Accurate System Calibration Research Articles

Multi-view fringe projection profilometry for surfaces with intricate structures and high dynamic range

Fringe projection profilometry plays an important role for quality control in production line. However, it is facing challenges in the measurement of objects with intricate structures and high dynamic range that involved in precision manufacturing and semiconductor packaging. In this paper, a multi-view fringe projection profilometry system, which deploys a vertical telecentric projector and four oblique tilt-shift cameras, is presented to address the “blind spots” caused by shadowing, occlusion and local specular reflection. A flexible and accurate system calibration method is proposed, in which the corrected pinhole imaging model is used to calibrate the telecentric projection, and the unified calibration is performed by bundle adjustment. Experimental results show that the 3D repeated measurement error and standard deviation are no more than 10 μm within a measurable volume of 70 × 40 × 20 mm3. Furthermore, a group of experiments prove that the developed system can achieve complete and accurate 3D measurement for high dynamic range surfaces with complex structures.

Flexible and accurate system calibration method in microscopic fringe projection profilometry.

The three-dimensional (3D) measurement task of complex microstructures holds paramount significance in the domains of precision manufacturing and inspection. The calibration of the 3D system heavily determines the final reconstruction accuracy. The widely adopted system calibration method is phase-height mapping (PHM) and stereo vision (SV) based. The former can be applied directly to the calculation without considering the imaging model of the system, but it relies on highly precise and expensive translation stages or standard blocks. The latter's accuracy cannot be guaranteed because it is difficult to accurately calibrate the projector. In this paper, we establish an optically coupled microscopic fringe projection profilometry system that consists of a Scheimpflug pinhole projector and a super-low distortion bi-telecentric camera. We introduce a simplified 3D system calibration approach that combines phase modulation transfer and ray propagation. Our method enables the simultaneous calibration of the system, including the calibration of the projector, camera, and the phase to a 3D coordinates relationship, using only a 2D target. The calibrated projector's external parameters are used to obtain the target's complete poses, and then the direct mapping coefficients of the phase to the 3D coordinates can be obtained through the optical geometry structure and phase labels. Comparable experiments verify the feasibility of the proposed method.

A Guide to Accurate System Calibration and Data Extraction to Increase Significance of Spectral Photon Emission Microscopy Measurements

Abstract This article presents and evaluates a calibration method that significantly improves the spectral information that can be extracted from photon emission signals obtained from semiconductor devices. Step-by-step instructions are given for calibrating photon emission microscopes for specific measurements such as device parameters and material band gap. The article also discusses the types of errors that can occur during calibration. Although the procedure presented is used on InGaAs sensors, it applies to all common photon emission detectors.

MULTI-MODAL TRIANGULATION FOR IN-SITU BORESIGHT CALIBRATION OF FRAME AND LINE CAMERAS ONBOARD GNSS/INS-ASSISTED UAVS

Abstract. Unmanned aerial vehicles (UAVs) equipped with integrated global navigation satellite systems/inertial navigation systems (GNSS/INS) together with RGB and hyperspectral (HS) cameras have become popular data acquisition tool for several applications. To derive accurately georeferenced products from such systems, the spatial and rotational offset between the onboard sensors and the GNSS/INS unit must be determined. While the spatial relationship can be measured manually, establishing the angular offset – i.e., boresight angles – is more challenging and requires a calibration strategy. Given that the majority of RGB cameras are based on frame imaging mechanism, boresight calibration of these sensors is usually conducted through automated triangulation of overlapping images. On the other hand, most of current HS cameras are based on push-broom technology – also known as line cameras – which capture 1D images at a time. Consequently, automated triangulation of these non-overlapping images is not possible and thus, current boresight calibration strategies for HS cameras are based on calibration missions where special targets are deployed in the study site and are manually measured in the imagery. Although boresight calibration missions can lead to accurate system calibration parameters, they are expensive and labor intensive activities. To address these limitations and motivated by the new trend of UAVs equipped with both RGB and HS cameras, this study proposes a multi-modal triangulation approach to conduct an in-situ boresight calibration for the two cameras, simultaneously. Experimental results over an agricultural field and an urban area show that the proposed approach results in orthophotos with high visual quality and geolocation accuracy.

