Conventional Calibration Methods Research Articles

Defining the medial-lateral axis of the femur: Medical imaging, conventional and functional calibration methods lead to differences in hip rotation kinematics for children with torsional deformities

Hip rotation during gait is a major indicator for femoral derotation osteotomy. However, repeatability of hip rotation is poor because of discrepancies in determining the medial-lateral axis of the femur. Combining 3D gait analysis with medical imaging allows in vivo evaluation of current clinical methods. We used the condylar axis, identified from low dose biplanar radiographs (EOS imaging Inc), as our reference to evaluate conventional, functional calibration and freehand 3D ultrasound methods to define the medial-lateral axis in children with lower-limb torsional deformities.Twenty participants underwent 3D gait analysis accompanied by freehand 3D ultrasound and biplanar radiographs. The condylar axis identified from biplanar radiographs provided the reference method used to construct the femoral coordinate system. This was used to evaluate a conventional, two functional calibration methods (axis transformation technique and 2DoFKnee) and freehand 3D ultrasound. We measured reliability of 3D localisation of skin markers and anatomical landmarks from the biplanar radiographs.Localisation of skin markers (SD 0.4 mm) and anatomical landmarks (SD 1.3 mm) from the biplanar radiographs were reliable, leading to a precision of 1° for the condylar axis after registration in the motion capture system. The freehand 3D ultrasound produced similar results to the biplanar radiographs reference, with internal hip rotation during gait of 18° and 19° respectively. The conventional and functional calibration methods were predominantly external compared to the reference, with average hip rotation of 4–6° internal.Freehand 3D ultrasound and biplanar radiographs provide reliable means to define the medial-lateral axis of femur for gait analysis, and aid clinical interpretation in children with torsional deformities.

Calibration of a 3D Hydrodynamic Meandering River Model Using Fully Spatially Distributed 3D ADCP Velocity Data

AbstractThis study used fully spatially distributed three-dimensional (3D) velocity data for the improved calibration of a 3D hydrodynamic model. The conventional calibration methods do not account...

A Robust and Accurate Calibration Method for Out-of-focus Camera

Conventional camera calibration methods regard camera as ideal pinhole model and require well-focused images, which can't be satisfied for long-range photogrammetry or low depth-of-field lens. In this paper, we propose a novel active calibration method for out-of-focus camera using LCD monitor. Firstly, we estimate the defocus map by the temporal coded binary-shift patterns, which makes our method more accurate. Secondly, based on the defocus map, we encode LCD pixel's coordinates into phase-shift patterns with optimal frequency and step properties, and then deblur captured patterns. Finally, deblurred patterns are decoded to generate dense phases map to extract accurate feature points coordinates. Our method significantly improves camera calibrations robustness to lens' defocus, noises, glass refraction compared with state-of-art methods. Experimental results demonstrate that our method is superior to conventional methods whether camera is in- or out-of- focus.

Review of Calibration Methods for Scheimpflug Camera

The Scheimpflug camera offers a wide range of applications in the field of typical close-range photogrammetry, particle image velocity, and digital image correlation due to the fact that the depth-of-view of Scheimpflug camera can be greatly extended according to the Scheimpflug condition. Yet, the conventional calibration methods are not applicable in this case because the assumptions used by classical calibration methodologies are not valid anymore for cameras undergoing Scheimpflug condition. Therefore, various methods have been investigated to solve the problem over the last few years. However, no comprehensive review exists that provides an insight into recent calibration methods of Scheimpflug cameras. This paper presents a survey of recent calibration methods of Scheimpflug cameras with perspective lens, including the general nonparametric imaging model, and analyzes in detail the advantages and drawbacks of the mainstream calibration models with respect to each other. Real data experiments including calibrations, reconstructions, and measurements are performed to assess the performance of the models. The results reveal that the accuracies of the RMM, PLVM, PCIM, and GNIM are basically equal, while the accuracy of GNIM is slightly lower compared with the other three parametric models. Moreover, the experimental results reveal that the parameters of the tangential distortion are likely coupled with the tilt angle of the sensor in Scheimpflug calibration models. The work of this paper lays the foundation of further research of Scheimpflug cameras.

