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  • 1.
    Benckert, Lars
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    A method to resolve the 180° ambiguity in speckle photography1991In: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 30, no 4, p. 376-378Article in journal (Other academic)
    Abstract [en]

    A new way of resolving the 180° ambiguity in speckle photography is presented. Results from a study of the deformation of a wooden block caused by drying illustrates the method

  • 2.
    Benckert, Lars
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    In-plane surface deformations measured by speckle photography: some examples1992In: Laser in der Technik: Vorträge des 10. Internationalen Kongresses Laser 91 = Laser in engineering / [ed] Wilhelm Waidelich, Berlin: Encyclopedia of Global Archaeology/Springer Verlag, 1992, p. 50-54Conference paper (Refereed)
  • 3.
    Benckert, Lars
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    White light speckle photography as a tool to study deformations in wood caused by drying1990In: Experimental mechanics : 9th International conference : Papers, Lyngby: Technical University of Denmark , 1990Conference paper (Refereed)
  • 4.
    Benckert, Lars
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Wood drying studies using white light speckle photography1992In: Measurement, ISSN 0263-2241, E-ISSN 1873-412X, Vol. 10, no 1, p. 24-30Article in journal (Refereed)
    Abstract [en]

    White light speckle photography is a powerful tool for measuring displacement fields in the sub-millimetre range. Here it has been utilised to study deformation and crack development in a block of wood during drying. The use of a series of single exposures made it possible to monitor the changes of the wood's surface with time. By combining two of the negatives the displacements over a given time interval were obtained for, at least in principle, all points on the surface.

  • 5.
    Benckert, Lars
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Forsberg, Lars
    Molin, Nils-Erik
    Fresnel diffraction of a Gaussian laser beam by polished metal cylinders1990In: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 29, no 3, p. 416-421Article in journal (Refereed)
    Abstract [en]

    A laser beam touching the periphery of a polished cylinder is subjected to both diffraction and reflection. Inthe area where diffracted light and reflected light interact the resulting intensity distribution differs from thepattern predicted by diffraction theory for a sharp edge. The difference increases with cylinder radius. Inthis paper it is shown that a good description of the resulting intensity pattern is obtained by adding thereflected light amplitude to the diffracted amplitude as predicted by the Fresnel-Kirchhoff theory for a sharpedge.

  • 6.
    Benckert, Lars
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Jonsson, Mikael
    Molin, Nils-Erik
    Measuring 3-D displacement of a surface using the white light speckle technique1986In: Proceedings of SPIE, the International Society for Optical Engineering, ISSN 0277-786X, E-ISSN 1996-756X, p. 207-210Article in journal (Refereed)
    Abstract [en]

    When measuring in plane surface deformation using white light speckle photography, errors will arise if an out of plane displacement is present. Here it is shown that stereoscopic photography resolves this problem and makes the measurement of true in plane displacements possible. A rigid body translation is also introduced to determine unambiguously the direction of the displacement

  • 7.
    Benckert, Lars
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Jonsson, Mikael
    Molin, Nils-Erik
    Measuring true in-plane displacements of a surface by stereoscopic white-light speckle photography1987In: Optical Engineering: The Journal of SPIE, ISSN 0091-3286, E-ISSN 1560-2303, Vol. 26, no 2, p. 167-169Article in journal (Refereed)
    Abstract [en]

    When in-plane surface deformations are measured using white-light speckle photography, errors arise if an out-of-plane displacement is present. Stereoscopic photography resolves this problem and makes possible the measurement of true in-plane displacements. A rigid-body translation is introduced to determine unambiguously the direction of the displacement

  • 8.
    Benckert, Lars
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Molin, Nils-Erik
    White light speckle: a non-contact method for the measurement of strain- and velocity fields1985Report (Other academic)
  • 9.
    Dahl, Jan
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Hermansson, Roger
    Gren, Per
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Benckert, Lars
    Temperatures and velocities near the gradient zone in a short term water heat storage1991In: Proceedings: Thermastock '91, International Conference on Thermal Energy Storage, Scheveningen, 13 - 16 May 1991, Utrecht: Novem , 1991Conference paper (Refereed)
  • 10. Dumortier, L.
    et al.
    Dahl, Jan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Benckert, Lars
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Kjellström, Björn
    Evaluation of particle tracking velocimetry for studies of the flow in a boiler furnace1999In: Applied Modelling and Simulation, International conference, 1999Conference paper (Refereed)
  • 11.
    Forsberg, Lars
    et al.
    Luleå tekniska universitet.
    Benckert, Lars
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Particle image velocimetry with a ruby laser to measure velocity fields in water1991Report (Other academic)
  • 12. Huntley, J.M
    et al.
    Benckert, Lars
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Measurement of Dynamic Crack Tip Displacement Field by Speckle Photography and Interferometry1992In: Fringe analysis '92, FASIG , 1992Conference paper (Refereed)
  • 13. Huntley, J.M.
    et al.
    Benckert, Lars
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Measurement of dynamic crack tip displacement field by speckle photography and interferometry1993In: Optics and lasers in engineering, ISSN 0143-8166, E-ISSN 1873-0302, Vol. 19, no 4-5, p. 299-312Article in journal (Refereed)
    Abstract [en]

