Large Calibration Errors Research Articles

High-precision binocular camera calibration method based on a 3D calibration object.

A high-precision binocular camera calibration method is proposed to address the issues of poor calibration accuracy and large calibration errors in current practical applications. This method uses a triangular stereo sphere as the calibration object and employs steps, such as ellipse fitting, Cholesky decomposition, homography matrix solution, and nonlinear optimization, to compute the intrinsic and extrinsic parameters, distortion parameters, and relative pose of the binocular camera. Moreover, this method simplifies the correspondences between primitives, enabling simultaneous calibration of multiple viewpoint cameras. This method is also suitable for both binocular cameras consisting of two different structured monocular cameras and those composed of two image sensors within the same structure. Experimental results showed that this method outperforms traditional algorithms in terms of binocular camera calibration accuracy, calibration errors between left and right cameras, and robustness, resulting in a significant improvement in overall algorithm performance.

Uniaxial MEMS-based 3D reconstruction using pixel refinement.

A uniaxial micro-electro-mechanical systems (MEMS) micro-vibration mirror can be used to construct a new type of fringe projection profilometry (FPP) system. In FPP system calibration, some pixels may be calibrated worse than other pixels due to various error sources, which will affect the final reconstruction accuracy. In addition, there are some difficulties in calibrating the MEMS-based system because a projector using the uniaxial vibration mirror does not have focusing optics and can only project unidirectional fringes. In this paper, we developed an FPP system using a uniaxial MEMS micro-vibration mirror. To solve the calibration problems, we propose a calibration model suitable for the MEMS-based system and a pixel refinement method. These pixels with relatively large calibration errors are called outlier-pixels, which will significantly increase the error of the following 3D mapping. Therefore, the pixel refinement method classifies all pixels based on a frequency distribution histogram of calibration errors during calibration and prevents outlier-pixels from participating in the following 3D mapping. The experimental results show that the proposed method can improve the accuracy of 3D reconstruction, and the feasibility of the self-developed system is verified.

A high-accuracy calibration method for fusion systems of millimeter-wave radar and camera

Multi-sensor information fusion is widely used in the field of unmanned aerial vehicles obstacle avoidance flight, particularly in millimeter-wave (MMW) radar and camera fusion systems. Calibration accuracy plays a crucial role in fusion systems. The low-angle measurement accuracy of the MMW radar usually causes large calibration errors. To reduce calibration errors, a high-accuracy calibration method based on a region of interest (ROI) and an artificial potential field was proposed in this paper. The ROI was selected based on the initial calibration information and the MMW radar’s angle measurement error range from the image. An artificial potential field was established using the pixels of the ROI. Two moving points were set at the left and right ends of the ROI initially. The potential forces of the two moving points are different because the pixels of the obstacle and the background are different in the image. The two moving points were iteratively moved towards each other according to the force until their distance was less than the iteration step. The new calibration point is located in the middle of the final position of the two moving points. In contrast to the existing calibration methods, the proposed method avoids the limitations of low angle measurement accuracy by using image pixels. The experimental results show that the calibration errors decrease by 83.95% and 75.79%, which is significantly improved compared to the traditional methods and indicates the efficiency of the proposed method.

Technical Note: Vendor miscalibration of preclinical orthovoltage irradiator identified through independent output check.

Accurate radiation dosimetry in radiobiological experiments is crucial for preclinical research in advancement of cancer treatment. Vendors of cell irradiators often perform calibration for end-users. However, calibration accuracy remains unclear due to missing detailed information on calibration equipment and procedures. In this study, we report our findings of a vender miscalibration of the radiation output and our investigation on the root cause of the discrepancy. Independent calibration verification for a commercial preclinical orthovoltage irradiator was conducted. Initially, in the absence of ionization chambers calibrated at kV energy, radiochromic films (EBT3) was first calibrated at MV energy. Energy correction factors from literature were used to create an in-house kV dosimetry system. The miscalibration identified with the in-house kV EBT3 dosimetry was later confirmed by ADCL calibrated ionization chambers (Exradin A1SL and PTW 30013) at kV energy. Ionization chambers were suspended in-air following TG-61 recommendation for output calibration. To investigate the root cause of the miscalibration, additional measurements were performed with ionization chambers placed on the shelf. A validated Monte Carlo simulation code was also used to investigate the impact of placing the ionization chamber on the shelf instead of suspending it in air during the vendor-performed calibration process. Up to a 6% dosimetry error was observed when comparing the vendor calibrated output of the preclinical irradiator with our independent calibration check. Further investigation showed incorrect setups in the vendor's calibration procedure which may result in dose errors up to 11% from the backscatter of the shelf board during calibration, and up to 5% from omitting temperature and pressure corrections to ionization chamber readings. Our study revealed large dose calibration errors caused by incorrect setup and the omission of temperature/pressure correction in the vendor's calibration procedure. The findings also highlighted the importance of performing an independent check of the dose calibration for preclinical kV irradiators. More absolute dosimetry training is needed for both vendors and end users for establishing accurate absolute dosimetry.

