Image-based Measurement Technique Research Articles

Image-Based Experimental Measurement Techniques to Characterize Velocity Fields in Blood Microflows.

Predicting blood microflow in both simple and complex geometries is challenging because of the composition and behavior of the blood at microscale. However, characterization of the velocity in microchannels is the key for gaining insights into cellular interactions at the microscale, mechanisms of diseases, and efficacy of therapeutic solutions. Image-based measurement techniques are a subset of methods for measuring the local flow velocity that typically utilize tracer particles for flow visualization. In the most basic form, a high-speed camera and microscope setup are the only requirements for data acquisition; however, the development of image processing algorithms and equipment has made current image-based techniques more sophisticated. This mini review aims to provide a succinct and accessible overview of image-based experimental measurement techniques to characterize the velocity field of blood microflow. The following techniques are introduced: cell tracking velocimetry, kymographs, micro-particle velocimetry, and dual-slit photometry as entry techniques for measuring various velocity fields either in vivo or in vitro.

Background oriented schlieren technique with fast Fourier demodulation for measuring large density-gradient fields of fluids

In order to measure the large density-gradient fields of fluids such as underwater shock waves, we employed a fast Fourier demodulation called Fast Checkerboard Demodulation (FCD) method in Background Oriented Schlieren (BOS) technique. BOS is a simple image-based measurement technique that detects the apparent displacement (local distortion) of a background image caused by the density gradient of the fluid in front of the background. The cross-correlation particle image velocimetry (PIV) method, which is commonly employed in the BOS technique for detecting the apparent displacement, uses a random-dot background, whereas FCD uses a periodic pattern as the background (e.g., checkerboard pattern, lattice grid pattern (grid scale)) to detect the apparent displacement as the phase change of the pattern in the Fourier space. In this study, we measured the apparent displacement, which is proportional to density gradient of fluid, of an underwater shock wave using FCD and PIV. The results showed that FCD can measure a displacement gradient of up to 2.5 times larger than that which PIV can measure. Furthermore, we systematically investigated the measurement limit of FCD-BOS by changing several parameters of the periodic patterns, such as the grid size. Also, we explored the related parameters in the Fourier space to understand the limitations. It is worth noting that FCD-BOS with lattice grid pattern (grid scale) can measure the apparent displacement as accurately as that with a custom-made pattern, indicating that large density-gradient fields of fluids can be measured using a simple setup with a commercially available (inexpensive) pattern.

A new PDE-based resolution enhancement technique for the analysis of low SNR particle displacement images

Speckle images have found multiple applications in image-based measurement techniques, where the objective of measurement is the estimation of speckle displacements between two often consecutive images. Depending on the characteristics of a measurement system and its measurands, the signal-to-noise ratio (SNR) of recorded images may become low and even marginal. One solution to improve the accuracy of speckle displacement estimation in such cases is the use of super-resolution methods. In this article a robust and versatile partial differential equation based resolution enhancement method is presented for the analysis of low SNR speckle displacement signals. The application of the method to synthetic one-dimensional and two-dimensional signals as well as actual particle displacement images indicate that the method is capable of reducing noise effects and partially recovering lost correlation, which in turn makes the selection of smaller interrogation windows and achieving higher resolution and accuracy of displacement estimation possible. The method is applicable to both speckle and particle displacement images.

TARGETLESS PHOTOGRAMMETRY NETWORK SIMULATION FOR INSPECTION PLANNING IN OIL AND GAS INDUSTRY

Abstract. The oil and gas offshore industry demands regular inspections of components and structures that are subjected to extreme operational and environmental conditions. In this context, risers are pipelines that transport mainly oil, gas, water, and cables between submarine structures and the surface offshore platform, in the portion not touching the ocean floor. The emerged part of these risers is typically inspected by industrial climbing, which is a very time-consuming activity, has high operational costs, is dangerous and has a strong dependence on inspector skills. Remotely Piloted Aircraft Systems (RPAS) have been recently used for visual inspection of risers, however, no quantitative or geometrical evaluation has been conducted using this kind of image acquisition yet. An image-based measurement technique, such as close-range photogrammetry, can provide a 3D reconstruction using images, but a series of requisites is mandatory to achieve good results as image acquisition sequence, overlap, camera positioning network, spatial resolution and object texture in non-prepared and targetless scenes. The analysis of different image acquisition strategies using a real RPAS is too difficult because it demands a lot of time, good weather, daylight, and a scene similar to where risers are installed. An alternative is to use simulation. In this paper a ROS/Gazebo simulation is described and used to create a realistic textured 3D virtual environment of the platform, risers and RPA, providing a fast and low-cost solution to simulate different RPA trajectories for photogrammetry image acquisition in targetless scenes. These trajectories are evaluated by comparing the measured risers through photogrammetry to its CAD/simulated model. Since the scene is not prepared, the RPA position/orientation or a stereo vision setup can be used to set scale to the measurement result. The best trajectory found during simulations was also evaluated in a real experiment.

