High-accuracy Calibration Research Articles

Fraunhofer line-based wavelength-calibration method without calibration targets for planetary lander instruments

High-accuracy wavelength calibration is critical for qualitative and quantitative spectroscopic measurements. Many spectrometers employed in planetary-exploration missions have onboard calibration sources, including standard lamps and calibration targets. However, such calibration sources are not always available because planetary missions, particularly landing missions, usually have limitations in size and mass. Thus, a wavelength calibration method without requiring hardware addition can be highly beneficial. In this study, we demonstrate a method for wavelength calibration using solar Fraunhofer lines observed in the reflectance spectra of planetary surfaces. Using a Raman spectrometer prototype developed for a Phobos rover, we measured the spectrum of the sunlight reflected from a spectral standard, manufactured to provide similar reflectance spectra to the surface of Phobos. We identified 35 Fraunhofer absorption lines in the wavelength range between 530 and 700 nm and utilized these features for the wavelength calibration of the spectrometer. This approach using Fraunhofer lines achieved good results (better than +0.04/−0.06 nm), comparable to the results achieved using a conventional Ne lamp. The wavelength accuracy corresponds to a wavenumber accuracy better than ±1.5 cm−1 in the 0–4000 cm−1 Raman shift (Stokes shift) range with a 532 nm excitation laser. This result enabled the estimation of the magnesium number (Mg#) of olivine, achieving a value more precise than 1.5% based on the Raman peak positions. In addition, we examined the number of solar Fraunhofer lines detectable at different wavelength resolutions by binning the solar spectrum acquired in this study. We found that more than 10 Fraunhofer lines could be detected as prominent absorption lines when the wavelength resolution is higher than 1 nm/pix (30 cm−1/pix at 1000 cm−1). This result suggests that the target-free wavelength-calibration method using solar Fraunhofer lines can be applied to other spectrometers simply by observing sunlit planetary surfaces.

Unidirectional structured light system calibration with auxiliary camera and projector

This paper presents a novel calibration method for a unidirectional structured light system, which utilizes a white planar surface as the calibration target instead of the conventional targets with physical features like circle dots or checker squares. The proposed method reconstructs the white planar surface by employing stereo vision with projected random patterns and plane fitting. To facilitate the calibration process, an auxiliary camera and an auxiliary projector are employed. Experimental results demonstrate that the high calibration accuracy is achieved by the proposed method for a unidirectional structured light system.

High accuracy calibration method for multi-line structured light three-dimensional scanning measurement system based on grating diffraction.

Multi-line structured light three-dimensional (3D) scanning measurement system enables to obtain the richer 3D profile data of the object simultaneously during one frame, ensuring high accuracy while structured light is deformed for the modulation by the object. Nevertheless, current calibration methods cannot fully take advantage of its high precision. In this paper, a fast and high-accuracy 3D measurement system based on multi-line lasers with a spatially precise structure via integrating a diffraction grating was proposed. This helps achieve precise calibration results of the light planes by introducing spatial constraint relations of the diffractive light, thus improving measurement accuracy. The operating principle and the workflow of the proposed system were described in detail. The measurement accuracy of the developed prototype was verified through contrastive experiments. At a working distance of 400 mm, the results show that the root mean square error (RMSE) of the proposed system is 0.083 mm, which is improved by 37.6% compared to the traditional calibration method of light planes for the ranging system. The system utilizing a grating that facilitates the integration of the device has great application value.

Construction and evaluation of a column chart model and a random forest model for predicting the prognosis of hydrodistention surgery in BPS/IC patients based on preoperative CD117, P2X3R, NGF, and TrkA levels

ObjectiveThis study seeks to investigate independent risk factors affecting the prognoses of patients with bladder pain syndrome/interstitial cystitis (BPS/IC) following hydrodistention surgery and to develop a column chart model and a random forest model to help predict clinical outcomes.MethodA retrospective analysis was conducted on the clinical data of 1006 BPS/IC patients who visited the urology department of the Fifth Hospital of Shanxi Medical University (Shanxi Provincial People's Hospital) between June 2012 and June 2022. The patients were randomly divided into a model group (n = 704) and a validation group (n = 302). In the model group, logistic regression analysis was used to identify independent risk factors, which were used to construct a prognostic nomogram. The nomogram was evaluated by analyzing the area under the curve (AUC), calibration curve, and decision curve. These results were subsequently validated via consistency analysis (n = 302). And based on the random forest algorithm, we calculate the same data and construct a random forest model.ResultMultivariate logistic regression analysis revealed that age and the expression of the biomarkers CD117, P2X3R, NGF, and TrkA were independent prognostic factors for patients with BPS/IC (P < 0.05). Using these five indicators, a nomogram was developed to predict the risk factors for BPS/IC (scores ranged from 0 to 400). Based on the indicators, the nomogram demonstrated good prognostic performance (AUC = 0.982 and 95% confidence interva is 0.960–0.100). The correction curve indicated a high level of differentiation in the model, and the decision curve suggested positive clinical benefits. The random forest model has high accuracy and good calibration in predicting the prognosis of patients with interstitial cystitis after hydrodistention surgery.ConclusionAge, CD117, P2X3R, NGF, and TrkA are independent prognostic factors for bladder pain syndrome/interstitial cystitis. The column chart model and random forest model constructed based on these indicators have good predictive performance for patient prognosis.

