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  • 101. Sarady, Istvan
    et al.
    Gren, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Sjödahl, Mikael
    Kaplan, Alexander
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Investigation of interaction mechanisms in laser processing using digital speckle photography2003Inngår i: Congress proceedings: Laser Materials Processing Conference [and] Laser Microfabrication Conference [at] ICALEO 2003, 22nd International Congress on Applications of Lasers & Electro-Optics, October 13 - 16, 2003, Adam's Mark Hotel, Jacksonville, Florida, USA / [ed] Xiangli Chen, Orlando, Fla: Laser institute of America , 2003, s. 314-321Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Digital, defocused speckle photography was used for the in-situ, real-time determination of the strain field round the focus spot of a pulsed Nd:YAG laser. These surface strains can cause cracking during laser cutting and drilling of advanced engineering ceramic materials. A CW HeNe laser was used for illumination, images being captured by CCD camera with a 633 nm band pass filter, synchronised to operate between the individual laser pulses. Targets included alumina and stainless steel.

  • 102. Sarady, Istvan
    et al.
    Yalukova, O.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Kaplan, Alexander
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Drilling of polymeric materials with pulsed lasers at short wavelengths2005Inngår i: 10th NOLAMP Conference: the 10th Nordic Laser Materials Processing Conference, 17-19 August 2005, Luleå Sweden / [ed] Alexander Kaplan, Luleå: Luleå tekniska universitet, 2005, s. 245-255Konferansepaper (Fagfellevurdert)
  • 103.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Accuracy in electronic speckle photography1997Inngår i: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 36, nr 13, s. 2875-2885Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Electronic speckle photography is an accurate, easy-to-use, video-based technique for the analysis of two- and three-dimensional deformation fields and in-plane strain fields, based on numerical cross correlation. Through the use of statistical optics, simulated speckle patterns, and experiments the accuracy in electronic speckle photography was found to depend on correlation, speckle size, window size, and correlation filter. The estimated correlation was found to be the combined effect of three mutually competing factors because of classical speckle correlation, subimage overlap, and displacement gradients. In many applications white-light speckle patterns provide a more accurate estimate of the displacement field than do laser speckle patterns

  • 104.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    An electronic speckle photography system for in plane deformation measurements1993Licentiatavhandling, med artikler (Annet vitenskapelig)
  • 105.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Calculation of speckle displacement, decorrelation, and object-point location in imaging systems1995Inngår i: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 34, nr 34, s. 7998-8010Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    My purpose here is to outline a method for calculating the fundamental behavior of speckle patterns in imaging systems. The theory of speckle displacement and decorrelation to include imaging at a general oblique angle is extended to more imaging systems, and explicit formulas are given for the image-point-object-point relationship that is important when defocused speckle is used. The intermediate results can be reused for optical systems other than those presented here. The image-speckle displacement analyzed in the three systems is expressed equivalently. The speckle decorrelation is in general larger in a single-lens system than in a two-lens system and can be minimized by proper design of the system

  • 106.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Digital speckle photography2000Inngår i: Proceedings, International Conference on Trends in Optical Nondestructive Testing: May 3 - 5, 2000, Hotel de la Paix, Lugano, Switzerland / [ed] Pramod K. Rostagi, Lugano: Ecole Polytechnique Fédérale de Lausanne , 2000Konferansepaper (Fagfellevurdert)
  • 107.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Digital speckle photography2000Inngår i: Trends in optical non-destructive testing and inspection, Amsterdam: Elsevier, 2000, s. 179-196Kapittel i bok, del av antologi (Annet vitenskapelig)
  • 108.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Digital speckle photography2000Inngår i: Digital Speckle Pattern Interferometry and Related Techniques, Chichester: John Wiley & Sons, 2000, s. 289-336Kapittel i bok, del av antologi (Annet vitenskapelig)
  • 109.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Dynamic properties of multispectral speckles in digital holography and image correlation2013Inngår i: Optical Engineering: The Journal of SPIE, ISSN 0091-3286, E-ISSN 1560-2303, Vol. 52, nr 10, artikkel-id 101908Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    I discuss dynamic properties of multispectral speckle in the context of digital holographic interferometry and image correlation. I outline the correlation of speckles in free space, in an imaging system, and, in the case of interferometric detection, caused by reflection off an inclined diffuse surface. It is shown that interferometric phase gradients and speckle movements are closely related where in fact the phase gradients are the generator of speckle movements in a defocused plane. The theory is exemplified by three typical situations encountered in image-plane digital holographic interferometry.

