Optical Correlation Method Research Articles

Visualization of Buckling on Thin-walled Cylindrical Shell by Digital Image Correlation Method

The problem of buckling stability is very important in thin-walled cylindrical shell structure. In order to investigate the buckling phenomena, digital image correlation (DIC) method was applied in this experiment. The specimens have a thickness of 0.131mm and the ratio of external diameter (R0) to thickness (H) R0/H is 252. Due to its curved and thin walled elements, measurement of displacement and strains by using the traditional methods is difficult, because there is not enough space for sticking necessary torsion gauges to measure the out-of-plane behaviour, even if it is possible, the rigidity of the gauge itself will have an effect on the buckling strength. However, by using the optical image correlation method, the problem is solved. DIC method is an optical instrument which utilizes the full-field and non-contact measurement to gauge the three-dimensional displacement and strains on materials and structural parts. In this paper three dimensional digital image correlation system is applied to observe the buckling generation process in details and make a record of the surface displacement during the compression test by using two high resolution digital cameras, then the test data are analyzed and compared by a special correlation technique which can determine the surface displacements of cylindrical shells and it is shown in colour contour in full-field region. The buckling strengths obtained by the theoretical and experimental methods are compared and discussed in this paper. The experimental study shows that the buckling strength of theoretical value is much higher than that of experimental one, because the buckling strength is highly dependent on the imperfections.

Measurement of in-plane displacements using the phase singularities generated by directional wavelet transforms of speckle pattern images

We present a method to determine micro and nano in-plane displacements based on the phase singularities generated by application of directional wavelet transforms to speckle pattern images. The spatial distribution of the obtained phase singularities by the wavelet transform configures a network, which is characterized by two quasi-orthogonal directions. The displacement value is determined by identifying the intersection points of the network before and after the displacement produced by the tested object. The performance of this method is evaluated using simulated speckle patterns and experimental data. The proposed approach is compared with the optical vortex metrology and digital image correlation methods in terms of performance and noise robustness, and the advantages and limitations associated to each method are also discussed.

Application of digital image correlation to full-field measurement of shrinkage strain of dental composites

Objectives: Polymerization shrinkage of dental composites remains a major concern in restorative dentistry because it can lead to micro-cracking of the tooth and debonding at the tooth-restoration interface. The aim of this study was to measure the full-field polymerization shrinkage of dental composites using the optical digital image correlation (DIC) method and to evaluate how the measurement is influenced by the factors in experiment setup and image analysis. Methods: Four commercial dental composites, Premise Dentine, Z100, Z250 and Tetric EvoCeram, were tested. Composite was first placed into a slot mould to form a bar specimen with rectangular-section of 4 mm×2 mm, followed by the surface painting to create irregular speckles. Curing was then applied at one end of the specimen while the other part were covered against curing light for simulating the clinical curing condition of composite in dental cavity. The painted surface was recorded by a charge-coupled device (CCD) camera before and after curing. Subsequently, the volumetric shrinkage of the specimen was calculated with specialist DIC software based on image cross correlation. In addition, a few factors that may influence the measuring accuracy, including the subset window size, speckle size, illumination light and specimen length, were also evaluated. Results: The volumetric shrinkage of the specimen generally decreases with increasing distance from the irradiated surface with a conspicuous exception being the composite Premise Dentine as its maximum shrinkage occurred at a subsurface distance of about 1 mm instead of the irradiated surface. Z100 had the greatest maximum shrinkage strain, followed by Z250, Tetric EvoCeram and then Premise Dentine. Larger subset window size made the shrinkage strain contour smoother. But the cost was that some details in the heterogeneity of the material were lost. Very small subset window size resulted in a lot of noise in the data, making it difficult to discern the general pattern in the strain distribution. Speckle size did not seem to have obvious effect on the volumetric shrinkage strain along specimen length; however, larger speckles resulted in higher level of noise or heterogeneity in the shrinkage distribution. Compared with bright illumination, dimmer lighting produced larger standard deviations in the measured shrinkage, indicating a higher level of noise. The longer the specimen, the greater was the rate of reduction with distance from the irradiated surface, especially for the longitudinal strain. Significance: The image correlation method is capable of producing full-field polymerization shrinkage of dental composites. The accuracy of the measurements relies on selection of optimal parameters in experimental setup and DIC analysis.