Extrinsic calibration of non-overlapping multi-camera system with high precision using circular encoded point ruler

In the field of visual metrology, achieving coordinate system unification among non-overlapping multi-camera systems in wide field-of-view scenarios poses significant challenges due to the absence of overlapping fields of view. This paper presents a novel calibration method for such multi-camera systems utilizing a ruler equipped with double-layer circular encoded landmarks. The fabrication of the ruler is simple, and its length covers the entire multi-camera measurement system, rendering the calibration process convenient. To calibrate the cameras, the ruler is positioned in front of the target cameras, and images of the ruler are captured by the multi-cameras. By establishing the initial transformation relationships between the non-overlapping cameras and the ruler's coordinate system, the proposed method efficiently determines the inter-camera transformations. Subsequently, by moving the ruler to different positions and capturing corresponding images, the distances between the encoded center points on the ruler and other encoded points, as captured by the non-overlapping cameras, are kept constant. An optimization-based objective function is formulated to minimize the three-dimensional distances between the measured encoded center points, thereby refining the initial transformation relationships. Combining the coplanarity constraint of the coding center points with the extrinsic results obtained through distance optimization, a secondary optimization was conducted to obtain the final transformation parameters. Experimental results demonstrate the simplicity and versatility of this method, which does not rely on complex constraints. Moreover, it exhibits high calibration accuracy, with maximum distance errors between ruler points and a fitting standard plane error both below 0.275 mm and 0.132 mm, respectively. Following multiple experimental iterations, the average disparity between the radius of the standard cylinder measured via line laser scanning and its true value was found to be 0.022 mm. The average maximum distance from the point cloud to the fitted cylinder surface was determined to be 0.281 mm, while the average distance, computed across multiple instances, was 0.048 mm. Consequently, the proposed approach proves effective for calibrating non-overlapping camera systems in various applications.

Accurate and robust calibration method for simultaneous Stokes polarimetry

The simultaneous Stokes polarimetry (SSP) plays a crucial role in various fields, including the characterization of suspended particles in air and water, as well as the measurement of the Mueller matrix of samples. System calibration is a critical step for improving the accuracy of the Stokes polarimetry. However, there is a lack of commercially available SSP, and the corresponding calibration method is not yet fully developed. In this paper, an accurate and robust calibration method for SSP based on division-of-amplitude is proposed, which includes intensity monitoring and geometric optimization. The optimal calibration method significantly can reduce measurement time while maintaining high calibration accuracy and robustness. The maximum root-mean-square error of the measured Stokes vector can be minimized to less than 0.0061, which represents approximately a tenfold improvement in accuracy compared to the traditional calibration method using a rotating quarter-wave plate. Additionally, the measurement errors of Stokes vectors are minimized globally to ensure robust measurement. Based on this foundation, the calibrated SSP is capable of tracking the changes in random Stokes vectors with fast speed. These results demonstrate the importance of selecting appropriate calibration polarization states and the significance of incorporating light intensity monitoring to enhance the accuracy and robustness of polarization calibration.

Absolute radiometric calibration evaluation of the thermal infrared spectrometer onboard SDGSAT-1

ABSTRACTA thermal infrared spectrometer is an important sensor onboard the SDGSAT-1 dedicated to serving the 2030 Agenda for Sustainable Development of the United Nations. Field campaigns were conducted from January to December 2022 to evaluate its radiometric calibrations. A radiance-based calibration method was used to derive the spectral radiance of the lake surface around the satellite overpass using a boat-deployed TIR spectrometer or radiometer. The radiometric calibration conditions were quantified by examining the temperature bias between the satellite-based brightness temperature and the predicted at-sensor brightness temperature. Our results indicated that the average brightness temperature bias reached 0.772 K, −1.032 K, and −0.300 K, with RMSEs of 1.00, 1.10, and 0.43 K in bands 1, 2 and 3, respectively. Eighty percent of the field experiments achieved a high calibration accuracy with temperature biases of less than 1 K, and the calibration accuracy of band 3 was better than that of other bands. Atmospheric conditions and surface characterization play important roles in determining the calibration accuracy through surface observations and atmospheric transfer simulations. With the increasing need for SDGSAT-1 products, periodic field experiments are needed to monitor and improve the operational radiometric calibration of satellite instruments to create high-resolution and reasonable scientific datasets.