Efficient material decomposition method for dual-energy X-ray cargo inspection system

Dual-energy X-ray inspection systems are widely used today for it provides X-ray attenuation contrast of the imaged object and also its material information. Material decomposition capability allows a higher detection sensitivity of potential targets including purposely loaded impurities in agricultural product inspections and threats in security scans for example. Dual-energy X-ray transmission data can be transformed into two basis material thickness data, and its transformation accuracy heavily relies on a calibration of material decomposition process. The calibration process in general can be laborious and time consuming. Moreover, a conventional calibration method is often challenged by the nonuniform spectral characteristics of the X-ray beam in the entire field-of-view (FOV). In this work, we developed an efficient material decomposition calibration process for a linear accelerator (LINAC) based high-energy X-ray cargo inspection system. We also proposed a multi-spot calibration method to improve the decomposition performance throughout the entire FOV. Experimental validation of the proposed method has been demonstrated by use of a cargo inspection system that supports 6 MV and 9 MV dual-energy imaging.

Improvement of Gaofen-3 Absolute Positioning Accuracy Based on Cross-Calibration.

The Chinese Gaofen-3 (GF-3) mission was launched in August 2016, equipped with a full polarimetric synthetic aperture radar (SAR) sensor in the C-band, with a resolution of up to 1 m. The absolute positioning accuracy of GF-3 is of great importance, and in-orbit geometric calibration is a key technology for improving absolute positioning accuracy. Conventional geometric calibration is used to accurately calibrate the geometric calibration parameters of the image (internal delay and azimuth shifts) using high-precision ground control data, which are highly dependent on the control data of the calibration field, but it remains costly and labor-intensive to monitor changes in GF-3’s geometric calibration parameters. Based on the positioning consistency constraint of the conjugate points, this study presents a geometric cross-calibration method for the rapid and accurate calibration of GF-3. The proposed method can accurately calibrate geometric calibration parameters without using corner reflectors and high-precision digital elevation models, thus improving absolute positioning accuracy of the GF-3 image. GF-3 images from multiple regions were collected to verify the absolute positioning accuracy after cross-calibration. The results show that this method can achieve a calibration accuracy as high as that achieved by the conventional field calibration method.

Sensitivity-Based Parameter Calibration and Model Validation Under Model Error

In calibrating model parameters, it is important to include the model discrepancy term in order to capture missing physics in simulation, which can result from numerical, measurement, and modeling errors. Ignoring the discrepancy may lead to biased calibration parameters and predictions, even with an increasing number of observations. In this paper, a simple yet efficient calibration method is proposed based on sensitivity information when the simulation model has a model error and/or numerical error but only a small number of observations are available. The sensitivity-based calibration method captures the trend of observation data by matching the slope of simulation predictions and observations at different designs and then utilizing a constant value to compensate for the model discrepancy. The sensitivity-based calibration is compared with the conventional least squares calibration method and Bayesian calibration method in terms of parameter estimation and model prediction accuracies. A cantilever beam example, as well as a honeycomb tube crush example, is used to illustrate the calibration process of these three methods. It turned out that the sensitivity-based method has a similar performance with the Bayesian calibration method and performs much better than the conventional method in parameter estimation and prediction accuracy.

Monitoring the metering performance of an electronic voltage transformer on-line based on cyber-physics correlation analysis

Metering performance is the key parameter of an electronic voltage transformer (EVT), and it requires high accuracy. The conventional off-line calibration method using a standard voltage transformer is not suitable for the key equipment in a smart substation, which needs on-line monitoring. In this article, we propose a method for monitoring the metering performance of an EVT on-line based on cyber-physics correlation analysis. By the electrical and physical properties of a substation running in three-phase symmetry, the principal component analysis method is used to separate the metering deviation caused by the primary fluctuation and the EVT anomaly. The characteristic statistics of the measured data during operation are extracted, and the metering performance of the EVT is evaluated by analyzing the change in statistics. The experimental results show that the method successfully monitors the metering deviation of a Class 0.2 EVT accurately. The method demonstrates the accurate evaluation of on-line monitoring of the metering performance on an EVT without a standard voltage transformer.

Azimuth-Variant Phase Error Calibration Technique for Multichannel SAR Systems

Multiple azimuth channels are usually employed to overcome the inherent limitation between high resolution and wide swath in synthetic aperture radar systems. However, unavoidable channel phase errors will significantly degrade the performance of ambiguity suppression. Conventional calibration methods usually regard these phase errors as constants during the whole observation time and ignore the azimuth-variant phase errors, which may not totally suppress azimuth ambiguities especially for very strong targets. This letter presents an azimuth-variant phase error calibration technique. The proposed technique first selects a strong point-like target as the calibration source. Then, the azimuth-variant phase errors can be estimated by comparing the phase of the calibration source and the corresponding steering vector in the range-compressed signals. Besides, a preprocessing method is presented to improve the calibration accuracy when the selected calibration source is affected by noise or interferences. Theoretical analysis and experiments demonstrate the feasibility of the proposed technique.