Relaxation of nonequilibrium nanoparticle velocities in liquid medium

An optical speckle correlation method has been used to measure the velocities of nonequilibrium nanoparticles suspended in a liquid medium. It is established that the excess velocity of nanoparticles upon the irradiation of their suspension by high-power laser pulses decreases with the time according to a law that is close to t−1 and the characteristic relaxation time reaches several dozen milliseconds.

Robust and discriminating method for face recognition based on correlation technique and independent component analysis model

We demonstrate a novel technique for face recognition. Our approach relies on the performances of a strongly discriminating optical correlation method along with the robustness of the independent component analysis (ICA) model. Simulations were performed to illustrate how this algorithm can identify a face with images from the Pointing Head Pose Image Database. While maintaining algorithmic simplicity, this approach based on ICA representation significantly increases the true recognition rate compared to that obtained using our previously developed all-numerical ICA identity recognition method and another method based on optical correlation and a standard composite filter.

An optical flow and inter-frame block-based histogram correlation method for moving object detection

Moving object detection is very important for video surveillance. In this paper, we present a new real time motion detection algorithm that is based on the integration of optical flow and inter-frame block-based histogram correlation method to achieve better performance. The optical flow method is used to detect any movement under the background and the inter-frame block-based histogram correlation method is used to eliminate the background information and separate the moving object from it. The biggest advantage of this algorithm is that it does not need to learn the background model from hundreds of images and can handle quick image variations without prior knowledge about the object size and shape. The algorithm has high capability of anti-interference and preserves high accurate rate detection at the same time. The effectiveness of the proposed algorithm for motion detection is demonstrated in a simulation environment and the evaluation results are reported in this paper.

A Robust Subpixel Motion Estimation Algorithm Using HOS in the Parametric Domain

Motion estimation techniques are widely used in todays video processing systems. The most frequently used techniques are the optical flow method and phase correlation method. The vast majority of these algorithms consider noise-free data. Thus, in the case of the image sequences are severely corrupted by additive Gaussian (perhaps non-Gaussian) noises of unknown covariance, the classical techniques will fail to work because they will also estimate the noise spatial correlation. In this paper, we have studied this topic from a viewpoint different from the above to explore the fundamental limits in image motion estimation. Our scheme is based on subpixel motion estimation algorithm using bispectrum in the parametric domain. The motion vector of a moving object is estimated by solving linear equations involving third-order hologram and the matrix containing Dirac delta function. Simulation results are presented and compared to the optical flow and phase correlation algorithms; this approach provides more reliable displacement estimates particularly for complex noisy image sequences. In our simulation, we used the database freely available on the web.

Measurement of the full-field polymerization shrinkage and depth of cure of dental composites using digital image correlation

Objectives The aim of this study was to measure the full-field polymerization shrinkage of dental composites using optical image correlation method. Methods Bar specimens of cross-section 4 mm × 2 mm and length 10 mm approximately were light cured with two irradiances, 450 mW/cm 2 and 180 mW/cm 2, respectively. The curing light was generated with Optilux 501 (Kerr) and the two different irradiances were achieved by adjusting the distance between the light tip and the specimen. A single-camera 2D measuring system was used to record the deformation of the composite specimen for 30 min at a frequency of 0.1 Hz. The specimen surface under observation was sprayed with paint to produce sufficient contrast to allow tracking of individual points on the surface. The curing light was applied to one end of the specimen for 40 s during which the painted surface was fully covered. After curing, the cover was removed immediately so that deformation of the painted surface could be recorded by the camera. The images were then analyzed with specialist software and the volumetric shrinkage determined along the beam length. Results A typical shrinkage strain field obtained on a specimen surface was highly non-uniform, even at positions of constant distance from the irradiation surface, indicating possible heterogeneity in material composition and shrinkage behavior in the composite. The maximum volumetric shrinkage strain of ∼1.5% occurred at a subsurface distance of about 1 mm, instead of at the irradiation surface. After reaching its peak value, the shrinkage strain then gradually decreased with increasing distance along the beam length, before leveling off to a value of approximately 0.2% at a distance of 4–5 mm. The maximum volumetric shrinkage obtained agreed well with the value of 1.6% reported by the manufacturer for the composite examined in this work. Using irradiance of 180 mW/cm 2 resulted in only slightly less polymerization shrinkage than using irradiance of 450 mW/cm 2. Significance Compared to the other measurement methods, the image correlation method is capable of producing full-field information about the polymerization shrinkage behavior of dental composites.

