Surface Of Test Object Research Articles

High-Accuracy 2D Digital Image Correlation Measurements with Bilateral Telecentric Lenses: Error Analysis and Experimental Verification

By comparing two digital images of a test planar specimen surface recorded in different configurations, two-dimensional digital image correlation (2D-DIC) provides full-field displacements to sub-pixel accuracy and full-field strains in the recorded images. For the 2D-DIC systems using an optical lens, a simple pinhole imaging model is commonly used to describe the linear relationship between the measured sensor plane displacements and the actual displacements in the object surface. However, in a practical measurement, various unavoidable disadvantageous factors, such as small out-of-plane motion of the test object surface occurred after loading, small out-of-plane motion of the sensor target due to the self-heating or temperature variation of a camera, and geometric distortion of the imaging lens, may seriously impair or slightly change the originally assumed linear correspondence. In certain cases, these disadvantages may lead to significant errors in displacements and strains measured by 2D-DIC. In this work, the measurement errors of 2D-DIC due to the above three disadvantageous factors are first described in detail. Then, to minimize the errors associated with these disadvantages, a high-accuracy 2D-DIC system using a bilateral telecentric lens is established. The performance of the established 2D-DIC system and other two 2D-DIC systems using a conventional lens and an object-side telecentric lens are investigated experimentally using easy-to-implement stationary, out-of-plane and in-plane rigid body translation tests. A detailed examination reveals that a high-quality bilateral telecentric lens is not only insensitive to out-of-plane motion of the test object and the self-heating of a camera, but also demonstrates negligible lens distortion. Uniaxial tensile tests of an aluminum specimen were also performed to quantitatively compare the axial and transversal strains measured by the proposed 2D-DIC system and those measured by strain gage rosettes. The perfect agreement between the two measurements further verifies the accuracy of the established 2D-DIC system.

重建彩色数字全息图时不同色光物光场的准确重叠研究

Regarding the test object surface as scattering plane,and using spherical wave as reference wave,with different wavelength illumination,reconstruction position of object plane wave field is studied by using Angular spectrum theory.The result shows that,when the physical meaning of the image is not considered,diffraction image center reconstruction with S-FFT(Single Fourier Transform) is corresponding to the frequency spectrum center of object wave field.Taking the S-FFT reconstruction image as a reference,the location of the object wave field spectrum can be well determined,then the reconstruction field of different wavelength can be accurate overlapped with adjustable magnification reconstruction Angular spectrum method.

Mean intensity gradient: An effective global parameter for quality assessment of the speckle patterns used in digital image correlation

Digital image correlation (DIC) is an image-based optical metrology for full-field deformation measurement. In DIC technique, the test object surface must be covered with a random speckle pattern, which deforms together with the object surface as a carrier of deformation information. In practice, the speckle patterns may show distinctly different intensity distribution characteristics and have an important influence on DIC measurements. How to assess the overall quality of different speckle patterns with a simple yet effective parameter is an interesting but confusing problem, and is also helpful to the optimal use of the technique. In this paper, a novel, simple, easy-to-calculate yet effective global parameter, called mean intensity gradient, is proposed for quality assessment of the speckle patterns used in DIC. To verify the correctness and effectiveness of the new concept, five different speckle patterns are numerically translated, and the displacements measured with DIC are compared with the exact ones. The errors are evaluated in terms of mean bias error and standard deviation error. It is shown that both mean bias error and standard deviation of the measured displacement are closely related to the mean intensity gradient of the speckle pattern used, and a so-called good speckle pattern should be of large mean intensity gradient.

Dynamic visualization of stress distribution on metal by mechanoluminescence images

We have successfully demonstrated that the stress distribution of a metal material can be directly displayed by coating the surface of test objects with an upgraded strong mechanoluminescence (ML) material of SrAl2O4:Eu (SAO). In this paper an aluminum plate with the SAO sensing film was applied to experimental analysis of stress concentrations. And the comparison with a numerical analysis showed that the ML intensity of SAO sensing film correlates linearly with stress on metal surface and the observed real-time ML images quantitatively reflect stress concentration. This novel visualisation technique can be applied to view stress concentration in various fields such as modelling, manufacturing and demonstration of industrial products as well as to point out danger areas in structural objects such as pipelines and bridges.