A Novel Calibration Board and Experiments for 3D LiDAR and Camera Calibration.

Aiming at the problems of feature point calibration method of 3D light detection and ranging (LiDAR) and camera calibration that are calibration boards in various forms, incomplete information extraction methods and large calibration errors, a novel calibration board with local gradient depth information and main plane square corner information (BWDC) was designed. In addition, the "three-step fitting interpolation method" was proposed to select feature points and obtain the corresponding coordinates of feature points in the LiDAR coordinate system and camera pixel coordinate system based on BWDC. Finally, calibration experiments were carried out, and the calibration results were verified by methods such as incremental verification and reprojection error comparison. The calibration results show that using BWDC and the "three-step fitting interpolation method" can solve quite accurate coordinate transformation matrix and intrinsic and external parameters of sensors, which dynamically change within 0.2% in the repeatable experiments. The difference between the experimental value and the actual value in the incremental verification experiment is about 0.5%. The average reprojection error is 1.8312 pixels, and the value changes at different distances do not exceed 0.1 pixels, which also show that the calibration method is accurate and stable.

Vegetation effects on temperature calibrations of branched glycerol dialkyl glycerol tetraether (brGDGTs) in soils

Distributions of branched glycerol dialkyl glycerol tetraether (brGDGT) lipids are sensitive to environmental parameters, which enables their use in paleoenvironmental studies. In particular, the degree of methylation of brGDGTs (MBT and MBT′) and the cyclization ratio of brGDGTs (CBT) are often used to estimate changes in temperature in paleoclimate studies. Application of these proxies requires reliable and precise calibrations to temperature. However, available calibrations of brGDGT proxies to temperature have large calibration errors in arid areas which suggests that other environmental variables influence brGDGT distributions in these areas. Here we analyze brGDGT distributions in soils from the Tibetan Plateau, an arid/semi-arid region with gradients in vegetation cover (forest, grassland, and desert) to examine: (1) the applicability of MBT′/CBT calibrations to reconstruct mean annual air temperature (MAAT); and (2) the effects of vegetation cover on temperature calibrations. We show that temperatures reconstructed using the global soil MBT′/CBT calibrations are warmer than instrumentally observed temperature, and that the calibrations are significantly influenced by the presence or absence of vegetation. Excluding sample sites without vegetation (bare soil) from temperature calibrations substantially improves the correlation between observed and reconstructed temperatures. Within the vegetated soils, we reanalyzed published global soil calibration datasets, including results of analyses that either did or did not separate 5- and 6-methyl brGDGT isomers, and find that the correlation between MBT′5Me and MBT′ and temperature varies for different vegetation types (grass and forest) and that the error of temperature calibrations is reduced if calibrations are separated by the type of vegetative cover (grass or forest). We test these new calibrations in sequences from the Lantian and Mangshan loess of the Chinese Loess Plateau and find that reconstructed temperatures from both modern and Last Glacial Maximum sediments are more consistent with other reconstructions and climate model simulations when vegetation-specific calibrations are applied to the brGDGT data. The results indicate that changes in vegetation should be taken into consideration when applying brGDGT proxies to reconstruct past changes in climate.

On Inertial Body Tracking in the Presence of Model Calibration Errors.