The transmission interferometric adsorption sensor

This paper describes a high-speed adsorption sensor based on thin-film interference at the interfaces. The sensor can be used as a stand-alone instrument or in combination with a direct surface force measurement, which yields a wide range of additional information on molecular interactions on adsorbed films. The achieved mass resolution of the presented method (1–10 ng cm−2 Hz−1/2) is comparable to or better than other modern bio-sensors. The dependence of mass resolution on various factors is presented and demonstrated in a number of relevant examples. The described method is suitable for the implementation of a low-cost bio-sensor with a minimal number of optical elements.The measurement spot size is one micrometre or more and sampling rates >10 Hz are readily possible. In contrast to other bio-sensors, the signal baseline has a remarkable long-term stability since the measured signal is virtually independent of refractive index changes in the fluid medium above the sensor surface.In combination with an optical spectral correlation method, the classical computer calculations are substituted by an optical calculator and a label-free real-time imaging adsorption sensor is realized. We demonstrate sensor operation both inside the extended surface forces apparatus as well as in a stand-alone bio-sensor configuration. As a final point, we illustrate the imaging capability of this new sensor technology on a patterned bio-functionalized surface.

Precision thickness and refractive index imaging of molecular films

We present an interferometric contrasting technique that allows visualizing the thickness and refractive index of a molecular film being part of an optical multilayer structure (etalon). Small wavelength shifts of the comblike etalon spectrum are transformed into measurable intensity variations using a second reference etalon (optical correlator), illuminated in series. A charge-coupled device camera acting as two-dimensional photometer is utilized to measure the optical spectral correlation (OSC) image. The performance of the here proposed optical spectral correlation method is demonstrated using very thin confined liquid films. We give a detailed signal-to-noise analysis. Subangstrom film thickness resolution is experimentally verified with single exposure images acquired at frame rates comparable to video standards (approximately 25 Hz). Finally, we describe the calibration procedures necessary to obtain an absolute quantification of the OSC image.

Validation of diffuse correlation spectroscopy for muscle blood flow with concurrent arterial spin labeled perfusion MRI

Calf blood flow was measured simultaneously in healthy human subjects (n = 7) during cuff inflation and deflation using near-infrared diffuse correlation spectroscopy (DCS) and arterial spin labeled perfusion MRI (ASL-MRI). The DCS and ASL-MRI data exhibited highly correlated absolute and relative dynamic flow responses in each individual (p < 0.001). Peak flow variations during hyperemia were also significantly correlated, though more for relative (p = 0.003) than absolute (p = 0.016) flow. Repeated measurement variation was less than 8% for both modalities. The results provide much needed quantitative blood flow validation of the diffuse optical correlation method in humans.

Diffuse optical correlation tomography of cerebral blood flow during cortical spreading depression in rat brain

Diffuse optical correlation methods were adapted for three-dimensional (3D) tomography of cerebral blood flow (CBF) in small animal models. The image reconstruction was optimized using a noise model for diffuse correlation tomography which enabled better data selection and regularization. The tomographic approach was demonstrated with simulated data and during in-vivo cortical spreading depression (CSD) in rat brain. Three-dimensional images of CBF were obtained through intact skull in tissues(~4mm) deep below the cortex.