Microwave-Induced Plasma Optical Emission Spectrometry (MIP OES) and Standard Dilution Analysis to Determine Trace Elements in Pharmaceutical Samples.

In this work, we evaluate the application of microwave-induced plasma optical emission spectrometry (MIP OES) to determine of Al, Cr, Co, Cu, Fe, Mn, Ni and Zn in children's cough syrup, eye drops, and oral antiseptic using standard dilution analysis (SDA). The SDA method is simple, with only two calibration solutions prepared per sample. The first solution (S1), composed of 50% sample +50% of a standard solution, is introduced into the plasma and the analytical signals are monitored in a time-resolved fashion. Then, the second solution (S2), composed of 50% sample +50% blank, is poured into the vial containing S1. As the solutions mix, the analytical signals gradually drop to a stable baseline. The calibration curve is computed by plotting the ratio of the analyte signal (SA) over the internal standard signal (which is also part of S1) (SIS) on the y-axis, versus the inverse of the IS concentration on the x-axis (i.e., SA/SIS versus 1/CIS). In this study, SDA results were compared with values obtained with the traditional methods of external calibration (EC), internal standardization (IS), and standard additions (SA) in MIP OES determinations. The precision (represented as percent RSD) for SDA showed values in the range of 2.50-8.00% for all samples, while conventional calibration methods showed RSDs in the range of 6.40-32.50% for EC, 8.30-21.80% for IS, and 5.20-17.40% for SA. The LODs calculated for SDA are below the maximum limits allowed by the major pharmaceutical regulatory agencies, and presents superior precision and accuracy compared to the traditional calibration methods. Considering its simplicity and efficiency, SDA is an important new tool for accurate analyses of pharmaceuticals.

Analytical and experimental studies to obtain electrical resistivity in a small-scaled laboratory test

Electrical resistivity surveys have been used to investigate soil behavior at the microscale, and thus they require a method for obtaining accurate electrical resistivity. The previously suggested geometric factor ignores the dimensions of the electrode due to the scale effect present in field conditions, thus necessitating a more appropriate method to capture reliable electrical resistivity for laboratory tests. Our objective is to suggest an analytical solution to obtain reliable electrical resistivity in laboratory testing. Models are verified through laboratory tests and statistical methods. The relationship between electrical resistance and electrical resistivity is analytically defined by Ohm’s law and Gauss’s flux theorem. Consequently, the underlying importance of electrode capacitance including electrode length and diameter for estimating electrical resistivity is evaluated. In addition to the electrical resistivity estimated based on Ohm’s law (EOL), capacitance based on the single-electrode model (CSM) and the multiple-electrode model (CMM), electrical resistivity based on the conventional calibration method is also addressed. Four-equally spaced electrode probe system is designed to measure the electrical resistance. The estimated electrical resistivity based on each model (EOL, CSM, CMM, and [Formula: see text]) is compared with the electrical resistivity estimated from the conductivity meter to verify the suggested models. The electrical resistivity estimated from EOL shows high reliability. Our results underline the significance of EOL model in the conversion of measured electrical resistance into electrical resistivity in laboratory tests.

Method for calibration accuracy improvement of projector-camera-based structured light system

Calibration is a critical step for the projector-camera-based structured light system (SLS). Conventional SLS calibration means usually use the calibrated camera to calibrate the projector device, and the optimization of calibration parameters is applied to minimize the two-dimensional (2-D) reprojection errors. A three-dimensional (3-D)-based method is proposed for the optimization of SLS calibration parameters. The system is first calibrated with traditional calibration methods to obtain the primary calibration parameters. Then, a reference plane with some precisely printed markers is used for the optimization of primary calibration results. Three metric error criteria are introduced to evaluate the 3-D reconstruction accuracy of the reference plane. By treating all the system parameters as a global optimization problem and using the primary calibration parameters as initial values, a nonlinear multiobjective optimization problem can be established and solved. Compared with conventional calibration methods that adopt the 2-D reprojection errors for the camera and projector separately, a global optimal calibration result can be obtained by the proposed calibration procedure. Experimental results showed that, with the optimized calibration parameters, measurement accuracy and 3-D reconstruction quality of the system can be greatly improved.