Robot line structured light vision measurement system: light strip center extraction and system calibration

We investigate the light strip center extraction and system calibration method of a robot line structured light vision measurement system to realize fast on-line measurement of complex objects. First, the model of a robot vision measurement system is established to analyze the key technologies involved for improving the measurement accuracy of the system. Second, on the basis of the traditional gray gravity method, a gray gravity method based on the normal direction is proposed to extract the center of the laser stripe. Considering the influence of camera distortion, the accuracy of extracting the center stripe can be further improved. Third, a two-dimensional checkerboard is applied to synchronously calibrate the camera parameters, laser plane parameters, and hand-eye relationship of the robot, thus improving the efficiency and accuracy of system calibration. Finally, three-dimensional topography and the geometric dimensions of a complex part are measured by the constructed system with high accuracy. The experimental results show that the average measurement error is <0.2 mm and the measurement accuracy is <1.3 % , indicating that the system can be effectively used in actual industrial measurement.

The calibration method of the circle-structured light measurement system for inner surfaces considering systematic errors

Light plane deviation and motion direction deviation of stage systems are the two main sources of systematic errors in circle-structured light inner surface measurement systems. In this paper, we propose a new two-step calibration method to compensate these systematic errors. In the first step, a more accurate initial light plane calibration result is obtained by multi-position binocular-structured light calibration. Then, in the second step, the initial result is optimized by measuring the inner radius of a standard ring gauge to obtain the optimal light plane calibration result. Based on this result, the real motion direction is also calibrated by optimization using the inner surface scanning measurement data of the standard ring gauge. The proposed method improves the system calibration accuracy through the second calibration step, making it convenient to operate without using complex calibration targets or expensive equipment. Therefore, this calibration method has the potential to be used in actual measurements. The experimental results show that the mean relative measurement error can reach 0.015% in our homemade measurement system and validate that the measurement system calibrated by our proposed method realizes high-precision and high-repeatability measurements for inner surfaces.

3D measurement system based on divergent multi-line structured light projection, its accuracy analysis

This paper proposes an effective three-dimensional(3D) measurement system based on divergent multi-line structured light projection, and analyzes the measurement accuracy of the system. In the system, the previous methods have been improved. The accuracy of system calibration and 3D measurement can be improved effectively by changing the calibration board, changing the relative distance between the camera and the laser. Firstly, the camera calibration and the light plane calibration are realized by projecting a circular calibration board and a blank board. Secondly, the Steger centerline extraction algorithm is used to complete the extraction of the centerline of the stripe. Finally, based on the system calibration parameters and the centerline extraction results, the surface 3D measurement of the object is realized, and the camera calibration, light plane calibration and 3D measurement accuracy are analyzed. Experiments show that the calibration method proposed in this paper can obtain higher accuracy than the checkerboard calibration method, which can reduce the camera calibration error by more than 3 times and the light plane calibration error by more than 1 times. In addition, compared with the previous calibration method, the blank board avoids the influence of the calibration pattern on the light stripe. Finally, the relative error of light plane calibration is controlled within 1% by changing the relative distance between the camera and the laser.

Multiscale Image Matching for Automated Calibration of UAV-Based Frame and Line Camera Systems

Unmanned aerial vehicles (UAVs) equipped with integrated global navigation satellite systems/inertial navigation systems together with frame and/or line cameras are used for a variety of applications. Geometric system calibration is crucial for delivering accurate products from UAV-based imaging systems. This article presents automated geometric calibration strategies for UAV-based frame and line camera systems to estimate accurate system calibration parameters without the need for ground control points or manual measurements of tie points. The matching strategy used in this article to establish conjugate features among overlapping frame camera images is based on a traditional Structure from Motion technique augmented with several layers of matching outlier removal. On the other hand, a new strategy relying on ortho-rectified images is introduced for automated feature matching in line camera scenes. Then, a general bundle adjustment procedure with system calibration capabilities for frame and line cameras is presented, where the derived automated tie points are used for estimating accurate geometric system calibration parameters. The proposed approach is evaluated using four datasets-two datasets captured by frame cameras and two datasets captured by line cameras. The results show that the developed automated calibration strategy is capable of producing the same level of absolute accuracy when compared to using manually measured tie points for both frame camera and line camera systems. Results also indicate that the presented automated system calibration approach can be applied to systems even with significant deviation of actual system parameters from their nominal values, and still produce accurate estimates of calibration parameters.