  • 110.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Electronic speckle photography applied to in-plane deformation and strain field measurements1995Doktoravhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    Non-contacting measurements of deformation and strain fields are of great importance in experimental mechanics. This thesis describes the development and applications of a video-based, computerized system called electronic speckle photography that measures the in-plane deformation of a strained body. The physical restrictions that limits the performance of the system have been carefully analysed. The system determines the deformation of a characteristic pattern, somehow attached onto the object surface. A FFT based cross-correlation of subimages from the two object states of interest is calculated. Subpixel accuracy is obtained through a Fourier serie expansion of the discrete correlation surface. For a pattern to be considered characteristic it has to be completely random, be of a high contrast and high frequent enough so that a small portion of the digitized image gets typical for its position. A good example of a characteristic pattern is a laser speckle pattern, but any pattern that fulfills the requirements can be used. For a good correlated pattern the system manages to calculate each displacement vector in the field with an accuracy of 1 % of a pixel. The accuracy decreases with increased speckle size and increased speckle decorrelation. An application in which the ESP-system has succesfully been used is to measure the deformation field caused by hygroexpansion in paper. Of primary interest in experimental mechanics is the strain field. There are two possibilities to determine the in-plane strain field with the use of the ESP-system. The first is to differentiate the measured deformation field obtained using either a laser speckle pattern or a white light speckle pattern. For this to be a feasable alternative the random error in the deformation field has to be decreased even further through the use of some filter. It seems difficult to obtain better accuracies than 100 μ strain in the strain fields through the differentiation process without losses in the spatial resolution. The second possibility relies on the motion of defocused laser speckle patterns. By combining the measured deformation fields obtained from four different illumination directions, all components of the in-plane strain field can be determined with an accuracy comparable to that of an electrical strain gauge (10 μ strain with retained spatial information. In measurements using laser speckle patterns to produce reliable results, a careful analysis of the behaviour of these patterns concerning speckle displacement, decorrelation and image point-object point correspondence have to be made. A telecentric imaging system has many positive qualities concerning these parameters for use in a defocused system and is the one that has been used in the measurements.

  • 111.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Electronic speckle photography: increased accuracy by nonintegral pixel shifting1994Inngår i: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 33, nr 28, s. 6667-6673Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Electronic speckle photography offers a simple and fast technique for measuring in-plane displacement fields in solid and fluid mechanics. An improved algorithm is presented and analyzed by use of both computer-simulated speckle patterns and real experiments. The idea of the improved algorithm is to maximize the correlation between correlated subimages from different images by shifting some of them by nonintegral pixel values. The improved algorithm was found to determine displacement components with an uncertainty of less than 1% of a pixel and with negligible systematic errors in ideal experimental conditions

  • 112.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Electronic speckle photography: measurement of in-plane strain fields through the use of defocused laser speckle1995Inngår i: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 34, nr 25, s. 5799-5808Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    High-accuracy, noncontact measurements of in-plane strain fields have been performed through the use of an electronic-speckle-photography system. The strain fields are extracted from the displacement of defocused laser speckle in a telecentric imaging system. Two different illumination configurations have been suggested, both of which use four illumination directions. Both configurations produce results of an accuracy according to Me/ΔL, where M is the demagnification of the telecentric imaging system, e is the random error in the speckle-displacement fields, and ΔL is the magnitude of the defocusing distance. The maximum defocusing distance possible was found to be restricted by the spatial resolution, especially at high magnifications. In experiments on a semicircularly and a rectangularly notched aluminum sheet, the principal strain field around the notch was measured with a random error in the strain field of less than 10 μstrain (μm/m)

  • 113.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Electronic speckle photography: some applications1994Inngår i: Laser in der Technik: Vorträge des 11. Internationalen Kongresses Laser 93 = Laser in engineering / [ed] Wilhelm Waidelich, Berlin: Encyclopedia of Global Archaeology/Springer Verlag, 1994Konferansepaper (Fagfellevurdert)
  • 114.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Gradient Correlation Functions in Digital Image Correlation2019Inngår i: Applied Sciences, E-ISSN 2076-3417, Vol. 9, nr 10, artikkel-id 2127Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The performance of seven different correlation functions applied in Digital Image Correlation has been investigated using simulated and experimentally acquired laser speckle patterns. The correlation functions were constructed as combinations of the pure intensity correlation function, the gradient correlation function and the Hessian correlation function, respectively. It was found that the correlation function that was constructed as the product of all three pure correlation functions performed best for the small speckle sizes and large correlation values, respectively. The difference between the different functions disappeared as the speckle size increased and the correlation value dropped. On average, the random error of the combined correlation function was half that of the traditional intensity correlation function within the optimum region.

  • 115.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Image and complex correlation near discontinuities2010Inngår i: Strain, ISSN 0039-2103, E-ISSN 1475-1305, Vol. 46, nr 1, s. 3-11Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In this paper, the effect of imposing continuity constraints in real and complex image correlation has been investigated. It is found that in regions where continuity can be assumed a second-order spline expansion of the underlying deformation field provides a globally smooth estimate of the deformation field. In general, only an estimate of the deformation at the subinterval centres needs to be provided from local correlation. If the assumption of continuity is violated multiple peaks in the correlation function are introduced. If the discontinuity exceeds one speckle diameter the correlation peaks will be separated, but with reduced correlation values. Otherwise, the different peaks will blend resulting in a deformed correlation function and a correlation value that approaches unity for small discontinuities. In this case, the Hessian is shown to provide a good estimate of the position of a discontinuity. Furthermore, a methodology to perform automatic remeshing is sketched. A subinterval edge is placed at the position of the detected discontinuity, the surrounding subintervals are reduced in size and the continuity constraint is relaxed. The methodology has proven successful on one-dimensional images but it can be transferred with reasonable effort to multi-dimensional images.