High-accuracy vanishing-constraints-based calibration of fringe projection vision sensor

In fringe projection profilometry (FPP), calibration of the vision sensor is crucial to ensure the accuracy of three-dimensional (3D) shape measurement. However, due to correspondence errors of planar calibration target (PCT) 3D points and its image points, and low mapping accuracy between projector pixels and camera pixels, traditional calibration methods based on the PCT sometimes cannot provide reliable and accurate results. Aiming at these problems, a high-accuracy calibration method based on vanishing constraints for fringe projection vision sensor (FPVS) is proposed in this paper. The intrinsic parameters are calibrated using the constraints of vanishing points, the rotation matrix is calibrated using the constraints of vanishing lines, and the optimal calibration parameters are estimated by the objective function established based on the 2D and 3D constraints of feature points. Experimental results demonstrated the performance of our calibration method.

A novel camera calibration method based on circle projection model

Camera calibration is one of the important factors affecting measurement accuracy in the field of visual measurement, and the camera calibration method with high precision and strong robustness is the prerequisite for ensuring high-precision visual measurement. Among the commonly used calibration methods, target-based calibration methods have high accuracy and good flexibility. However, the feature extraction of checkerboard targets is prone to noise interference. Circle targets have higher feature extraction accuracy, but will introduce eccentricity errors in the center of the circle projection. In order to eliminate the eccentricity error of circle targets, this paper proposes a camera calibration method based on the circle projection model, which does not rely on the central projection coordinates, but directly constructs the mapping between the circle equation on the target and the elliptic equation in the camera image, and solves the optimal mapping parameters through nonlinear optimization algorithm. Through simulation and experiments, it has been proven that the calibration method based on the circle projection model has high calibration accuracy and anti-noise interference ability, providing a guarantee for high-precision visual measurement.

A 12-bit single slope ADC with multi-step structure and ramp calibration technique for image sensors

This paper presents a novel multi-step single slope analog to digital converter (MS SS ADC). The proposed ADC splits the 12-bit conversion into 3 steps, including 1-bit half section decision, 4-bit coarse conversion, and finally 8-bit fine conversion with 1-bit redundancy. The conversion step is substantially reduced from conventional 4096 to 276. The linearity of the ADC is guaranteed by a stable common mode voltage of the comparator varying only within 31.25 mV. Moreover, two high-accuracy ramp calibration techniques are utilized to correct the current variation in the fine ramp and the slope error between the coarse and fine ramps. A hybrid comparator is utilized in the ADC to save power while maintaining the performance. The ADC is designed and simulated in 0.18 μm CMOS process with 7.5 μm width. The differential nonlinearity and integral nonlinearity of the proposed ADC are reduced from +0.3/-1 LSB and +3.1/-2.55 LSB to +0.3/-0.3 LSB and +1/-0.45 LSB at 61.7 kS/s sample rate by the calibrations.

Localized Equipment Calibration Technology for Substation Relay Protection Considering Electromagnetic Interference

At present, the conventional localized equipment calibration technology mainly adopts the manual way to check the equipment parameters, which leads to low calibration accuracy due to the lack of analysis of the calibration error sources. In this regard, the localized equipment calibration technology for substation relay protection considering electromagnetic interference is proposed. The calibration error value is calculated by simulating a high-precision localized equipment unit, designing the pre-circuit for fast voltage and current sampling, and analyzing the calibration error source. The experimental results show that the proposed calibration technique can effectively reduce the delay characteristic error value of the equipment and has a high calibration accuracy. The delay characteristic error of the average calibration is lower than 23%.

