Yun Pan

Bio: Yun Pan is an academic researcher from Nanjing University of Aeronautics and Astronautics. The author has contributed to research in topics: Digital holography & Speckle pattern. The author has an hindex of 2, co-authored 3 publications receiving 13 citations.

Accurate full-edge detection and depth measurement of internal defects using digital speckle pattern interferometry

Abstract: In this paper, a new method, Moment-Stiffness Method (MSM), is proposed for measuring the edge and depth of internal defects which can be applied to any structures with known stiffness. MSM is based on the relation of the differences of displacements between flawed and flawless areas caused by the change of stiffness. Theoretical, numerical and experimental results are presented. In the method, digital speckle pattern interferometry (DSPI) is used to measure the out-of-plane displacement of a plate with an internal defect. In the detection of the first specimen, the distributions of first- and second-order radial derivatives of the out-of-plane displacement are obtained by the radial numerical differential. Then the full edge and depth of the defect is accurately determined using MSM. All the results indicate that the proposed method is an effective tool to measure internal defects precisely.

Research and application of dual-camera dynamic in-line digital holography using a two-step phase-shifting cepstrum technique.

Abstract: Due to limits in the properties of digital cameras, in-line digital holography is commonly used to take full advantage of the sampling space of the camera. To realize the dynamic high-resolution measurement of in-line digital holography, dual-camera dynamic in-line digital holography is proposed. By means of a two-step phase-shifting cepstrum algorithm and a dual-camera parallel phase-shifting recording optical path, the complex amplitude of the object wave can be reconstructed without its zero-order and conjugate terms. Meanwhile, a novel spherical wave interference calibration method is also developed for the dual-camera recording system, and image correction is carried out via rotation, translation, and diffraction, with an average error of phase correction of 0.1107 rad. Finally, the feasibility and effectiveness of the proposed technique is well demonstrated by a practical application of dynamic temperature field measurement in a transparent medium.

Dynamic measurement of three-dimensional absolute displacements using dual-camera multiplexed digital holography.

Abstract: Digital holography has been frequently used to measure the micro-deformation in mechanical tests due to its full-field measurement with high resolution and accuracy. To measure dynamic three-dimensional absolute displacements without a known reference displacement, a new technique based on the combination of off-axis multiplexed digital holography and stereo photogrammetry is proposed. Under the illumination of two different wavelength lasers along various directions, two off-axis multiplexed holograms recorded by the dual-camera system are used to extract four phase maps with different sensitivity vectors simultaneously. Meanwhile, the variation of sensitivity vectors and registration of phase maps are carried out by the object shape measured by the dual-camera system. By the four registered phases with four varying sensitivity vectors, three-dimensional absolute displacements can be determined. The feasibility of our method is well demonstrated by a quantitative experiment and finite element analysis, and the dynamic measurement of a resistor undergoing thermal expansion is presented.

Edge detection of internal defects based on the hidden singularity of gradient streamlines obtained by continuous wavelet transform

Abstract: A method based on the measurement of out-of-plane displacement by the phase-shifting digital speckle pattern interferometry is proposed to detect the edge of internal defects. The out-of-plane displacement is scanned by the scanning method of gradient streamlines first, which can provide paths through the defect edge with the best signal-to-noise ratio everywhere. Then, continuous wavelet transform (CWT) combined with the weighted structural intensity of the wavelet maxima is applied to detect the hidden singularities of the gradient streamlines that form the defect edge later. Moreover, the weighted structural intensity method is optimized to adapt to the detection of the hidden singularity by CWT. The results of theory, simulation, and experiments agree well, and the proposed method is efficient in internal defect detection.

Dynamic Measurement of 3d Displacements Using Dual-Camera Lensless Fourier Transform Digital Holography

Abstract: Dynamic 3D displacement field measurement is an effective means to characterize the electrical performance stability and structural soundness of microsystems. Combining off-axis lensless Fourier transform multiplexed digital holography and multi-illumination profilometry, a dual-wavelength dual-camera optical setup with a multi-illumination system is developed to simultaneously acquire four phase images with different sensitivity vectors, as well as the object shapes. Meanwhile, the shared reference wave in off-axis lensless Fourier holography gives a convenient way for sensitivity vector modification and phase image registration, which are based on the stereo checkerboard calibration method. The dynamic 3D displacement fields and the strain maps of an energized integrated circuit board reveal that the inhomogeneous thermal expansion may cause some damage to chips, such as pin desoldering and microstructure fracture.