Gradient-based point tracking method and its application in the modal test of a solar array model

Structural dynamic testing is of great importance for a built-up aerospace structure. The response acquisition is commonly time and labor-consuming when the size of the test structure is large. The recently developed image-based measurement techniques, such as point tracking (PT) method, are well-suited to obtain the responses of lightweight and flexible membrane structures in structural dynamic testing. This paper develops two new PT methods: the improved PT method and the gradient-based PT method. In order to access the accuracy of the two methods and the original PT method, a comparison experiment of a cantilever beam is conducted, where the laser Doppler vibrometer signal is set as the reference. For the testing of a solar array model, the gradient-based PT method is applied to extract the responses of the 254 points on the surface. The first five modes of the solar array are estimated based on the extracted responses from the gradient-based PT method, which show agreement with the results from the traditional measurement technique. The gradient-based PT method shows advantages in reducing testing time, saving labor, and providing high spatial resolution in mode shapes.

Video Processing Techniques for the Contactless Investigation of Large Oscillations

The experimental acquisition of large vibrations presents various technical difficulties. Especially in the case of geometric nonlinearities, dealing with very flexible, very light structures causes minimal variations in mass or stiffness to affect severely the dynamical response. Thus, sensors’ added masses change the behaviour of the structure with respect to the unloaded condition. Moreover, the most common tools regularly employed for acquisition in vibration analysis - that is to say, laser vibrometers and accelerometers - are often designed with small amplitudes in mind. Their recordings are known to lack accuracy when the investigated structure undergoes large or very large motions, due to geometrical reasons. Image-based measurement techniques offer a valid solution to this problem. Here, an ensemble of three video processing techniques are benchmarked against each other and tested as viable options for the non-contact dynamic characterisation of slender beam-like structures. The methods have been applied to the case study of an aluminium spar for a highly-flexible airwing prototype and compared to the measurements recorded by a laser velocimeter and several Raspberry PI Inertial Measurement Units (IMUs), which also proved to be minimally invasive.

Accurate assessment and prediction of noise in clinical CT images.

The objectives of this study were (a) to devise a technique for measuring quantum noise in clinical body computed tomography (CT) images and (b) to develop a model for predicting that noise with high accuracy. The study included 83 clinical image sets at two dose levels (clinical and 50% reduced dose levels). The quantum noise in clinical images was measured by subtracting sequential slices and filtering out edges. Noise was then measured in the resultant uniform area. The noise measurement technique was validated using 17 clinical image cases and a turkey phantom. With a validated method to measure noise in clinical images, this noise was predicted by establishing the correlation between water-equivalent diameter (Dw) and noise in a variable-sized phantom and ascribing a noise level to the patient based on Dw estimated from CT image. The accuracy of this prediction model was validated using 66 clinical image sets. The error in noise measurement was within 1.5 HU across two reconstruction algorithms. In terms of noise prediction, across the 83 clinical image sets, the average discrepancies between predicted and measured noise were 6.9% and 6.6% for adaptive statistical iterative reconstruction and filtered back projection reconstruction, respectively. This study proposed a practically applicable method to assess quantum noise in clinical images. The image-based measurement technique enables automatic quality control monitoring of image noise in clinical practice. Further, a phantom-based model can accurately predict quantum noise level in patient images. The prediction model can be used to quantitatively optimize individual protocol to achieve targeted noise level in clinical images.

Kalisphera: an analytical tool to reproduce the partial volume effect of spheres imaged in 3D

In experimental mechanics, where 3D imaging is having a profound effect, spheres are commonly adopted for their simplicity and for the ease of their modeling. In this contribution we develop an analytical tool, ‘kalisphera’, to produce 3D raster images of spheres including their partial volume effect. This allows us to evaluate the metrological performance of existing image-based measurement techniques (knowing a priori the ground truth). An advanced application of ‘kalisphera’ is developed here to identify and accurately characterize spheres in real 3D x-ray tomography images with the objective of improving trinarization and contact detection. The effect of the common experimental imperfections is assessed and the overall performance of the tool tested on real images.

Optical imaging for low-cost structural measurements

For many structures, the easiest first step in trying to understand its condition is to take a good look. Often more detailed investigations are required really to characterise the behaviour and performance, enabling assessment of performance against design models, future repair needs and likely in-service lifetimes. If these investigations could be largely carried out in a similar manner to ‘taking a good look’ using digital still and video cameras, then considerable time and cost savings could be made. By processing images captured this way using specialist image correlation software, the systems investigated can create reliable and accurate measurements that could become part of the civil engineer's toolkit. This study shows that the optical image-based measurement techniques do indeed provide reliable data across a range of typical civil engineering surfaces and can provide both high precision and large area measurements providing useful information about the behaviour of civil engineering structures (in this case bridges), which might be useful for short and long-term monitoring.