A calibration method of ground-airborne frequency domain electromagnetic receving system based on magnetic source excitation

In the ground-airborne frequency domain electromagnetic detection, the receving system converts the magnetic field signal into an electrical signal representing the magnetic field signal. In order to accurately obtain the relationship between the magnetic field signal and the electrical signal, it is necessary to avoid the influence of environment and other factors on the system accuracy through on-site calibration. This requires high calibration accuracy, simple operation, portable equipment and strong adaptability to the environment. However, the existing calibration methods put forward higher requirements for instruments and environment. For example, the calibration of uniform magnetic field generated by Helmholtz coil needs to shield external magnetic field interference. In this paper, an overall calibration method of measurement system based on magnetic source excitation is proposed to meet the requirements of electromagnetic field accurate measurement in ground-airborne frequency domain. Firstly, based on the electromagnetic measurement method in ground-airborne frequency domain, the overall calibration method and principle of the detection system are introduced. Then, based on the computable standard magnetic field generated by the magnetic source transmitting coil, determine the measured voltage obtained by the receiving system, get the calibration coefficient of the receiving system. At last,the source of standard uncertainty related to calibration is analyzed, and the uncertainty is evaluated. In addition, the method is verified by comparing the calibration results with those obtained by Braunbeck coil calibration.

A Calibration Algorithm of 3-D Laser Scanner Based on KMPE Loss Function

Aiming at solving the estimation problem of measurement model parameters of 3D laser scanner, a kernel mean p-power error (KMPE) loss function-based calibration algorithm is proposed in this paper. The KMPE loss function is robust to measurement noise and outliers. Thus, a KMPE loss function-based nonlinear optimization model of measurement model parameters of 3D laser scanner is firstly established according to the distance constraint between each laser scanning point on the calibration sphere and the center of the calibration sphere. Then, to conduct calibration of the model parameters, the optimization model is solved by combining the success-history based parameter adaptation for differential evolution (SHADE) algorithm with the Levenberg-Marquardt (LM) algorithm. Experimental results show that the proposed algorithm can achieve high calibration accuracy of the measurement model parameters of the 3D laser scanner by effectively suppressing the influence of measurement noise and outliers.

High-accuracy and high-efficiency calibration method for determining voltage-phase characteristics of LCVR based on a Wollaston prism

Liquid crystal variable retarder (LCVR), as a promising and precision device in applications of light field modulations and manipulations, has been widely used to change the phase and thus the polarization state of light based on electronically controlled rotations of liquid crystal molecules. However, the LCVR usage inevitably involves an important task, i.e., the calibration, since the phase retardation of LCVR changes with the temperature of the environment and the wavelength of the incident light. In this paper, based on using a Wollaston prism, we propose an effective calibration method to tailor the voltage-phase characteristic curves of LCVR. In the experiment, given a wavelength of light to be calibrated, similar calibration results are always obtained, regardless of the angle of polarization (AoP) of the incident light and the angle of LCVR placement. Experimental results show that the root-mean-square error (RMSE) of the voltage-phase calibration is no more than 0.132 radian. Moreover, excellent robustness and efficiency of this LCVR calibration method are also demonstrated and verified.

Study on the calibration of full polarization imager

Polarization is one of the fundamental properties of light, which has a wide range of applications and is developing rapidly. To meet the needs of polarization detection, different types of polarization instruments came into being. The precision of the polarization detection instruments is vital to the result analysis. In this paper, a full polarization imager is designed, and the radiometric calibration and polarization calibration of this instrument are studied. In radiometric calibration, the different numbers lights are set to verify the light intensity response of the imager. The mathematical model was constructed for numerical fitting, and the correlation between the fitted values and the measured values in the 490 nm, 550 nm, and 670 nm bands was above 0.99. Fixed the radiance of the integrating sphere, and adjusted exposure times. The correlation of the three bands is above 0.99, which verifies that the radiative stability of the imager is good. The polarimetric calibration system adopts the adjustable degree of polarization reference light source (APOL). The theoretical and measured values of the degree of polarization of reference light sources in three different bands are analyzed. The results show that the measurement accuracy of the 490 nm band is less than 2%. The precision of polarization measurement in the 550 nm band is less than 1.5%, and the precision of polarization calibration in the 670 nm band is less than 1%. The imager is verified to have high polarization calibration accuracy and meets the requirements of high-precision polarization detection.