  • 116.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Mapping of refractive index by pulsed digital holographic image field correlation2008Inngår i: Laser Applications to Chemical, Security and Environmental Analysis: OSA Technical Digest (CD), Optical Society of America, 2008, artikkel-id PDPJMA2Konferansepaper (Fagfellevurdert)
    Abstract [en]

    The possibility to use digital holographic reconstructions to map three-dimensional refractive index fields is presented. The technique uses depth reconstructions in combination with speckle movements and phase information in an imaging system.

  • 117.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Robust information extraction in interferometry: overview and applications2010Inngår i: 9th Euro-American Workshop on Information Optics: WIO 2010; Helsinki; 12 July 2010 through 16 July 2010, IEEE Communications Society, 2010, artikkel-id 5582504Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Optical interferometry is one of the most versatile and accurate measurement techniques available for the scientist and engineer but is also known to be easily disturbed when used outside a well controlled research lab. Recent advances in theory and availability of photonic hardware and calculation efficiency have, however, increased the applications in which interferometry can be used considerably. In this presentation the fundamental theory of information extraction from interferometry will be reviewed and a few examples of measurements made possible by these advances will be presented

  • 118.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Robustness of objective and subjective speckle patterns in the speckle strain gauge1997Inngår i: Optical Review, ISSN 1340-6000, E-ISSN 1349-9432, Vol. 4, nr 2, s. 270-276Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The robustness to rigid body object motions of three optical systems used in the speckle strain gauge were experimentally investigated and compared with analytical results of the correlation. It was found that an out-of-plane motion of the object damaged the reliability of the strain measure when recording the objective speckle patterns while subjective speckle patterns were more robust. Besides out-of-plane object motions, the robustness of a free-space geometry and an afocal imaging configuration are approximately the same, while a telecentric imaging system is more robust to rigid body motions but more sensitive to deformation gradients (basically in-plane rotation and tilt). Results from a measurement of the relaxation in a lead-tin alloy used in organ pipes is also presented

  • 119.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Roubustness of objective and subjective speckle patterns in the speckle strain gaugl1996Konferansepaper (Fagfellevurdert)
  • 120.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Some recent advances in electronic speckle photography1998Inngår i: Optics and lasers in engineering, ISSN 0143-8166, E-ISSN 1873-0302, Vol. 29, nr 2-3, s. 125-144Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Major advances in electronic speckle photography are reviewed. Topics include the correlation properties of laser speckles, the principles and expected accuracy of electronic speckle photography, the effect of the digital recording and evaluation, and some recent applications. These applications involve two techniques for the measurement of 3D deformation fields, measurement of the in-plane strain field components using defocused laser speckle, and measurement of object shape.

  • 121.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Strain field measurements using electronic speckle photography: a comparison1994Inngår i: Recent advances in experimental mechanics: proceedings of the 10th International Conference on Experimental Mechanics, Lisbon, Portugal, 18 - 22 July 1994 / [ed] J.F. Silva Gomes, Rotterdam: Balkema Publishers, A.A. / Taylor & Francis The Netherlands , 1994Konferansepaper (Fagfellevurdert)
  • 122.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Whole field speckle strain sensor1999Inngår i: Optical engineering for sensing and nanotechnology: (ICOSN '99), [International Conference on Optical Sensing and Nanotechnology], 16 - 18 June 1999, Yokohama, Japan / [ed] Ichirou Yamaguchi, Bellingham, Wash.: SPIE - International Society for Optical Engineering, 1999, s. 84-87Konferansepaper (Fagfellevurdert)
    Abstract [en]

    The construction of a whole field speckle strain sensor is reported. The sensor makes use of the in-plane motion of defocused laser speckles in a telecentric imaging system. To distinguish between the contribution to the speckle motion from surface displacement, strain, rotation and tilt three different illumination directions were used. For illumination three pigtailed laser diodes of slightly different wavelength was used. To allow for simultaneous acquisition of the three speckle patterns, the different wavelengths were splitted with the aid of dichroic mirrors and directed onto separate synchronized detectors. The motion of the three speckle patterns is calculated using digital speckle photography and combined to give the three in-plane components of the strain tensor. A discussion about sources of random and systematic errors are included

  • 123.
    Sjödahl, Mikael
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Amer, Eynas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Quantitative measurement of thin phase objects: comparison of speckle deflectometry and defocus-variant lateral shear interferometry2018Inngår i: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 57, nr 14, s. 3645-3652Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The two techniques of lateral shear interferometry and speckle deflectometry are analyzed in a common optical system for their ability to measure phase gradient fields of a thin phase object. The optical system is designed to introduce a shear in the frequency domain of a telecentric imaging system that gives a sensitivity of both techniques in proportion to the defocus introduced. In this implementation, both techniques successfully measure the horizontal component of the phase gradient field. The response of both techniques scales linearly with the defocus distance, and the precision is comparative, with a random error in the order of a few rad/mm. It is further concluded that the precision of the two techniques relates to the transverse speckle size in opposite ways. While a large spatial coherence width, and correspondingly a large lateral speckle size, makes lateral shear interferometry less susceptible to defocus, a large lateral speckle size is detrimental for speckle correlation. The susceptibility for the magnitude of the defocus is larger for the lateral shear interferometry technique as compared to the speckle deflectometry technique. The two techniques provide the same type of information; however, there are a few fundamental differences. Lateral shear interferometry relies on a special hardware configuration in which the shear angle is intrinsically integrated into the system. The design of a system sensitive to both in-plane phase gradient components requires a more complex configuration and is not considered in this paper. Speckle deflectometry, on the other hand, requires no special hardware, and both components of the phase gradient field are given directly from the measured speckle deformation field.