Reflective fringe pattern technique for non-destructive subsurface flaw detection

This paper presents a novel optical technique, Reflective Fringe Pattern (RFP), for non-destructive testing of objects having specularly or semi-specularly reflective surfaces. The principles are based on measuring the surface deformation of a test object and identifying deformation anomalies caused by subsurface flaws. In the set-up, a computer-generated fringe pattern displayed on a computer monitor is placed in front of the test object surface (which acts as a mirror) and forms a mirror image of the fringe pattern. The fringe pattern is perturbed according to the surface slope distribution. In the measurement, the fringe phase distributions before and after deformation are separately determined, and the difference of the phase distributions depicts the change of surface slope due to the deformation. In this paper, experimental demonstrations of the present technique to subsurface crack detection and adhesive bonding evaluation are presented. Two methods are also designed to enhance the flaw visibility on the output ofRFP.

A novel optical method without phase unwrapping for subsurface flaw detection

This paper presents a novel optical method without phase unwrapping for subsurface flaw detection, based on the computer-vision reflection grating technique. In the setup, a computer-generated fringe pattern displayed on a computer-monitor is placed in front of the test object surface and forms a mirror image behind the reflective surface. The fringe pattern is perturbed according to the surface slope distribution. Unlike the current fringe pattern analysis techniques, which require one/several images, the analysis algorithm of the present method involves a sequence of fringe patterns having multiple frequencies to deduce the ‘pixel-slope’ information which is directly related to the surface slope of the test object. The novel method offers independent analysis for each pixel of the data array, and thus the problem of phase discontinuity in phase unwrapping can be eliminated. In this paper, experimental demonstration of the novel optical method to subsurface flaw detection is presented.

Simultaneous Flatness and Surface Roughness Measurement of a Plastic Sheet Using a Fan-Shaped Laser Beam Scanning System

A fan-shaped laser beam scanning measuring system based on the triangulation principle, was newly developed in this study to measure simultaneously the flatness and the surface roughness of a surface plate with good reflectivity after a suitable calibration. The developed system mainly consists of a micro-focus fan-shaped line laser, an image processing system, a PC-based 3-axis machine, and a PC. The height value of the object can be calculated, based on the offset between the position projected onto the object and the position projected onto the platform, after carrying out the height calibration with gauge blocks. According to the scattering theorem, a correlation between the surface roughness of the test object measured with a precision stylus instrument and the average fringe width projected onto the test object surface, has been constructed.

A Novel Technique for Viewing Stress Distribution with Mechanoluminescence Materials

We have successfully demonstrated that the stress distribution of a metal substrate can be directly displayed by coating SrAl2O4:Eu (SAO), a representative of strong mechanoluminescent materials, on the surface of test objects. An aluminum plate with SAO sensing film had been applied to experimental analysis of stress concentrations, and a numerical calculation via a finite element method confirmed that the observed real time mechanoluminescence images displayed the stress distribution. As a result, visualization of stress distribution on metal surface has been realized by ML images using SAO sensing film, and this novel visualization technique can be applied for viewing the stress concentration in various fields such as modeling, manufacturing and demonstration of an industrial product.

Discussion on Transmission and Reception of Shear Vertical Wave Propagating along Surface of Test Object

It is shown that a shear vertical (SV) wave propagating along a surface of a test object can be transmitted and received by angle probes whose angle of refraction is close to a critical angle. In order to investigate this phenomenon, simulation is carried out using the finite difference time domain method. According to the acoustic fields obtained by the simulation, the beam width of a SV wave increases with propagation, and energy associated with the SV wave exists at the surface. Therefore, the transmitted pulse can be received. The phenomenon of SV wave propagation is referred to as SVC (SV wave for a critical angle). This simulation result is verified by an experiment, which showed that the transmitted pulse is applicable to the measurement of SV wave velocity.