Diffuse optical measurement of blood flow in breast tumors

Blood flow contrast between tumor and normal tissues in patients with malignant and benign breast cancer was measured by diffuse optical correlation methods. The measurements were carried out with a hand-held optical probe that was manually scanned over the tumor-bearing breast. Increased blood flow was observed in tumor regions relative to healthy tissue, and control subjects did not exhibit significant blood flow heterogeneity. The measurements introduce a new optical contrast for diffuse optical mammography.

Durability studies for aerospace applications using peel and wedge tests

Measurements of peel strength are reported for some small, 2mm wide, unstressed aluminium adherend peel specimens based on aerospace adhesives AF163-2K06 and Redux 775 immersed in distilled water at 35C for over 7 years. Predictions of water distribution across the specimen width were made as a function of time using a measured diffusion coefficient and assuming Fickian diffusion. In the absence of primary corrosion, specimens after 7 years immersion showed no significant change in peel strength relative to strengths obtained in their initial dry state. This was after almost complete saturation of the adhesive.A brief review of the literature is given on the use of the Boeing wedge and DCB type tests for assessing the environmental durability of adhesive joints. Some of the assumptions used in the analysis of the wedge test have also been examined. An optical image correlation method is presented which permits accurate measurement of the adherend displacement. So long as sufficiently stable features can be identified on the adherend edge, then the position where the adherend displacement is zero may be used to give a well-defined effective crack length, and therefore fracture energy, even in the presence of extensive damage zones typical of those associated with tough structural adhesives.A beam on elastic foundation model is also given which demonstrates that apparent crack growth in the wedge test, associated with movement of the zero adherend displacement position, may be explained by relaxation of the adhesive.

Velocity field of the “La Clapière” landslide measured by the correlation of aerial and QuickBird satellite images

Two displacement maps of the “La Clapière” landslide (France) have been derived over two periods of 4 years (1995–1999 and 1999–2003) by correlation of aerial photographs and a QuickBird satellite image. The movement of the landslide ranges from 2.5 m to 20 m per year. Those values have been validated over 13 points monitored by conventional tacheometric measurements. Three areas with significant differences in velocity field have been mapped. Limits of those areas are in good agreement with in situ observations. Velocity maps show the low long term temporal variability of the landslide movement and its spatial variability. The optical correlation method using images derived from various sensors (airborne and spatial) is a promising technique for improving the spatial resolution of velocity field observation of landslides over several years.

Nonlinear morphological correlation: optoelectronic implementation

An optoelectronic implementation of the nonlinear morphological correlation by use of a threshold-decomposition technique and a joint transform correlator architecture is presented. This nonlinear morphological correlation provides improved image detection compared with standard linear optical pattern-recognition correlation methods. It also offers a more robust detection of low-intensity images in the presence of high-intensity patterns to be rejected.

Optical syntactic pattern recognition by fuzzy scoring

A novel syntactic approach is introduced to treat particular problems in pattern recognition. The procedure is implemented by the use of optical correlation methods for identifying the various primitives that appear in the input pattern, and their importance is determined by fuzzy relational scoring. Robust pattern recognition with tolerance to normal variations is demonstrated, indicating an efficient new approach for optical pattern recognition.

Optical correlation method for studying disperse media

A new optical correlation method is developed for measuring the scattering-particle size distribution function. The method is based on the transformation of the transverse coherence function of the scattered field that forms the scattering-particle images. A device for measuring the scattering-particle size distribution function is proposed that has a response time of about 10 s and can be applied to measure particles with sizes in the range of 3 to 500 µm to within 10%.

Dynamic strain measurement by optical speckle correlation method using LCTV panels

A new noncontacting dynamic strain measurement technique using optical speckle correlation is presented. Two LCTV panels placed symmetrically with respect to the incident laser beam are modulated with the initial speckle patterns and used as shadow filters. Speckle movement can be converted into light intensity variation. A strain wave excited under a shock loading through impact on an aluminum cantilever was measured. The results agreed well with the strain wave monitored by an electrical strain gauge.

Optical correlation method for measuring spatial complexity in optical fields

An analog method for measuring spatial complexity in optical fields generated by randomly phased objects is proposed. The method is used to make a device for high-speed real-time optical field measurements.