  • 124.
    Sjödahl, Mikael
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Benckert, Lars
    Electronic speckle photography: analysis of an algorithm giving the displacement with subpixel accuracy1993Inngår i: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 32, nr 13, s. 2278-2284Artikkel i tidsskrift (Fagfellevurdert)
    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

  • 125.
    Sjödahl, Mikael
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Benckert, Lars
    Systematic and random errors in electronic speckle photography1994Inngår i: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 33, nr 31, s. 7461-7471Artikkel i tidsskrift (Fagfellevurdert)
    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

  • 126.
    Sjödahl, Mikael
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Benckert, Lars
    Molin, Nils-Erik
    Marklund, O.
    Optical metrology at Luleå University of Technology, LTU, Sweden1999Inngår i: Recent research developments in optical engineering, Trivandrum: Research Signpost, 1999, s. 279-293Kapittel i bok, del av antologi (Annet vitenskapelig)
  • 127.
    Sjödahl, Mikael
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Gren, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Sarady, Istvan
    Miroshnicova, Natalia
    Luleå tekniska universitet.
    Laser hole drilling process studied using laser speckle correlation2006Inngår i: Speckle06: speckles, from grains to flowers ; 13 - 15 September 2006, Nimes, France ; [proceedings] / [ed] Pierre Slangen; Christine Cerruti, Bellingham, Wash.: SPIE - International Society for Optical Engineering, 2006, s. 63412M-Konferansepaper (Fagfellevurdert)
    Abstract [en]

    This paper revisits defocused speckle correlation as a tool for measuring the response in metal sheets during percussion laser drilling processing. For the processing the 4th harmonic Nd:YAG wavelength (266 nm) was used in pulsed mode. It is shown that the method provides a cost efficient and robust alternative to speckle interferometry for the study of the small deformations that appear during laser processing. The accuracy was shown to be in the order of a few tens of μrads for the tilt component being measured which translates to a few nm in deflection when being spatially integrated. In the measurements deflections up to 50 nm was detected on the backside of silver and a copper sheet, respectively.

  • 128.
    Sjödahl, Mikael
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Gren, Per
    Tatar, Kourosh
    Olsson, Erik
    Laser metrology in experimental mechanics2005Inngår i: 10th NOLAMP Conference: the 10th Nordic Laser Materials Processing Conference, 17-19 August 2005, Luleå Sweden / [ed] Alexander Kaplan, Luleå: Luleå tekniska universitet, 2005, s. 41-52Konferansepaper (Fagfellevurdert)
  • 129.
    Sjödahl, Mikael
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Hallstig, Emil
    Optronic Partner Dp AB.
    Khodadad, Davood
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Multi-spectral speckles: Theory and applications2012Inngår i: Speckle 2012: : V International Conference on Speckle Metrology : 10 - 12 September 2012, Vigo, Spain / [ed] Ángel F. Doval, Bellingham, Wash.: SPIE - International Society for Optical Engineering, 2012Konferansepaper (Fagfellevurdert)
    Abstract [en]

    The objective of this paper is to discuss the properties and a few applications of multi-spectral speckles. The paper starts with a theoretical section where the correlation properties of multi-spectral speckles are detailed for the case of reflective imaging geometry. Both a free-space geometry and an imaging geometry are detailed. As an application example effects and possibilities provided by the theory in a measurement of surface shape of a generally shaped object from a dual-wavelength holographic recording are detailed. It is showed that the same phase profile is obtained from integration of speckle movements and phase unwrapping and they are therefore exchangeable quantities

  • 130.
    Sjödahl, Mikael
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Larsson, Linda
    Luleå tekniska universitet.
    Monitoring microstructural material changes in paper through microscopic speckle correlation rate measurements2004Inngår i: Optics and lasers in engineering, ISSN 0143-8166, E-ISSN 1873-0302, Vol. 42, nr 2, s. 193-201Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A system for real-time digital speckle correlation rate measurements is developed and applied on printing paper in the micro-scale. The system consists of a continuous laser and a CCD camera connected to a PC. To measure on objects in the micro-scale the system is connected to a stereomicroscope where one of the channels is used for imaging and the other channel is used for illumination of the object surface. The system is evaluated through measurements on printing paper over an area of 1 mm2 and with a spatial resolution of 30 μm on the object. The results show that the system is able to monitor the dynamics in paper during both wetting and drying in real-time over a time scale ranging from one-tenth of a second to several hours

