Showing papers on "Digital image correlation published in 1994"

Optimal image correlation in experimental mechanics

Abstract: The integration of optimization techniques for use in digital image correlation problems in experimental mechanics is discussed. Stereo sets of images of a speckle pattern on a loaded body are correlated using four types of optimization routines. The results obtained using the gradient-based first-order method and nongradient-based pattern search, simplex, and Powell's methods compare extremely well with the original experimental results. The effectiveness and accuracy of these different optimal image processing techniques have also been analyzed and found suitable for this example

Submicron Deformation Field Measurements II: Improved Digital Image Correlation

Abstract: This is the second paper in a series of three devoted to the applicaiton of Scanning Tunneling Microscopy to mechanics problems. In this paper improvements to the Digital Image Correlation method are outlined, a technique that compares digital images of a specimen surface before and after deformation to deduce its (2-D) surface displacement field and strains. The necessity of using the framework of large deformation theory for accurately addressing rigid body rotations to reduce associated errors in the strain components is pointed out. In addition, the algorithm is extended to compute the three-dimensional surface displacement field from Scanning Tunneling Microscope data; also, significant improvements are achieved in the rate as well as the robustness of the convergence. For Scanning Tunneling Microscopy topographs the resolution yields 4.8 nm for the in-plane and 1.5 nm for the out-of-plane displacement components spanning an area of 10 μm x 10 μm.

Three-dimensional image correlation for surface-displacement measurement

Abstract: This paper describes a 3D surface profile and displacement measurement system capable of micron level accuracy using moderately priced off-the-shelf equipment. A non-linear optimization based calibration system is presented. The calibration system determines the position and operating characteristics of the cameras as well as correcting for lens distortion. Also presented is a surface profile and displacement measurement system base on projections into space of subsets of the recorded images. This method provides information about both the location and orientation in space of the subset. The accuracy of the system is established through a series of experiments. The calibration is assessed and the results are expressed using several different error measurements including a new error measurement proposed by the authors. The baseline accuracy of the measurement system was determined through a series of profile and translation tests. The system is capable of measurements to an accuracy of 0.003 mm over a 14 mm X 18 mm field from a distance of 416 mm using a 512 X 480 CCD camera and a magnification factor of 27 pixels/mm. The system was also used to measure the bending of a circular plate under pressure loading. The experimental results are analyzed and compared with theoretical prediction.

Deformation Measurements at the Sub-Micron Size Scale: II. Refinements in the Algorithm for Digital Image Correction

Abstract: Improvements are proposed in the application of the Digital Image Correlation method, a technique that compares digital images of a specimen surface before and after deformation to deduce its sureface (2-D) displacement field and strains. These refinements, tested on translations and rigid body rotations were significant with regard to the computer efficiency and covergence properties of the method. In addition, the formulation of the algorithm was extended so as to compute the three-dimensional surface displacement field from Scanning Tunneling Microscope tomographies of a deforming specimen. The reolsution of this new displacement measuring method at the namometer scale was assessed on translation and uniaxial tensile tests and was found to be 4.8 nm for in-plane displacement components and 1.5 nm for the out-of-plane one spanning a 10 x 10 μm area.

Motion vector generation using interleaved subsets of correlation surface values

Abstract: A system for generating motion vectors from image data calculates correlation surface values representing image correlation between temporally adjacent images. The system provides a stream of correlation surface values comprising interleaved subsets of correlation surface values from differing correlation surfaces, each subset being a portion of a correlation surface. The system also provides a correlation surface analyzer for receiving the stream of correlation surface values and determining partial result values for motion vectors, a buffer memory for storing the partial result values, and an apparatus for reading out final result values of motion vectors for a given correlation surface.

Correlation-feedback approach to computation of optical flow

Abstract: The optical flow techniques have been developed for more than one decade. Once the optical flow field is computed accurately, this measurement of image velocity can be used widely in many tasks in computer vision area. Current computer vision techniques require that the relative errors in the optical flow be less than 10%. However, to reduce error in determination of the optical flow is still a difficult problem. In this paper, firstly, errors occurring in the correlation-based approaches to optical flow computation are analyzed. Through understanding how the errors arise, we developed a new approach to computation of optical flow named the correlation-feedback approach. It is based on the idea of feedback and the correlation-based approach. In this approach, a virtual continuous image is obtained by a bilinear interpolation applied to a digital image. The idea of feedback is used so that errors in determining optical flow are reduced considerably in the iterative procedure. It is proved that the algorithm is convergent generally. Several experiments working on real image sequences in the laboratory demonstrate that our correlation-feedback algorithm performs better than the gradient-based and correlation-based algorithms in terms of accuracy. >