In inertial body tracking, the human body is commonly represented as a biomechanical model consisting of rigid segments with known lengths and connecting joints. The model state is then estimated via sensor fusion methods based on data from attached inertial measurement units (IMUs). This requires the relative poses of the IMUs w.r.t. the segments—the IMU-to-segment calibrations, subsequently called I2S calibrations—to be known. Since calibration methods based on static poses, movements and manual measurements are still the most widely used, potentially large human-induced calibration errors have to be expected. This work compares three newly developed/adapted extended Kalman filter (EKF) and optimization-based sensor fusion methods with an existing EKF-based method w.r.t. their segment orientation estimation accuracy in the presence of model calibration errors with and without using magnetometer information. While the existing EKF-based method uses a segment-centered kinematic chain biomechanical model and a constant angular acceleration motion model, the newly developed/adapted methods are all based on a free segments model, where each segment is represented with six degrees of freedom in the global frame. Moreover, these methods differ in the assumed motion model (constant angular acceleration, constant angular velocity, inertial data as control input), the state representation (segment-centered, IMU-centered) and the estimation method (EKF, sliding window optimization). In addition to the free segments representation, the optimization-based method also represents each IMU with six degrees of freedom in the global frame. In the evaluation on simulated and real data from a three segment model (an arm), the optimization-based method showed the smallest mean errors, standard deviations and maximum errors throughout all tests. It also showed the lowest dependency on magnetometer information and motion agility. Moreover, it was insensitive w.r.t. I2S position and segment length errors in the tested ranges. Errors in the I2S orientations were, however, linearly propagated into the estimated segment orientations. In the absence of magnetic disturbances, severe model calibration errors and fast motion changes, the newly developed IMU centered EKF-based method yielded comparable results with lower computational complexity.

Evaluation of Collision Cross Section Calibrants for Structural Analysis of Lipids by Traveling Wave Ion Mobility-Mass Spectrometry.

Collision cross section (CCS) measurement of lipids using traveling wave ion mobility-mass spectrometry (TWIM-MS) is of high interest to the lipidomics field. However, currently available calibrants for CCS measurement using TWIM are predominantly peptides that display quite different physical properties and gas-phase conformations from lipids, which could lead to large CCS calibration errors for lipids. Here we report the direct CCS measurement of a series of phosphatidylcholines (PCs) and phosphatidylethanolamines (PEs) in nitrogen using a drift tube ion mobility (DTIM) instrument and an evaluation of the accuracy and reproducibility of PCs and PEs as CCS calibrants for phospholipids against different classes of calibrants, including polyalanine (PolyAla), tetraalkylammonium salts (TAA), and hexakis(fluoroalkoxy)phosphazines (HFAP), in both positive and negative modes in TWIM-MS analysis. We demonstrate that structurally mismatched calibrants lead to larger errors in calibrated CCS values while the structurally matched calibrants, PCs and PEs, gave highly accurate and reproducible CCS values at different traveling wave parameters. Using the lipid calibrants, the majority of the CCS values of several classes of phospholipids measured by TWIM are within 2% error of the CCS values measured by DTIM. The development of phospholipid CCS calibrants will enable high-accuracy structural studies of lipids and add an additional level of validation in the assignment of identifications in untargeted lipidomics experiments.

Significantly Improving Regional Seismic Amplitude Tomography at Higher Frequencies by DeterminingS‐Wave Bandwidth

Characterizing regional seismic signals continues to be a difficult problem due to their variability. Calibration of these signals is very important to many aspects of monitoring underground nuclear explosions, including detecting seismic signals, discriminating explosions from earthquakes, and reliably estimating magnitude and yield. Amplitude tomography, which simultaneously inverts for source, propagation, and site effects, is a leading method of calibrating these signals. A major issue in amplitude tomography is the data quality of the input amplitude measurements. Pre‐event and prephase signal‐to‐noise ratio (SNR) tests are typically used but can frequently include bad signals and exclude good signals. The deficiencies of SNR criteria, which are demonstrated here, lead to large calibration errors. To ameliorate these issues, we introduce a semiautomated approach to assess the bandwidth of a spectrum where it behaves physically. We determine the maximum frequency (denoted as F max) where it deviates from this behavior due to inflections at which noise or spurious signals start to bias the spectra away from the expected decay. We compare two amplitude tomography runs using the SNR and new F max criteria and show significant improvements to the stability and accuracy of the tomography output for frequency bands higher than 2 Hz by using our assessments of valid S ‐wave bandwidth. We compare Q estimates, P / S residuals, and some detailed results to explain the improvements. For frequency bands higher than 4 Hz, needed for effective P / S discrimination of explosions from earthquakes, the new bandwidth criteria sufficiently fix the instabilities and errors so that the residuals and calibration terms are useful for application.

Parallel/Opposed: IMRT QA using treatment log files is superior to conventional measurement-based method.

Parallel/Opposed: IMRT QA using treatment log files is superior to conventional measurement-based method.

Calibration error analysis of electro-optical detection systems

We present detailed analysis of calibration process error for electro-optical detection systems, which can be simplified as the plane rotation around a non-orthogonal axis. By means of octonions it firstly proves that the plane rotation around a non-orthogonal axis can be decomposed into rotations around two perpendicular axes. The rotation is further divided into three steps, and the calibration error is hence discussed and obtained. The simulation and test results indicate that there are large calibration errors in calibration process. The pointing error can be effectively improved after separating error components, which provides a more accurate set data for further compensation.