  • 131.
    Sjödahl, Mikael
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Larsson, Linda
    Luleå tekniska universitet.
    Speckle correlation used for measuring microstructural changes in paper2003Inngår i: Speckle metrology 2003: proceedings ; 18 - 20 June 2003, Trondheim, Norway / [ed] Kay Gastinger, Bellingham, Wash: SPIE - International Society for Optical Engineering, 2003, s. 33-38Konferansepaper (Fagfellevurdert)
    Abstract [en]

    In this paper the use of laser speckle correlation as a mean to measure the micro-structural changes in paper exposed to a changing environment will be presented. Of particular interest will be changes in humidity and crack propagation. Speckle correlation is a technique that calculates the local structural change in a speckle pattern as the object deforms in real time. The response of the technique is the local rate of micro-structural change in the material. Speckle correlation is hence a complement to other speckle metrology techniques. The paper starts with a short introduction to speckle correlation but will focus on applications of the technique on paper. Results from studies of wetting of printing paper at the flock scale, dynamic response in copy paper when exposed to a sudden change in humidity and crack propagation in a notched sheet of Kraftliner are included.

  • 132.
    Sjödahl, Mikael
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Li, Yiling
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Local strain field measurement using phase gradient estimation from holographic speckle correlation2015Inngår i: SPECKLE 2015: VI International Conference on Speckle Metrology : 24-26 August 2015, Guanajuato, México / [ed] Fernando Mendoza Santoyo, Washington: SPIE - International Society for Optical Engineering, 2015, artikkel-id 966004Konferansepaper (Fagfellevurdert)
    Abstract [en]

    This paper introduces a new approach to process holographic specklegrams that gives direct access to local phase gradients. For a full in-plane strain mapping at least three different sensitivity vectors have to be used, if only one principal strain component is of interest two sensitivity vectors suffices. The fields in two defocused planes are calculated by propagating the focused field a known distance numerically. Based on the fields in the three planes a correlation function is formulated that is used to estimate the local phase gradients generated by stressing the sample. The phase and phase gradients for the different sensitivity vectors are then further combined to give the in-plane deformation and strain fields, respectively. The technique was demonstrated on a plate stressed uniaxially. Two illumination directions were used symmetrically oriented in the horisontal plane in relation to the surface normal to the plate. A significant reduction in phase noise was demonstrated. In addition the in-plane strain was estimated to be 0.8 mstrain with a standard deviation of 10 μstrain using an evaluation window size of 15x15 pixels. The average speckle correlation for this case was found to be 0.92. This technique promises to be a valuable tool whenever high quality measurements of strain fields or surface tilts are of interest

  • 133.
    Sjödahl, Mikael
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Olsson, Erik
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Amer, Eynas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Gren, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Depth resolution of phase gradients using pulsed digital holography2010Inngår i: Biomedical optics (BIOMED): digital holography and three-dimensional imaging (DH) ; April 11 - 14, 2010, Miami, Florida, USA ; technical digest, Washington, DC: Optical Society of America, 2010Konferansepaper (Fagfellevurdert)
    Abstract [en]

    technique to gain depth information from a single pair image-plane Digital Holographic recording of a transient phase object positioned between a diffuser and an imaging system has been demonstrated

  • 134.
    Sjödahl, Mikael
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Olsson, Erik
    Amer, Eynas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Gren, Per
    Depth-resolved measurement of phase gradients in a transient phase object field using pulsed digital holography2009Inngår i: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 48, nr 34, s. H31-H39Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A technique to gain depth information from an image-plane digital holographic recording of a transient phase object positioned between a diffuser and an imaging system is demonstrated. The technique produces telecentric reconstructions of the complex amplitude throughout the phase volume using numerical lenses and the complex spectrum formulation of the diffraction integral. The in-plane speckle movements as well as the phase difference between the disturbed field and a reference field are calculated in a finite number of planes using a cross-correlation formulation. It is shown that depth information about in-plane phase gradients can be determined in two planes using reconstructed speckle fields from four different depths. In addition, the plane of optimum reconstruction for calculating the phase difference with maximum contrast is detected from the technique. The method is demonstrated on a measurement of a laser ablation process.

  • 135.
    Sjödahl, Mikael
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Oreb, B.F.
    CSIRO, Lindfield, NSW.
    Stitching interferometric measurement data for inspection of large optical components2002Inngår i: Optical Engineering: The Journal of SPIE, ISSN 0091-3286, E-ISSN 1560-2303, Vol. 41, nr 2, s. 4003-408Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    An algorithm for stitching together a set of individual phase measurements into a unified larger phase map is presented. The set of individual phase maps results from scanning a component over the interferometer aperture and acquiring a measurement at each position. The algorithm is based on an iterative approach, where singular value decomposition is used to solve the rigid body movement problem between the different phase maps. The algorithm has been tested in numerical experiments and has been successfully applied to real data