    Speckle interferometry has been used to measure the transient displacement field around a crack due to stress wave loading. Double exposure interferograms are recorded with a pulsed ruby laser, and correlation fringes formed by spatial filtering of the developed film. A new technique is presented for reducing the noise level in spatially filtered speckle interferograms. Independent fringe patterns, obtained from one interferogram but with the filtering aperture sampling different regions of the Fourier transform plane, are digitised by a CCD camera and combined numerically. It is demonstrated that accurate analysis of fringe patterns by the Fourier transform method can be carried out, even if no carrier fringes are present. The measurements are compared with those obtained by speckle photography. Least-squares fitting of the theoretical displacement field is used to calculate the stress intensity factor at the crack tip as a function of time.

  • 14.
    Huntley, J.M.
    et al.
    Cavendish Laboratory, University of Cambridge.
    Benckert, Lars
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Speckle interferometry: noise reduction by correlation fringe averaging1992In: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 31, no 14, p. 2412-2414Article in journal (Refereed)
    Abstract [en]

    A method for noise reduction in double-exposure speckle interferometry is proposed, based on averaging independent spatially filtered correlation fringe patterns

  • 15.
    Huntley, J.M.
    et al.
    Cavendish Laboratory, University of Cambridge.
    Goldrein, H.T.
    Cavendish Laboratory, University of Cambridge.
    Benckert, Lars
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Parallel processing system for rapid analysis of speckle-photography and particle-image-velocimetry data1993In: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 32, no 17, p. 3152-3155Article in journal (Refereed)
    Abstract [en]

    An automated system has been constructed to process double-exposure speckle-photography andparticle-image-velocimetry images. A 3 × 3 array of laser beams probes the photograph, forming ninefringe patterns in parallel; these are then analyzed sequentially by digital computer and the use of atwo-dimensional Fourier-transform method. Results are presented showing that the random errors inthe measured displacements from such a system approach the expected speckle-noise-limited performance,with a total analysis time per displacement vector of 160 ms.

  • 16. Huntley, J.M.
    et al.
    Withworth, A.
    Field, A.
    Benckert, Lars
    Sjödahl, Mikael
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Thesken, J.C.
    Henriksson, A
    High resolution moir photography: application to impact and dynamic fracture on polymers and composites1995In: Impact and dynamic fracture of polymers and composites: papers presented at the ESIS European Symposium on Impact and Dymnamic Fracture of Polymers and Composites, Porto Cervo, Sardinia, Italy, Institution of Mechanical Engineers, 1995Conference paper (Refereed)
  • 17. Johansson, Eva-Lena
    et al.
    Benckert, Lars
    Gren, Per
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Particle image velocimetry (PIV) measurements of velocity fields at an organ pipe labium2003In: Proceedings of SMAC 03: Stockholm Music Acoustics Conference, August 6-9, 2003 / [ed] Roberto Bresin, Stockholm: Kungl. Tekniska Högskolan , 2003Conference paper (Refereed)
    Abstract [en]

    A pilot experiment is presented that measures velocity fields in two planes at a blown organ pipe labium for a fundamental tone at 260 Hz. The sound pressure is also measured at the same time as the velocity fields are registered. This makes it possible to follow the change of the velocity field as the sound pressure varies with time. The methods used are stereoscopic particle image velocimetry (PIV) and two-dimensional particle image velocimetry. The difference between the methods is that the first one measures three velocity components by use of two CCD cameras and the second one measures two velocity components with one camera. A double pulsed Nd:YAG laser is used as illuminating source. It gives short light pulses (~13 ns) necessary to resolve the quite high air velocity (~10 m/s). Results show that it is possible to follow a travelling vortex at the organ pipe labium in time as the sound pressure changes. The stereoscopic measurements show that the velocity fields are three-dimensional. The measurements have shown to be repeatable.