A Fast Calibration Method for Magnetometer Array and the Application of Ferromagnetic Target Localization

The vector magnetometer array is commonly used in ferromagnetic target detection and localization. Due to technological limitations, the accuracy of measurements is restricted by magnetometer errors, misalignment errors between magnetometers and the interference of soft iron and hard iron. In order to correct the measurement errors, a calibration method with less computation time is proposed. A magnetometer array is designed for ferromagnetic target detection and localization, which is calibrated by the proposed method. The results of our empirical evaluations confirm that the proposed method outperforms conventional calibration method both in terms of computation cost and accuracy. Different test specimens are used to test the detection range and the localization accuracy of the system. The array system compensated by the proposed method has a larger detection range.

Assessment of Applicability of Portable HPGe Detector with In Situ Object Counting System based on Performance Evaluation of Thyroid Radiobioassays

Background: Different cases exist in the measurement of thyroid radiobioassays owing to the individual characteristics of the subjects, especially the potential variation in the counting efficiency. An In situ Object Counting System (ISOCS) was developed to perform an efficiency calibration based on the Monte Carlo calculation, as an alternative to conventional calibration methods. The purpose of this study is to evaluate the applicability of ISOCS to thyroid radiobioassays by comparison with a conventional thyroid monitoring system. Materials and Methods: The efficiency calibration of a portable high-purity germanium (HPGe) detector was performed using ISOCS software. In contrast, the conventional efficiency calibration, which needed a radioactive material, was applied to a scintillator-based thyroid monitor. Four radioiodine samples that contained 125I and 131I in both aqueous solution and gel forms were measured to evaluate radioactivity in the thyroid. ANSI/HPS N13.30 performance criteria, which included the relative bias, relative precision, and root-mean-squared error, were applied to evaluate the performance of the measurement system. Results and Discussion: The portable HPGe detector could measure both radioiodines with ISOCS but the thyroid monitor could not measure 125I because of the limited energy resolution of the NaI(Tl) scintillator. The 131I results from both detectors agreed to within 5% with the certified results. Moreover, the 125I results from the portable HPGe detector agreed to within 10% with the certified results. All measurement results complied with the ANSI/HPS N13.30 performance criteria. Conclusion: The results of the intercomparison program indicated the feasibility of applying ISOCS software to direct thyroid radiobioassays. The portable HPGe detector with ISOCS software can provide the convenience of efficiency calibration and higher energy resolution for identifying photopeaks, compared with a conventional thyroid monitor with a NaI(Tl) scintillator. The application of ISOCS software in a radiation emergency can improve the response in terms of internal contamination monitoring.

A novel calibration strategy based on background correction for quantitative circular dichroism spectroscopy

When using circular dichroism (CD) spectroscopy for quantitative analysis, the samples to be analyzed must be free of light-absorbing interferences. However, in real-world samples, the presence of background absorbers is practically unavoidable. The difference in the matrices between the real-world samples to be analyzed and the standard samples (on which either univariate or multivariate calibration model was built) would result in systematic errors in the quantification results of CD spectroscopy. In this contribution, a novel calibration strategy for quantitative CD spectroscopic analysis was proposed. The main idea of the proposed calibration strategy is to project the CD spectra of both the standard samples and the real-world sample to be analyzed onto a projection space orthogonal to the space spanned by the background CD spectrum of the real-world sample and then build a multivariate calibration model on the transformed CD spectra of the standard samples. The performance of the proposed calibration strategy was tested and compared with conventional univariate and multivariate calibration methods in the quantification of Pb2+ in cosmetic samples using CD spectroscopy in combination with a G-quadruplex DNAzyme (e.g. PS2.M). Experiments results revealed that the proposed calibration strategy could mitigate the influence of the difference in the matrices between the standard samples and cosmetic samples and realized quantitative analysis of Pb2+ in cosmetic samples, with precision and accuracy comparable to atomic absorption spectroscopy. The proposed calibration strategy has the features of simplicity and effectiveness, its combination with CD spectroscopic probes can realize accurate and precise quantification of analytes in complex samples using CD spectroscopy.

Out-of-focus color camera calibration with one normal-sized color-coded pattern

Conventional camera calibration methods always require well-focused target images for accurate feature detection. This requirement brings many challenges for long-range vision systems, due to the difficulty of fabricating an equivalent size target to the sensing field-of-view (FOV). This paper presents an out-of-focus color camera calibration method with one normal-sized color-coded pattern as the calibration target. The red, green and blue (RGB) channels of the color pattern are encoded with three phase-shift circular grating (PCG) arrays. The PCG centers used as feature points are extracted by ellipse fitting of the 2π-phase points. Experiments demonstrate that the proposed method can achieve accurate calibration results even under severe defocus.