  • 136.
    Sjödahl, Mikael
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Saldner, Henrik O.
    Luleå tekniska universitet.
    Three-dimensional deformation field measurements with simultaneous TV holography and electronic speckle photography1997Inngår i: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 36, nr 16, s. 3645-3648Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The ordinary recordings of TV holography (phase-stepped electronic speckle interferometry), arranged for out-of-plane sensitivity, are recombined and also analyzed by use of electronic speckle photography for simultaneous measurements of all three components of the deformation vector field. The principles of this technique are presented. Experimental results showing the opening of a slot in a perspex plate are also presented

  • 137.
    Sjödahl, Mikael
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Siviour, Clive
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Forsberg, Fredrik
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Digital volume correlation applied to compaction of granular materials2012Inngår i: Procedia IUTAM, ISSN 2210-9838, E-ISSN 2210-9838, Vol. 4, s. 179-195Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Compaction of powders and granular materials is an important process used to manufacture products including pharmaceuticals, ceramics, metals and explosives. In order to ensure mechanical integrity of the final product, an understanding of the compaction process, with particular reference to the homogeneity of the compacted bed, is required. In particular, it is necessary to have an improved understanding of the physics of the process, which can only be achieved through appropriate experimental measurements. X-ray tomography offers the opportunity to make full-field measurements of displacements in-situ: without removing the specimens from the die. This allows researchers to better understand the processes that take place at different stages in the compaction, and to more rigorously test numerical models.In this paper, we present data obtained using Digital Volume Correlation (DVC) to measure displacements in a compacted bed of sugar, with data obtained using X-ray microtomograpy. Instead of adding tracer particles, the natural, random, microstructure of the bed is used as the pattern for the volume correlation. The displacement data obtained are then used to calculate strain fields in the bed. A novel Finite Element (FE) based smoothing technique is applied to robustly smooth the data, allowing accurate and continuous strains to be calculated. These strains are compared to those calculated from the unsmoothed data. The paper presents details of both the DVC and FE smoothing routines

  • 138.
    Sjödahl, Mikael
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Synnergren, Per
    Luleå tekniska universitet.
    Measurement of shape by using projected random patterns and temporal digital speckle photography1999Inngår i: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 38, nr 10, s. 1990-1997Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Projected random patterns have been used to measure the shape of discontinuous objects. A sequence of independent random patterns are projected onto the object. These images are analyzed by use of the technique called temporal digital speckle photography (DSP) that is introduced here. With temporal DSP the spatial resolution of the shape measurement is improved considerably compared with previously reported results with projected random patterns. A calibration procedure is described that uses a sequence of independent random patterns to calibrate measurement volume. As a result, independent space coordinates for each subimage are obtained. The accuracy is of the order of 1/1000 of the field of view where a subimage size of 8 pixels seems to be a good compromise between reliability and spatial resolution. The technique is illustrated with a measurement of an electrical plug and a 9-V battery

  • 139.
    Sjödahl, Mikael
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Synnergren, Per
    Johnson, P.
    Volvo Aero Corporation.
    Applications of digital speckle photography in experimental mechanics1997Inngår i: Optical inspection and micromeasurements II: [conference held as a part of the European Symposium on Lasers and Optics in Manufacturing], 16-19 June 1997, Munich, FRG, SPIE - International Society for Optical Engineering, 1997Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Some recent applications of digital speckle photography in experimental mechanics are presented. Topics include the measurement of the in-plane deformation close to a triangular notch through an optical microscope, the 3D- deformation field of a sheet of paper subjected to increasing heat, the deformation of cardboard subjected to uniaxial stress, the shape of a curled sheet of paper, and the in-plane strain fields in an uniaxially stressed sheet of aluminium around a rectangular notch and a circular hole, respectively

  • 140.
    Sjödahl, Mikael
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Yamaguchi, I.
    Strain and torque measurements on cylindrical objects using the laser speckle strain gauge1996Inngår i: Optical Engineering: The Journal of SPIE, ISSN 0091-3286, E-ISSN 1560-2303, Vol. 35, nr 4, s. 1179-1186Artikkel i tidsskrift (Fagfellevurdert)
  • 141. Svanbro, Angelica
    et al.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Complex amplitude correlation for compensation of large in-plane motion in digital speckle pattern interferometry2006Inngår i: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 45, nr 34, s. 8641-8647Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The use of complex amplitude correlation to compensate for large in-plane motion in digital speckle pattern interferometry is investigated. The result is compared with experiments where digital speckle photography (DSP) is used for compensation. An advantage of using complex amplitude correlation instead of intensity correlation (as in DSP) is that the phase change describing the deformation is retrieved directly from the correlation peak, and there is no need to compensate for the large movement and then use the interferometric algorithms. A discovered drawback of this method is that the correlation values drop quickly if a phase gradient larger than π is present in the subimages used for cross correlation. This means that, for the complex amplitude correlation to be used, the size of the subimages must be well chosen or a third parämeter in the cross-correlation algorithm that compensates for the phase variation is needed. Correlation values and wrapped phase maps from the two techniques (intensity and complex amplitude correlation) are presented.