Industrial Applications of Image Based Measurement Techniques in Aerodynamics – Problems, Progress and Future Needs

Since two decades technological progress at lasers, video techniques, optoelectronics, computers and evaluation algorithms allows to extract quantitative information from images of flows, even in complex environments. Continuous improvement of such image based measurement techniques and decreasing costs of equipment enabled many research groups to exploit these techniques for extraction of 2-dimensional or even 3-dimensional data mainly for fundamental research. Since a decade ago many image based measurement techniques have found interest in aerodynamics and are even used as a matter of routine in industrial applications, especially in large wind tunnels or at in-flight testing. Application is mainly performed in the scope of large industrial projects in European co-operation. For this purpose mobile measurement systems have been developed, which can be flexibly adjusted to particular testing environments. All data is acquired non-intrusively so that no interference of the flow field by the measurement is to occur. In consequence, the methods developed are particularly suited for the aero-dynamical and aero-acoustical analysis of complex, unsteady three-dimensional flow fields. The paper will report on the state-of-the-art of the application of image based measurement techniques in aerodynamics and will describe some of the current problems and future needs.

Research on Crack Measurement Technique in Solids Material with Digital Image Correlation (DIC)

The measurement of crack propagation in solids is of vital importance for the research of the fatigue characteristic of solid materials. By analysis of the image based measurement technique, a contact-free crack measurement approach was proposed based on image cross correlation algorithm. Experiment was performed to verify the validity of this method and it was found that the method is of high precision. The approach distinguished itself from traditional methods by several characteristics: it has relatively high measuring accuracy, the crack propagation can be continuously measured with this method, the amount of information that the approach provide is much more abundant than that of other methods. The method is proved to be an efficient method for crack measurement and can be applied in fatigue induced material cracking researches.

Comparison of CCD, CMOS and intensified cameras

The properties of digital cameras are often important to obtain accurate results with image based measurement techniques. Unfortunately, a detailed comparison of the sensor specifications and performance is not accessible as this information is normally not provided by the manufacturers and no generally accepted comparison standard exists. Therefore, a detailed quantitative comparison was performed to evaluate and assess the characteristics of state-of-the-art CCD, CMOS and intensified CMOS sensors. These results may be of assistance when selecting the appropriate sensor for a desired application.

Modern Image-based Experimental Methods ForApplication In Fluid Dynamics

Experimental methods for the investigation of complex flow phenomena have primarily been developed for applications to problems of industrial interest such as aerodynamics. Mostly point-wise methods to determine flow velocity, pressure, temperature, density etc. have been utilized in the past. One disadvantage of these methods is that they require a probe to be inserted in the flow field, i.e. the flow phenomenon to be studied may be influenced by the presence of the probe. Due to the recent technological progress (lasers, optoelectronics, high resolution high-sensitive video sensors, processing speed and memory capacity of computers, development of advanced evaluation and post-processing algorithms) image based measurement techniques have been developed which allow to observe flow phenomena from a large distance. These image based methods have found application as standard and robust measurement techniques for aerodynamic investigations in industrial wind tunnels in the past decade. Inparallel they are more and more often used for the investigation of flows in nature as weIl. An overview of the most important image based measurement techniques as weIl as an assessment oftheir potential to be used for flows in nature will be given.

Recent Developments of Image Based Measurement Methods for Application to Transonic Flows in Industrial Wind Tunnels

The experimental investigation of unsteady complex flow fields in wind tunnels requires advanced measurement techniques. The most important of such image based measurement techniques are those for the measurement of planar flow velocity fields, planar pressure distribution, model location and deformation, model temperature and quantitative high speed flow visualization. The applications as carried out by DLR range from low speed flows to transonic flows, from high lift configurations to propellers and rotors, from wake vortex investigations in catapult facilities and water towing tanks to investigations of vortex break down phenomena on delta wings. The capability to use image based measurement techniques in transonic flows requires dedicated technical developments and experienced scientists due to the special environment of a transonic wind tunnel. In this paper an overview of the state-of-the art of the application of image based measurement techniques in transonic flows as performed by DLR's Institute of Aerodynamics and Flow Technology will be given.

Study on large deformation characteristics of knitted fabric reinforced thermoplastic composites at forming temperature by digital image-based strain measurement technique

The continuous fiber reinforced thermoplastic composites, such as textile composites, have many attractive properties and functions and they are used widely nowadays in industrial applications. The important point in the design and manufacturing is that the composites as well as the reinforcing textiles undergo large deformation during thermoforming, which is much influential on the overall properties. Hence, the large deformation characteristics must be investigated at thermoforming temperature. For this sake, the digital image-based measurement technique of large strain distribution is developed. It is applicable to arbitrary complex shaped components with the help of 3D-CAD. As the large strain definition, Biot's strain is adopted that is calculated by the polar decomposition of the deformation gradient tensor based on continuum mechanics. The developed strain measurement technique is applied to the deep-drawing of aramid rib-knitted fabric reinforced polypropylene composites. The comparison between rib and plain-knitted fabrics reinforced composites is studied and many differences are found. The large deformation characteristics are also investigated from microscopic point of view under uniaxial tension at thermoforming temperature.