CrowdFusion: Multisignal Fusion SLAM Positioning Leveraging Visible Light

With the fast development of location-based services, an ubiquitous indoor positioning approach with high accuracy and low calibration has become increasingly important. In this work, we target on a crowdsourcing approach with zero calibration effort based on visible light, magnetic field and WiFi to achieve sub-meter accuracy. We propose a CrowdFusion Simultaneous Localization and Mapping (SLAM) comprised of coarse-grained and fine-grained trace merging respectively based on the Iterative Closest Point (ICP) SLAM and GraphSLAM. ICP SLAM is proposed to correct the relative locations and directions of crowdsourcing traces and GraphSLAM is further adopted for fine-grained pose optimization. In CrowdFusion SLAM, visible light is used to accurately detect loop closures and magnetic field to extend the coverage. According to the merged traces, we construct a radio map with visible light and WiFi fingerprints. An enhanced particle filter fusing inertial sensors, visible light, WiFi and floor plan is designed, in which visible light fingerprinting is used to improve the accuracy and increase the resampling/rebooting efficiency. We evaluate CrowdFusion based on comprehensive experiments. The evaluation results show a mean accuracy of 0.67m for the merged traces and 0.77m for positioning, merely replying on crowdsourcing traces without professional calibration.

Simple and precise calibration of the line-structured light vision system using a planar target.

When calibrating a line-structured light vision system using a planar target, noise easily affects the solution of the coordinates of light stripe points at the camera coordinate frame. Therefore, the planar target must be placed in the measurement space many times to capture more target images for improving calibration stability and achieving relatively high calibration accuracy. This complicates the calibration process. This paper proposes a calibration method considering the measurement baselines of a planar target. The planar target is placed only two times, and two target images are captured correspondingly. A three-point subset is made up of the two calibration points that form the measurement baseline with the longest 2D projection and any other calibration point. In this way, it is less affected by noise when using the three-point subsets to establish the equations. Then, we use the lengths of the measurement baselines provided by all three-point subsets and their 2D projections to solve the coordinates of light stripe points at the camera coordinate frame more accurately to calibrate the line-structured light vision system. Both the simulation and actual experiment results demonstrate the feasibility of our method. Based on our calibration method, the RMS error is 0.035mm for length measurement and 0.054mm for height measurement. Compared with other existing methods, our method needs only two target images. It can also achieve more accurate calibration results than the other methods. In addition, our calibration method increases the applicability of the line-structured light measurement method by reducing the number of target swings.

High-Accuracy Patternless Calibration of Multiple 3-D LiDARs for Autonomous Vehicles

This paper proposes a new method for estimating the extrinsic calibration parameters between any pair of multi-beam LiDAR sensors on a vehicle. Unlike many state-of-the-art works, this method does not use any calibration pattern or reflective marks placed in the environment to perform the calibration; in addition, the sensors do not need to have overlapping fields of view. An iterative ICP-based process is used to determine the values of the calibration parameters, resulting in better convergence and improved accuracy. Furthermore, a setup based on the CARLA simulator is introduced to evaluate the approach, enabling quantitative assessment with ground-truth data. The results show an accuracy comparable with other approaches that require more complex procedures and have a more restricted range of applicable setups. This work also provides qualitative results on a real setup, where the alignment between the different point clouds can be visually checked. The open-source code is available at https://github.com/midemig/pcd_calib.

A touchless user interface based on a near-infrared-sensitive transparent optical imager

Touchless user interfaces that are based on gestures typically rely on near-infrared cameras. However, such systems are often hampered by their limited field of view and high-accuracy calibration requirements. Here we report a touchless user interface that is based on a visually transparent near-infrared-sensitive organic photodetector array and can be used on top of a display. Optical transparency is achieved by using a printed copper grid as a bottom transparent conductive electrode and an array of patterned organic photodetector subpixels. Electro-optical modelling is used to optimize the design of the image sensor, leading to a photodetectivity of approximately 1012 Jones at 850 nm and a high visible-light transmittance of 70%. We show that the imager can be used as a penlight-controlled and gesture-controlled touchless user interface when combined with a commercial display.

A visual guidance calibration method for out-of-focus cameras based on iterative phase target

Calibration of out-of-focus cameras is crucial for some special 3d reconstruction applications. An essential factor in improving the calibration result is avoiding the solution's singularity which tends to pose the calibration plate at a large inclination. However, the depth of field (DOF) of cameras is limited, especially in the defocused scenes, and the blurred corners will significantly reduce the calibration accuracy. A vision-guided calibration method is proposed, including an optimal pose selection algorithm and an iterative phase-target technology. Compared with other phase-target methods, this method simultaneously optimizes the mathematical properties of the solution and the angular uncertainty of the corners. Compared with the Chessboard-target or Circle-target, this method has a higher calibration accuracy at three different distances that gradually change from focusing to defocusing. In addition, experiments also prove the effectiveness of the two submodules: Gaussian iteration and vision guidance. Finally, its code is available in the URL: https://github.com/SHU-FLYMAN/Visual-Guidence-Calibration.