algorithms are developed in which the complex phasor of each pixel is measured and analyzed as a function of time. The developed algorithms are demonstrated in profile measurement of step heights. Experimental results show that the proposed phase retrieval algorithms for DHI perform well compared with conventional methods.