Non-contact optical dynamic measurements at different ranges: a review

Abstract: Optical dynamic measurements are widely used for non-contact vibration, continuous deformation, or moving objects. Various measurement techniques were developed for different deformation amplitudes. This paper reviews three types of technique for different measurement ranges: interferometric techniques for deformation or vibration (nanometer to sub-millimeter amplitude) whose measurement accuracies rely on phase extraction of interferometric signal; imaging based techniques for deformation or vibration (micrometer to centimeter amplitude) with the aid of moire, structured light, and man-made speckles, whose sensitivities is from 1/100 to 1/10 pixel; and videometrics for large deformation or movement detection (greater than centimeter amplitude). Many research groups have improved measurement capabilities for these three techniques to meet particular industrial application requirements.

Influence of edge effects on laser-induced surface displacement of opaque materials by photothermal interferometry

Abstract: We demonstrate the influence of edge effects on the photothermal-induced phase shift measured by a homodyne quadrature laser interferometer and compare the experiments with rigorous theoretical descriptions of thermoelastic surface displacement of metals. The finite geometry of the samples is crucial in determining how the temperature is distributed across the material and how this affects the interferometer phase shift measurements. The optical path change due to the surface thermoelastic deformation and thermal lens in the surrounding air is decoded from the interferometric signal using analytical and numerical tools. The boundary/edge effects are found to be relevant to properly describe the interferometric signals. The tools developed in this study provide a framework for the study of finite size effects in heat transport in opaque materials and are applicable to describe not only the phase shift sensed by the interferometer but also to contribute to the photothermal-based technologies employing similar detection mechanisms.

Adaptive local threshold segmentation for Fourier spatial filtering in automatic analysis of digital speckle interferogram

Abstract: Automated extraction of the +1 term spectrum can greatly benefit the real-time measurement of dynamic deformation in the digital speckle pattern interferometry (DSPI). However, most spatial filtering methods are not satisfactory for automatic analysis because they need manual intervention or are unable to accurately extract multiple +1 term spectrums in one frequency distribution image. We propose an adaptive local threshold segmentation method for the Fourier spatial filtering of a three-dimensional DSPI system. This method first uses global thresholding to coarsely recognize the spectral regions and then adopts local thresholding to accurately determine the spatial filtering windows for each spectrum. Experimental result shows that the proposed adaptive local threshold segmentation can accurately extract multiple desired +1 term spectrums even if the intensities of different spectral regions differ greatly. In addition, experiments of the comparison between the automatic and manual spatial filtering and automatic analysis of dynamic deformation demonstrate that the proposed method can be used for the real-time measurement.

Improved depth characterization of internal defect using the fusion of shearography and speckle interferometry

Abstract: A hybrid nondestructive testing (NDT) approach based on the fusion of shearography and digital speckle pattern interferometry (DSPI) is proposed for depth characterization of internal defect. To this end, a novel dual-sensitive speckle interferometry system is developed for both shearography and DSPI measurements. On one hand, shearography imaging results are used to identify the internal defect location, shape, and boundary qualitatively for the preparation of subsequent depth characterization. On the other hand, an improved mechanical model combined with bending theory and DSPI imaging results is built to perform the internal defect depth prediction. Finally, the hybrid NDT experiment for a thin metallic plate with an internal planar defect is conducted for feasibility validation of the proposed fusion method. The results indicate that the relative error of defect depth detection is less than 5% compared to the existing method and show that the proposed fusion method can successfully perform improved depth characterization of internal defect.

Single-frame reconstruction for improvement of off-axis digital holographic imaging based on image interpolation

Abstract: We present a method of single-frame reconstruction in off-axis digital holography with Kronecker-product interpolation to maximize spatial-frequency extraction. The Kronecker-product interpolation operated on a one-frame off-axis digital hologram can generate several aliasing spectra in its Fourier domain, where the zero-order aliasing spectra in the extrapolation region can be suppressed effectively. After adding all aliasing spectrum regions and substituting each aliasing spectrum in place with their sum, spectrum interception with a larger scope for filtering can be made in one of the aliasing spectrum regions. The experimental results demonstrate that the method is valid to improve the resolution in off-axis digital holography.