  • 142.
    Synnergren, Per
    et al.
    Luleå tekniska universitet.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Mechanical testing using digital speckle photography2000Inngår i: Proceedings of SPIE, the International Society for Optical Engineering, ISSN 0277-786X, E-ISSN 1996-756X, nr 4101, s. 399-408Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Some recent applications of stereoscopic and two-dimensional digital speckle photography in experimental mechanics are presented. The examples include uses of both laser speckles and white light speckles. Topics covered are: control of the flow front when manufacturing fibre reinforced polymer composites by a closed molding process, measurement of internal deformations at high strain rates, and in opaque specimens, using flashed x-ray radiation. It is also shown how defocused laser speckles can be used to measure in-plane strain fields without the need of numerical differentiation and how laser speckles can be used to monitor the extent of the plastic zone in materials.

  • 143. Synnergren, Per
    et al.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Optical in-plane strain field sensor2002Inngår i: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 41, nr 7, s. 1323-1329Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A whole-field speckle strain sensor is presented. The speckle strain sensor allows the measurement of all three in-plane components of the strain field simultaneously without touching the surface of the sample. The strain fields are extracted from the in-plane motion of defocused laser speckles in a telecentric imaging system. To distinguish the contribution to the speckle motion from surface translation, rotation, and strain, the speckle motion from three lasers with different illumination directions and wavelengths has to be analyzed separately. Simultaneous acquisition of the three individual speckle patterns is achieved by means of splitting the light from the lasers onto separate but synchronized detectors with the aid of dichroic mirrors. The motion of the speckles is calculated with digital speckle photography (speckle correlation), which enables the strain sensor to measure strain fields with noise levels as low as 10 μstrain

  • 144.
    Synnergren, Per
    et al.
    Luleå tekniska universitet.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Stereoscopic digital speckle photography system for 3-D displacement field measurements1999Inngår i: Optics and lasers in engineering, ISSN 0143-8166, E-ISSN 1873-0302, Vol. 31, nr 6, s. 425-443Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Stereoscopic digital speckle photography offers a technique to measure object shapes and 3-D displacement fields in experimental mechanics. The system measures the displacement of a random white light speckle pattern, which somehow is present on the object surface, using digital correlation. This paper describes a general physical model for stereo imaging systems. A camera calibration algorithm, which takes the distortion in the lenses into account, is also presented and evaluated by real experiments. Standard deviations of small deformations as low as 1% of the pixel size for in-plane deformations and 6% of the pixel size for the out-of-plane component are reported. Using the calibration algorithm described, the main source of errors is random errors originating from the correlation algorithm.

  • 145.
    Thuvander, Fredrik
    et al.
    Luleå tekniska universitet.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Berglund, Lars A.
    Luleå tekniska universitet.
    Measurements of crack tip strain field in wood at the scale of growth rings2000Inngår i: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 35, nr 24, s. 6267-6275Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The fracture mechanisms of wood have often been interpreted on the scale of cell walls. Although this scale is important, the scale of growth rings needs to be considered in the same context. In the present study, the crack tip strain field of radial TR cracks at the scale of growth rings is measured by electronic speckle photography. The methodology is discussed in detail as well as the data reduction scheme. The tip is in the earlywood layer and the crack plane of the TR crack is perpendicular to the stiffer latewood layer. Increasing opening mode load is applied in-situ as the crack is observed by reflected light optical microscopy. Strains are measured on direct images of the microstructure. In contrast to some other methodologies, this allows direct correlation between strain field and microstructure. In the softer earlywood, tangential strains extend considerable distances in the tangential direction. Due to the stiff latewood, the strain is heavily constrained in the radial direction. This nature of the local strain field has been largely neglected, despite its obvious significance to TR crack growth mechanisms.

  • 146.
    Tjörhammar, Staffan
    et al.
    Department of Applied Physics, Royal Institute of Technology (KTH).
    Eklöf, Finn Klemming
    Department of Applied Physics, Royal Institute of Technology (KTH).
    Yu, Zhangwei
    Department of Applied Physics, Royal Institute of Technology (KTH).
    Khodadad, Davood
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Hällstig, Emil
    Fotonic.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Laurell, Fredrik
    Department of Applied Physics, Royal Institute of Technology (KTH).
    Multiwavelength laser designed for single-frame digital holography2016Inngår i: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 55, nr 27, s. 7517-7521Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In this paper, we present a tailored multiwavelength Yb-fiber laser source in the 1.03 μm spectral region for spatially multiplexed digital holographic acquisitions. The wavelengths with bandwidths below 0.1 nm were spectrally separated by approximately 1 nm by employing fiber Bragg gratings for spectral control. As a proof of concept, the shape of a cylindrically shaped object with a diameter of 48 mm was measured. The holographic acquisition was performed in single-shot dual-wavelength mode with a synthetic wavelength of 1.1 mm, and the accuracy was estimated to be 3% of the synthetic wavelength.