A three-dimensional measurement system calibration method based on red/blue orthogonal fringe projection

Structured light measurement system that consists of a camera and a projector has been widely used in three-dimensional (3-D) shape measurement. The premise of measurement is to calibrate the structured light system, and the calibration accuracy directly affects the accuracy of 3-D measurement system. The traditional calibration board-based methods suffer from either complex operations or uncompetitive accuracy. To overcome these limitations, this paper proposes a novel calibration method by introducing a liquid crystal display (LCD) with holographic projection film as a phase target to simplify the operations and improve the speed and accuracy of system calibration. The proposed calibration procedures are given as follows: Firstly, the LCD is turned on and kept still to display the red/blue orthogonal fringe pattern, and it is then turned off to reflect the projected fringe pattern with the holographic projection film, the brightness and frequency of the two patterns are adjusted and captured; Secondly, the accurate absolute phase maps are obtained using optimized phase calculation and overall error compensation method; Finally, the above two steps are repeated at several different positions and orientations in the measured field of view. The one-to-one correspondence between the camera and the projector pixel coordinate system and the phase target world coordinate system is established by using the phase equality, and the intrinsic and extrinsic parameters of the system are obtained to reconstruct 3-D shape of the objects. The experiment results demonstrate that the proposed method is feasible and the equipment is simple, which can effectively reduce the overall error, ensure the synchronous calibration of the camera and the projector, avoid the problem of time-consuming, and eventually improve the accuracy of the system calibration.

Study on Comprehensive Calibration and Image Sieving for Coal-Gangue Separation Parallel Robot

Online sorting robots based on image recognition are key pieces of equipment for the intelligent washing of coal mines. In this paper, a Delta-type, coal gangue sorting, parallel robot is designed to automatically identify and sort scattered coal and gangue on conveyor belts by configuring the image recognition system. Robot calibration technology can reduce the influence of installation error on system accuracy and provides the basis for the robot to accurately track and grab gangue. Due to the fact that the angle deflection error between the conveyor belt coordinate system and the robot coordinate system is not considered in the traditional conveyor belt calibration method, an improved comprehensive calibration method is put forward in this paper. Firstly, the working principle and image recognition and positioning process of the Delta coal gangue sorting robot are introduced. The scale factor parameter Factorc of the conveyor encoder is adopted to characterize the relationship between the moving distance of the conveyor and the encoder. The conveyor belt calibration experiment is described in detail. The transformation matrix between the camera, the conveyor belt, and the robot are obtained after establishment of the three respective coordinate systems. The experimental results show that the maximum cumulative deviation of traditional calibration method is 13.841 mm and the comprehensive calibration method is 3.839 mm. The main innovation of the comprehensive calibration is such that the accurate position of each coordinate in the robot coordinate system can be determined. This comprehensive calibration method is simple and feasible, and can effectively improve system calibration accuracy and reduce robot installation error on the grasping accuracy. Moreover, a calculation method to eliminate duplicate images is put forward, with the frame rate of the vision system set at seven frames per second to avoid image repetition acquisition and missing images. The experimental results show that this calculation method effectively improves the processing efficiency of the recognition system, thereby meeting the demands of the grab precision of coal gangue separation engineering. The goal revolving around “safety with few people and safety with none” can therefore be achieved in coal gangue sorting using robots.

Deformation Detection Method of Mine Tunnel Based on Mobile Detection System.