ROBUST ADAPTIVE BEAMFORMING AGAINST ARRAY CALIBRATION ERRORS

Adaptive beamforming methods degrade in the presence of model mismatch. In this paper, we develop a modifled interference covariance matrix reconstruction based beamformer that is robust against large array calibration errors. The calibration errors can come from the element position errors, and/or amplitude and phase errors, etc.. The proposed method is based on the fact that the sample covariance matrix can approximate the interference covariance matrix properly when the desired signal is small, and a reconstructed covariance matrix based on the Capon spectral will be better than the sample covariance matrix when the desired signal is large. A weighted summation of two covariance matrices in references is used to reconstruct the interference covariance matrix. Moreover, a computationally e-cient convex optimization-based algorithm is used to estimate the mismatch of the steering vector associated with the desired signal. Several simulation cases are applied to show the superiority of the proposed method over other robust adaptive beamformers.

Joint Multi-Layer Segmentation and Reconstruction for Free-Viewpoint Video Applications

Current state-of-the-art image-based scene reconstruction techniques are capable of generating high-fidelity 3D models when used under controlled capture conditions. However, they are often inadequate when used in more challenging environments such as sports scenes with moving cameras. Algorithms must be able to cope with relatively large calibration and segmentation errors as well as input images separated by a wide-baseline and possibly captured at different resolutions. In this paper, we propose a technique which, under these challenging conditions, is able to efficiently compute a high-quality scene representation via graph-cut optimisation of an energy function combining multiple image cues with strong priors. Robustness is achieved by jointly optimising scene segmentation and multiple view reconstruction in a view-dependent manner with respect to each input camera. Joint optimisation prevents propagation of errors from segmentation to reconstruction as is often the case with sequential approaches. View-dependent processing increases tolerance to errors in through-the-lens calibration compared to global approaches. We evaluate our technique in the case of challenging outdoor sports scenes captured with manually operated broadcast cameras as well as several indoor scenes with natural background. A comprehensive experimental evaluation including qualitative and quantitative results demonstrates the accuracy of the technique for high quality segmentation and reconstruction and its suitability for free-viewpoint video under these difficult conditions.

Visual Servoing Invariant to Changes in Camera-Intrinsic Parameters

This paper presents a new visual servoing scheme which is invariant to changes in camera-intrinsic parameters. Current visual servoing techniques are based on the learning of a reference image with the same camera used during the servoing. With the new method, it is possible to position a camera (with eventually varying intrinsic parameters), with respect to a nonplanar object, given a "reference image" taken with a completely different camera. The necessary and sufficient conditions for the local asymptotic stability show that the control law is robust in the presence of large calibration errors. Local stability implies that the system can accurately track a path in the invariant space. The path can be chosen such that the camera follows a straight line in the Cartesian space. Simple sufficient conditions are given in order to keep the tracking error bounded. This promising approach has been successfully tested with an eye-in-hand robotic system.

Retrieval of the columnar aerosol phase function and single-scattering albedo from sky radiance over land: simulations

We present a retrieval scheme that can be used to derive the aerosol phase function and single-scattering albedo from the sky radiance over land. The retrieval algorithm iteratively corrects the aerosol volume scattering function, the product of the single-scattering albedo and the phase function, based on the difference between the measured sky radiance and the radiance calculated by solving the radiative transfer equation. It is tested first under ideal conditions, i.e., the approximations made in the retrieval algorithm totally agree with actual conditions assumed in creating the pseudodata for sky radiance. It is then tested under more realistic conditions to assess its susceptibility to measurement errors and effects of conditions not recognized in the retrieval algorithm, e.g., surface horizontal inhomogeneity, departures of the surface from Lambertian, and aerosol horizontal inhomogeneity. These simulations show that, in most cases, this scheme can retrieve the aerosol single-scattering albedo with high accuracy (within 1%) and can therefore be used to identify strongly absorbing aerosols. It can also produce meaningful retrievals of most aerosol phase functions: less than 5% error at 865 nm and less than 10% at 443 nm in most cases. Typically, the error in the volume scattering function is small for scattering angles ?90 degrees , then increases for larger angles. Disappointing results in both the single-scattering albedo and the scattering phase function occur at 443 nm, either when there are large calibration errors in the radiometer used to measure the sky radiance or when the land reflection properties are significantly inhomogeneous.