  • 147. Wahba, Hamdy
    et al.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Gren, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Olsson, Erik
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Digital holographic microscopy for the study of nano-fibers2013Inngår i: Optical Measurement Systems for Industrial Inspection VIII / [ed] Peter H. Lehmann; Wolfgang Osten; Armando Albertazzi, SPIE - International Society for Optical Engineering, 2013Konferansepaper (Fagfellevurdert)
    Abstract [en]

    The advantages of digital holographic microscopy to record not only the intensity but also the optical phase are employed. The experimental arrangement comprises a Mach-Zehnder type interferometer with a microscopic objective of magnification 100x. The used camera is a 5 Mpixels Allied Vision Guppy Pro F-503 with a pixel pitch of 2.2 μm. The lateral magnification is set to about 200x based on the standard MIL-STD-150A 1951 USAF resolution test target. The dimensions of the aggregated natural cellulose nanowhisker fibers used are in the range of some hundreds of nanometers, which are positioned in the front of the microscopic objective using a 3D translation stage in the object arm of the holographic setup. The recorded off-axis holograms are refocused using the angular spectrum method. The reconstructed complex field is used to calculate optical phase and intensity distributions of the object at different reconstructions depths. The dimensions and orientation of the fibers can be evaluated from the optical field at different depths. Then, the shape and textures along the aggregated natural cellulose nanowhisker fiber can be presented in 3D space. The nano fiber found to have the dimensions of mean width 223 nm, depth 308 nm and length of 8.1 μm. Further, the mean local refractive index of the nano fibers can be calculated (n=1.501).

  • 148.
    Wahba, H.H.
    et al.
    Department of Physics, Faculty of Science, Damietta University, New Damietta City.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Gren, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Olsson, Erik
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    High resolution digital holographic microscopy for the study of aggregated natural cellulose nanowhisker fibers2015Inngår i: Optics and lasers in engineering, ISSN 0143-8166, E-ISSN 1873-0302, Vol. 73, s. 69-74Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In this paper, digital holographic (DH) microscopy demonstrates its ability to perform a full characterization of nanofibers. The high resolution and magnification of the presented method to study the nanofibers are tested using standard MIL-STD-150A 1951 USAF resolution test target. In this investigation, aggregated natural cellulose nanowhisker fibers are positioned in the front of the microscopic objective using a 3D translation stage in the object arm of DH setup. The recorded off-axis holograms are refocused using the angular spectrum method. The reconstructed complex field is used to calculate optical phase and intensity distributions of the object at different reconstruction depths. A simple algorithm is used to define the focused image with suitable accuracy. The dimensions and orientation of the fibers can be evaluated from the optical field at different depths. Then, the shape and textures along the aggregated natural cellulose nanowhisker fiber can be presented in a 3D space.

  • 149.
    Wahl, Joel
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Gren, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Bergström, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Lindbäck, Torbjörn
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Magnusson, Malin
    GKN Aerospace Sweden AB.
    Domeij, Kristina
    GKN Aerospace Sweden AB.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Comparison between process simulation and deformation measured by defocused speckle photography2018Inngår i: Speckle 2018: VII International Conference On Speckle Metrology / [ed] M. Kujawińska ; L. R. Jaroszewicz, SPIE - International Society for Optical Engineering, 2018, Vol. 10834, artikkel-id 108341KKonferansepaper (Fagfellevurdert)
    Abstract [en]

    Defocused laser speckle photography is used as a tool to measure the heat responses in a titanium component during laser heating. The evolution of the response is compared with a set of preprocessed Finite Element Simulations of the corresponding process with the aim to verify the simulation model and to find the simulation settings that best resemble the experimental results. The titanium component consists of a 300 x 100 mm2 substrate of thickness 3.2 mm on which a 200 x 30 x 11 mm3 ridge is built up using the laser metal deposition by wire process. The component is heated on the top of the ridge by a 300 W laser for 10 s and the deformation of the subtrate is followed throughout the heating-cooling cycle. The simulated deformation gradient is shown to resemble the measured response, and the magnitude of the response indicates that about 70 % of the laser power transferres into heat in the metal.

  • 150.
    Wahl, Joel
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Sjödahl, Mikael
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Ramser, Kerstin
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Signaler och system. Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Single-Step Preprocessing of Raman Spectra using Convolutional Neural Networks2019Inngår i: Applied Spectroscopy, ISSN 0003-7028, E-ISSN 1943-3530, ISSN 0003-7028Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Preprocessing of Raman spectra is generally done in three separate steps: (1) cosmic ray removal, (2) signal smoothing, and(3) baseline subtraction. We show that a convolutional neural network (CNN) can be trained using simulated data tohandle all steps in one operation. First, synthetic spectra are created by randomly adding peaks, baseline, mixing of peaksand baseline with background noise, and cosmic rays. Second, a CNN is trained on synthetic spectra and known peaks.The results from preprocessing were generally of higher quality than what was achieved using a reference based onstandardized methods (second-difference, asymmetric least squares, cross-validation). From 105 simulated observations,91.4% predictions had smaller absolute error (RMSE), 90.3% had improved quality (SSIM), and 94.5% had reduced signal-tonoise(SNR) power. The CNN preprocessing generated reliable results on measured Raman spectra from polyethylene,paraffin and ethanol with background contamination from polystyrene. The result shows a promising proof of concept forthe automated preprocessing of Raman spectra.

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