  • 18. Johansson, Eva-Lena
    et al.
    Benckert, Lars
    Sjödahl, Mikael
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Evaluation of phase objects by pulsed TV holography and defocused speckle displacement2003In: Optical Measurement Systems for Industrial Inspection III: 23 - 26 June 2003, Munich, Germany / [ed] Wolfgang Osten, Bellingham, Wash.: SPIE - International Society for Optical Engineering, 2003, p. 556-561Conference paper (Refereed)
    Abstract [en]

    An optical measurement method is presented for the evaluation of phase objects. Pulsed TV holography in combination with digital speckle photography gives the possibility to study transient events. The phase object is placed in front of a diffuser, on which a CCD camera is focused. Holograms are recorded, with and without the phase object present. The speckle patterns are reconstructed and transformed to different imaging planes using a phase field for propagation. Speckle displacement fields are then calculated for the different imaging planes, which visualise the change of refractive index or phase gradients caused by the phase object. Together with the phase information in the hologram even more information is obtained. A flow of helium gas and a positive weak lens are used as phase objects in two test experiments

  • 19. Johansson, Eva-Lena
    et al.
    Benckert, Lars
    Sjödahl, Mikael
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Improving optical phase acqusition by numerical refocusing of pulsed TV interferograms2004In: Proceedings of ICEM 12, 2004Conference paper (Refereed)
    Abstract [en]

    In optical measurement situations where holographic interferometry is used the optical phase information maybe poor because of too large object deformation. The speckle pattern displacement must be less than the specklesize in the image plane to obtain high contrast fringes. A phase object that is out of focus in a laser speckle fieldmay give large speckle displacements in comparison to the situation where no phase object is present. Acombination of pulsed TV holography, digital speckle photography and a numerical refocusing technique isshown to improve the optical phase information. A flow of helium gas has been used as a test object. When thegas is out of focus an interferogram of low contrast fringes is obtained, but after refocusing high contrast fringesare obtained. The phase gradients may also be determined from the improved phase data. These phase gradientsare compared to the ones obtained in another way from two speckle displacement fields in two different imagingplanes after numerical refocusing of the speckle fields. The two phase gradients are similar

  • 20. Johansson, Eva-Lena
    et al.
    Benckert, Lars
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Sjödahl, Mikael
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Improving the quality of phase maps in phase object digital holographic interferometry by finding the right reconstruction distance2008In: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 47, no 1, p. 1-8Article in journal (Refereed)
    Abstract [en]

    Improved quality of phase maps in pulsed digital holographic interferometry is demonstrated by finding the right reconstruction distance. The objective is to improve the optical phase information when the object under study is a phase object and when it is out of focus, leading to low contrast fringes in the phase map. A numerical refocusing is performed by introducing an ideal lens as a multiplication by a phase field in the Fourier domain, and then a region of maximum speckle correlation is found by comparing undisturbed and disturbed sub-images in different refocused imaging planes. After finding the right reconstruction distance, a phase map of high visibility is constructed. By this technique a 30 % reduction of the phase error for a flow of Helium gas and a 50 % reduction of the phase error for a weak thin lens were obtained, which resulted in a significant improvement of the visual appearance of the phase maps.

  • 21. Johansson, Eva-Lena
    et al.
    Benckert, Lars
    Sjödahl, Mikael
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Phase object data obtained by pulsed TV holography and defocused laser speckle displacement2004In: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 43, no 16, p. 3235-3240Article in journal (Refereed)
    Abstract [en]

    Transient events in optically transparent media occur in many engineering applications. Using pulsed TV holography to capture a laser speckle field propagated through an optical disturbance makes it possible to obtain both the position and the phase gradients of the disturbance. The technique depends on the fact that speckles transmitted through an optical disturbance will be displaced by an amount that depends on the relation of the defocus to the object. First the speckle field is captured holographically, without and with disturbance present. Then the recorded fields are numerically refocused in a computer to a number of different focal planes. With a cross-correlation technique a number of speckle displacement fields are obtained, and from them the data about the disturbance are obtained. So far the technique has been shown to work for thin objects

  • 22. Johansson, Eva-Lena
    et al.
    Benckert, Lars
    Sjödahl, Mikael
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Phase object data obtained from defocused laser speckle displacement2004In: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 43, no 16, p. 3229-34Article in journal (Refereed)
    Abstract [en]