Subway structure safety detection is an important method to ensure the safe operation of trains. Efficient, high-precision, and automatic tunnel clearance detection is the key to ensure safe operations. This study introduces a mobile tunnel scanning system that integrates a scanner, an inertial measurement unit (IMU), and a rail car. Global Navigation Satellite System (GNSS) time and system hardware calibration are used to synchronize time and space information of the system; the attitude and speed are corrected using the control points from the tunnel to improve the accuracy of absolute positioning. The section coordinate system is converted using the control points and system calibration parameters to complete the tunnel clearance inspection, and the distance between the nearest point of the section and the clear height of the vault is given. Taking Fengxi Road’s Bashan tunnel section of Chongqing Metro Line 5 as an example, the overall system accuracy was tested. The accuracy of chord line measurements was within 1 mm, the internal coincidence accuracy of repeated measurements of the vault clear height was 1.1 mm, the internal coincidence accuracy of repeated measurements of the closest gauge point was 4.8 mm, and the system calibration accuracy was approximately 2 mm. Compared with the existing scheme, the system combines absolute measurement and relative measurement mode to judge the structural safety of tunnel section from multiple angles, high precision, and high efficiency.

A Miniaturized UWB Microwave Dual-Comb Dielectric Spectroscopy System

This article presents a miniaturized microwave dual-comb spectroscopy (DCS) system for the characterization of materials in an ultrawideband (UWB) frequency range. The proposed UWB DCS system consists of two comb generators, a sensor, a mixer, a low-pass filter, and an oscilloscope. The output of the DCS system is generated by mixing of two frequency combs with slightly different repetition rates, while one of the combs contains UWB dielectric properties of the material under test (MUT). Thus, the microwave UWB properties of the MUT is mapped to the baseband frequency significantly reducing the receiver complexity. All the sources and the oscilloscope are phase-locked to enable the measurement of both magnitude and phase of the system output. To decrease the averaging time and increase the system calibration accuracy while maintaining the system sensitivity, only nine samples of the system output in the time domain are captured and used for calibration. Through calibration, the DCS system behavior is described with the magnitude and phase responses of 13 reference MUTs. Using such system description, $\epsilon '$ and $\epsilon ''$ of six unknown MUTs are estimated with errors less than $0.055|\epsilon ^{*}|$ in the frequency range from 3 to 10 GHz. This proposed DCS system is accurate, fast, and low cost with the possibility of being self-sustained and independent of the laboratory facility.

Accurate Calibration Scheme for a Multi-Camera Mobile Mapping System

Mobile mapping systems (MMS) are increasingly used for many photogrammetric and computer vision applications, especially encouraged by the fast and accurate geospatial data generation. The accuracy of point position in an MMS is mainly dependent on the quality of calibration, accuracy of sensor synchronization, accuracy of georeferencing and stability of geometric configuration of space intersections. In this study, we focus on multi-camera calibration (interior and relative orientation parameter estimation) and MMS calibration (mounting parameter estimation). The objective of this study was to develop a practical scheme for rigorous and accurate system calibration of a photogrammetric mapping station equipped with a multi-projective camera (MPC) and a global navigation satellite system (GNSS) and inertial measurement unit (IMU) for direct georeferencing. The proposed technique is comprised of two steps. Firstly, interior orientation parameters of each individual camera in an MPC and the relative orientation parameters of each cameras of the MPC with respect to the first camera are estimated. In the second step the offset and misalignment between MPC and GNSS/IMU are estimated. The global accuracy of the proposed method was assessed using independent check points. A correspondence map for a panorama is introduced that provides metric information. Our results highlight that the proposed calibration scheme reaches centimeter-level global accuracy for 3D point positioning. This level of global accuracy demonstrates the feasibility of the proposed technique and has the potential to fit accurate mapping purposes.