P3-20 Discussion on Transmission and Reception of SV Wave Propagating along Surface of Test Object(Poster session 3)

P3-20 Discussion on Transmission and Reception of SV Wave Propagating along Surface of Test Object(Poster session 3)

Measurement of surface figure of plane optical surfaces with polarization phase-shifting Fizeau interferometer

A Fizeau interferometer based set up for measurement of surface forms of plane optical surfaces has been discussed. Phase shifting interferometry has been applied using polarization phase shifter. A linearly polarized (632.8 nm) He–Ne laser has been used as the source. Light reflected from the object and the reference/master surfaces are made circularly polarized in opposite senses by means of two properly oriented quarter wave retardation plates placed at appropriate positions, one inside and other outside the interference cavity of the interferometer, and phase shifts are introduced between the object and the reference/master waves by varying angular orientation of a polarizer/analyzer. Final result is made free from any residual wave-front aberrations introduced by the (intra-cavity) wave plate by subtracting phase values obtained by PSI technique between a high optical quality master surface and the reference surface from that obtained for the test object surface with respect to the same reference surface for each point of the interference field. Results are shown for a plane surface. Advantages of the technique presented are linearity and high accuracy in phase stepping, no perturbation of the interference cavity during the phase shifting and possibility of real time or dynamic interferometry.

Accuracy of orbscan II slit-scanning elevation topography

Purpose To establish the accuracy of Orbscan II (Orbtek Inc.) slit-scanning elevation topography in analyzing the anterior surface of complex test objects. Setting Discipline of Ophthalmology, University of Auckland, Faculty of Medical and Health Sciences, Auckland, New Zealand. Methods Six test objects were created from 2 materials: standard calibration poly(methyl methacrylate) (PMMA) (Orbtek Inc.) and a research PMMA material. The test objects were produced with spherical (radii of curvature 6.00 mm, 7.67 mm, and 8.88 mm), aspherical (apical radius 7.67, eccentricity 0.5, Q −0.25), and toroidal (7.67/7.92 mm radii of curvature) surfaces. The accuracy of the test surfaces was established by Form Talysurf Analysis. A single calibrated Orbscan II device was used to obtain 20 separate anterior elevation maps of each test object. The data obtained from Orbscan II, at 0.2 mm intervals along the chosen meridian, were directly compared with the Talysurf values for each test surface. Results Orbscan II measurements of all test objects were statistically significantly different from the Talysurf values ( P < .001). The test objects produced from standard calibration material were more accurately measured by Orbscan II than the objects produced from the research material. Data obtained by Orbscan II from the central 3.5 mm of all test objects were more accurate than peripheral data when compared with the Talysurf values ( P = .001). Conclusions Orbscan II anterior surface elevation measurements differed significantly from Form Talysurf Analysis of complex test surfaces. However, the magnitude of the errors in the measurement of standard test objects was small, less than 0.20 μm centrally and 0.70 μm peripherally. Clinically, if similar accuracy of measurement is confirmed in the human eye, anterior surface elevation maps can be considered accurate representations of corneal shape.

Automatic measurement of surface height with the interferoscope

Using a grazing incidence interferometer, the interferoscope, an automatic absolute measurement of a test object surface height can be made by counting the number of interference fringes when varying the surface incident angle. This is accomplished by combining the inter- feroscope with a computer-controlled stepping motor along with an im- age processing system. By transforming the equations describing the interferoscope, the sought depth can be regarded as a ''frequency,'' which can be attained using Fourier transforms or other frequency evalu- ation algorithms. © 1996 Society of Photo-Optical Instrumentation Engineers.

SGEMP Response Investigations with Exploding-Wire Photons

The exploding-wire photon experiments produced space-charge-limited boundary layers of thickness as small as a few centimeters. Comparison between the experimental results and state-of-the-art SGEMP calculations revealed the following features. Peak electric fields, surface currents and currents coupled into simple geometries were calculated within a factor of 1.5, with no normalization other than photoemission diode measurements of the electron current and average energy. Absolute agreement at low fluxes indicate that the electron-emission coefficients are measured by the diodes to an accuracy of better than 30%. The larger scatter in data at high fluxes on the disk experiment may be due to a combination of photon spectrum and rise time variations (especially in a soft component) between pulses and could possibly be affected by differences in minor contaminants on the test object and diode surfaces. The ratio of currents on the back and front of the disk is calculated to be a factor of 1.5 greater than measured. This discrepancy cannot depend on electron emission, and may indicate a need for finer gridding in calculating the edge of a disk. The leakage of electrons into a shadowed aperture is calculated within a factor of 1.4, but appears to persist somewhat longer than the calculations indicate.