    An optical technique that is based on defocused digital speckle photography is proposed for the evaluation of phase objects. Phase objects are different kinds of transparent or semi-transparent media that allow light to be transmitted. A phase object inserted in a laser speckle field introduces speckle displacement, from which information about the object may be extracted. It is shown that one may use speckle displacements to determine both the phase gradients and the positions of phase objects. As an illustration the positions and focal lengths of two weak lenses have been derived from defocused laser speckle displacement

  • 23. Jonsson, Mikael
    et al.
    Benckert, Lars
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Measurements of residual deformations in butt-welded plates by use of the white light speckle technique1986In: Journal of Strain Analysis for Engineering Design, ISSN 0309-3247, E-ISSN 2041-3130, Vol. 21, no 4, p. 243-246Article in journal (Refereed)
    Abstract [en]

    A method to measure residual deformation in welded plates is presented. An optical technique, called white light speckle, is used to record the released residual deformations. The method is destructive. Two experiments have been performed. The results are compared with corresponding calculated values. A short description of the calculations is given

  • 24.
    Melin, L. Gunnar
    et al.
    Luleå tekniska universitet.
    Thesken, J.C.
    FFB.
    Nilsson, S.
    FFB.
    Benckert, Lars
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Tensile impact delamination of a cross-ply interface studied by Moire photography1995In: Fatigue & Fracture of Engineering Materials & Structures, ISSN 8756-758X, E-ISSN 1460-2695, Vol. 18, no 10, p. 1101-1114Article in journal (Refereed)
    Abstract [en]

    Impact induced delamination along a cross-ply interface in carbon fiber/epoxy laminates is studied by high resolution moire photography. The specimens were loaded in a tensile split-Hopkinson bar giving mode I dominated fracture, and a high speed camera captures images during loading and delamination. The resulting moire fringes are analysed to produce full field displacement maps of the area around the loaded and propagating crack tips. The displacement map prior to failure shows good agreement with numerical solutions, calculated using a 3D self-adaptive p-version of the finite element method. The calibrated finite element solutions are then used to give further information about the matrix cracking zone size around the crack tip and the energy release rate. In comparison to quasi-static loading, tensile impact loading was found to increase the failure load and the resulting energy release rates; some physical explanations for this behaviour are discussed. It was concluded that the procedures presented have good potential for further determination of rate dependent material properties in carbon/fiber epoxy composites

  • 25.
    Sjödahl, Mikael
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Benckert, Lars
    Electronic speckle photography: analysis of an algorithm giving the displacement with subpixel accuracy1993In: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 32, no 13, p. 2278-2284Article in journal (Refereed)
    Abstract [en]

    Replacing photographic recording by electronic processing has some obvious advantages. An algorithm used for electronic speckle pattern photography is presented, and the reliability and accuracy is analyzed by using computer-generated speckle patterns. The algorithm is based on a two-dimensional discrete cross correlation between subimages from different images. Subpixel accuracy is obtained by a Fourier series expansion of the discrete correlation surface. The accuracy of the algorithm was found to vary in proportion to σ/n(1-δ)2, where σ is the speckle size, n is the subimage size, and δ is the amount of decorrelation, with negligible systematic errors. For typical values the uncertainty in the displacement is approximately 0.05 pixels. The uncertainty is found to increase with increased displacement gradients

  • 26.
    Sjödahl, Mikael
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Benckert, Lars
    Systematic and random errors in electronic speckle photography1994In: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 33, no 31, p. 7461-7471Article in journal (Refereed)
    Abstract [en]

    Electronic speckle photography offers a simple and fast technique for measuring in-plane displacement fields in solid and fluid mechanics. Errors from undersampling, illumination divergence, and displacement magnitude have been analyzed and measured. The nature of the systematic error is such that a drift toward the closest integral pixel value is introduced. Because of the finite extent of the sensor area, considerable undersampling is tolerable before systematic errors occur. The random errors are mainly dependent on the effective f-number of the imaging system and speckle decorrelation introduced by object displacement. When sampling at a rate of ~70% of the Nyquist frequency, we avoided systematic errors and minimized random errors

  • 27.
    Sjödahl, Mikael
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Benckert, Lars
    Molin, Nils-Erik
    Marklund, O.
    Optical metrology at Luleå University of Technology, LTU, Sweden1999In: Recent research developments in optical engineering, Trivandrum: Research Signpost, 1999, p. 279-293Chapter in book (Other academic)
1 - 27 of 27
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