An Iterative Distortion Compensation Algorithm for Camera Calibration Based on Phase Target.

Camera distortion is a critical factor affecting the accuracy of camera calibration. A conventional calibration approach cannot satisfy the requirement of a measurement system demanding high calibration accuracy due to the inaccurate distortion compensation. This paper presents a novel camera calibration method with an iterative distortion compensation algorithm. The initial parameters of the camera are calibrated by full-field camera pixels and the corresponding points on a phase target. An iterative algorithm is proposed to compensate for the distortion. A 2D fitting and interpolation method is also developed to enhance the accuracy of the phase target. Compared to the conventional calibration method, the proposed method does not rely on a distortion mathematical model, and is stable and effective in terms of complex distortion conditions. Both the simulation work and experimental results show that the proposed calibration method is more than 100% more accurate than the conventional calibration method.

Robust Channel Phase Error Calibration Algorithm for Multichannel High-Resolution and Wide-Swath SAR Imaging

High-resolution and wide-swath synthetic aperture radar (SAR) imaging can be achieved by the azimuth multichannel system. The minimum variance distortionless response (MVDR) beamformer can be utilized to suppress azimuth ambiguities. However, the presence of channel phase errors significantly deteriorates the performance of the azimuth multichannel SAR system. Instead of employing subspace techniques, this letter proposes a robust channel phase error calibration algorithm via maximizing the MVDR beamformer output power. Compared with the conventional subspace-based calibration methods, there is no redundancy of channels required to estimate the subspaces in the proposed algorithm. Also, the proposed algorithm is relatively robust, because it avoids the subspace swap phenomenon, which probably takes place at low signal-to-noise ratios for the subspace techniques. Moreover, the proposed method has the advantage of estimating the channel phase errors without covariance matrix decomposition, which reduces the computation load. The simulation experiments and the real data processing validate the effectiveness of the proposed calibration method.

Automatic Model Calibration: A New Way to Improve Numerical Weather Forecasting

AbstractWeather forecasting skill has been improved over recent years owing to advances in the representation of physical processes by numerical weather prediction (NWP) models, observational systems, data assimilation and postprocessing, new computational capability, and effective communications and training. There is an area that has received less attention so far but can bring significant improvement to weather forecasting—the calibration of NWP models, a process in which model parameters are tuned using certain mathematical methods to minimize the difference between predictions and observations. Model calibration of the NWP models is difficult because 1) there are a formidable number of model parameters and meteorological variables to tune, and 2) a typical NWP model is very expensive to run, and conventional model calibration methods require many model runs (up to tens of thousands) or cannot handle the high dimensionality of NWP models. This study demonstrates that a newly developed automatic model calibration platform can overcome these difficulties and improve weather forecasting through parameter optimization. We illustrate how this is done with a case study involving 5-day weather forecasting during the summer monsoon in the greater Beijing region using the Weather Research and Forecasting Model. The keys to automatic model calibration are to use global sensitivity analysis to screen out the most important parameters influencing model performance and to employ surrogate models to reduce the need for a large number of model runs. Through several optimization and validation studies, we have shown that automatic model calibration can improve precipitation and temperature forecasting significantly according to a number of performance measures.

Hybrid dynamic radioactive particle tracking (RPT) calibration technique for multiphase flow systems

The radioactive particle tracking (RPT) technique has been utilized to measure three-dimensional hydrodynamic parameters for multiphase flow systems. An analytical solution to the inverse problem of the RPT technique, i.e. finding the instantaneous tracer positions based upon instantaneous counts received in the detectors, is not possible. Therefore, a calibration to obtain a counts-distance map is needed. There are major shortcomings in the conventional RPT calibration method due to which it has limited applicability in practical applications. In this work, the design and development of a novel dynamic RPT calibration technique are carried out to overcome the shortcomings of the conventional RPT calibration method. The dynamic RPT calibration technique has been implemented around a test reactor with 1foot in diameter and 1 foot in height using Cobalt-60 as an isotopes tracer particle. Two sets of experiments have been carried out to test the capability of novel dynamic RPT calibration. In the first set of experiments, a manual calibration apparatus has been used to hold a tracer particle at known static locations. In the second set of experiments, the tracer particle was moved vertically downwards along a straight line path in a controlled manner. The obtained reconstruction results about the tracer particle position were compared with the actual known position and the reconstruction errors were estimated. The obtained results revealed that the dynamic RPT calibration technique is capable of identifying tracer particle positions with a reconstruction error between 1 to 5.9 mm for the conditions studied which could be improved depending on various factors outlined here.