Digital frequency domain correlator

Abstract: PURPOSE: To perform image correlation in real time between, specially, a video image and various target subject images given in the field of video image input by digitally performing correlation in a Fourier frequency area. CONSTITUTION: An electronic image for correlation with a matched filter image is obtained by a camera 2. The obtained subject image is digitized by a digitizing circuit 3 and enhanced by an image processor 4. After a composition multiplier performs multiplication by a specific window coefficient, row converting processors perform conversion by rows of a 1st frame first and then an FFT processor 9 processes the frame by Fourier transformation. Then the Fourier-transformed frame is multiplied by matched filters in order at the composition multiplier 10. Then the result is processed through the inverse fast Fourier transformation of an inverse FFT transformer 13 and a decision processor 14 takes an analysis for a correlation spot.

Reconstruction of surface topography using Fourier phase of structured light

Abstract: A variety of techniques have been previously developed for surface topography reconstruction using CCD video images and various image processing algorithms. These include passive stereo disparity estimation using sub-area image correlation, confocal imaging (depth from focus), as well as structured light techniques. These approaches are compared on the basis of theoretical height error and algorithm complexity. A simple post-processing scheme based on the use of Fourier phase of structured light is then described, and results are shown from height measurements of tens of microns over areas on the order of a centimeter. A diode laser source is used in conjunction with a fan-beam refractive element, appropriate optical filtering, microscope, and CCD camera. This approach has application to long working-distance microscopy, closed-loop numerical control of machining, and retinal surface topography for early disease detection. Our approach offers a simple, low-cost, and real-time method of surface topography visualization and closed-loop machine tool control. Reflected laser beam quality and associated digital image filtering are considered with respect to the nature of possible surface materials measured.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Broad optical stabilized image

Abstract: In this paper, the optical image system is abstracted to the dynamic optical system. Through analyzing and researching, the dynamic imaging process is ascribed to an image stabilized course, then make the method and formula of image stabilized system/scanning and tracking system/optical imitating system and so on unified.

High-precision flow motion analysis by means of photochromic dyes and image processing techniques

Abstract: Flow motion detection and analysis of 2-D and 3-D velocity fields in a fluid is a very important task in many applications, for example the study of fluidodynamics phenomena in microgravity conditions, i.e. in space environment. An extremely interesting technique useful to analyse fluids flow without introducing significative disturbances employs the change in optical density of some specific dyes when lighted with ultraviolet light. In this paper a complete processing system is described that uses the dye method to analyze fluid flow by means of digital image processing techniques.

A Laser Speckle Image Correlation Instrument for High-Temperature Surface Strain and Deformation Measurements

Abstract: Advanced high temperature structural materials for the National Aero-Space Plane (NASP) program are subjected to high level thermomechanical loadings during laboratory testing. This paper describes the technical performance of a laser speckle image correlation instrument for obtaining optical strain measurements at elevated temperatures of 1100°C and above. The technical approach to this project involved the development of an optical strain sensor incorporating laser speckle imaging and digital cross-correlation techniques for uni-axial strain and deformation measurements. This PC-based instrumentation system consists of three main modules: three high-resolution, line-scan, charge-coupled device (CCD) cameras with special optics; three electromechanical shutters for triple-beam sequential illuminations by a continuous wave probing laser; two digital signal processing (DSP) system boards and DSP analysis software using Hypersignal-Windows Block Diagram programming modules. Surface strain and deformation measurements along the central axis were performed for nickel-based alloy and silicon-carbide coated carbon-carbon composite samples in a cantilever fixture under thermomechanical loading. The comparative simulation study was performed using analytical/graphical computation of thermomechanically loaded test samples. The experimental test results showed that the acquired speckle images used for the cross-correlation calculations suffered insignificant degradation or drift due to the presence of extreme thermal conditions. This optical strain gage sensor requires no surface preparation or application of micro-indentation marks or targets. The multi-level operating software consists of several modules, each performing a different task such as hardware initiation, image acquisition, digital signal processing, strain/deformation analysis data storage, and graphical presentation. Application of the digital laser speckle cross-correlation method has been extended for in situ monitoring and characterization of laser-processed refractory ceramic coating film formation.