Development of a field system for measurement of tropospheric OH radical using laser-induced fluorescence technique

A ground-based system for measuring tropospheric OH radical based on laser-induced fluorescence (AIOFM-LIF) was developed in this work. In this system, ambient air is expanded through a 0.4 mm nozzle to low pressure in a detection chamber, where OH radical is irradiated by the 308 nm laser pulse at a repetition rate of 8.5 kHz. Then, the resultant fluorescence corresponding to the A2Σ+(υ'=0)←X2Πi(ν''=0) transition at 308 nm is detected using gated photon counting. The AIOFM-LIF system was integrated into a mobile observing platform for the field observation following the series of laboratory characterization. A portable standard OH radical source by water photolysis-ozone actinometry was established and optimized for accurate system calibration. The factors affecting the system sensitivity were quantified. It was shown that the ultimate system sensitivity is 9.9 × 10-8 cps (molecules cm-3)-1 mw-1; the minimum detection limits are (1.84 ± 0.26) × 105 cm-3 and (3.69 ± 0.52) × 105 cm-3 at night and noon, respectively; and the whole error of AIOFM-LIF system is about 16%. Then, the system was deployed in Shenzhen, China, during the "A comprehensive STudy of the Ozone foRmation Mechanism in Shenzhen" (STORM) campaign. Valid OH radical concentrations for 31 days were obtained, and the peak of the daily average concentration was 6.6 × 106 cm-3 around 12:00. And a high correlation (R2 = 0.77) between OH and j(O1D) was also observed in this field campaign. The relationship between OH concentration and NOx was attentively discussed. The deployment of AIOFM-LIF system in STORM campaign has demonstrated its capability of measuring tropospheric OH radical with high sensitivity and accuracy in a polluted environment.

Polarization modeling and predictions for Daniel K. Inouye Solar Telescope part 4: calibration accuracy over field of view, retardance spatial uniformity, and achromat design sensitivity

Modern observatories and instruments require optics fabricated at larger sizes with more stringent performance requirements. The Daniel K. Inouye Solar Telescope (DKIST) will be the world’s largest solar telescope at 4.0-m aperture delivering a 300 W beam and a 5 arc min field. Spatial variation of retardance is a limitation to calibration of the full field. Three polarimeters operate seven cameras simultaneously in narrow bandpasses from 380 to 1800 nm. The DKIST polarization calibration optics must be 120 mm in diameter at Gregorian focus to pass the beam and operate under high heat load, UV flux, and environmental variability. Similar constraints apply to the three retarders for modulation within the instrument suite with large beams near focal planes at F/18 to F/62. We assess how design factors can produce more spatial and spectral errors simulating elliptical retardance caused by polishing errors. We measure over 5-deg net circular retardance and spectral oscillations over ±2 deg for optics specified as strictly linear retarders. Spatial variations on scales >10 mm contain 90% of the variation. Different designs can be a factor of 2 more sensitive to polishing errors with dissimilar spatial distributions even when using identical retardance bias values and materials. The calibration of the on axis beam is not impacted once circular retardance is included. The calibration of the full field is limited by spatial retardance variation unless techniques account for this variation. We show calibration retarder variation at amplitudes of 1-deg retardance for field angles greater than roughly 1 arc min for both quartz and MgF2 retarders at visible wavelengths with significant variation between the three DKIST calibration retarders. We present polishing error maps to inform calibration techniques attempting to deliver absolute accuracy of system calibration below effective cross talk levels of 1 deg retardance.

Measurement of shot noise in magnetic tunnel junction and its utilization for accurate system calibration

This work presents a technique to calibrate the spin torque oscillator (STO) measurement system by utilizing the whiteness of shot noise. The raw shot noise spectrum in a magnetic tunnel junction based STO in the microwave frequency range is obtained by first subtracting the baseline noise, and then excluding the field dependent mag-noise components reflecting the thermally excited spin wave resonances. As the shot noise is guaranteed to be completely white, the total gain of the signal path should be proportional to the shot noise spectrum obtained by the above procedure, which allows for an accurate gain calibration of the system and a quantitative determination of each noise power. The power spectral density of the shot noise as a function of bias voltage obtained by this technique was compared with a theoretical calculation, which showed excellent agreement when the Fano factor was assumed to be 0.99.

Modeling of a metallic truncated cone for electromagnetic capacitive sensors

This paper presents an electromagnetic modeling of a grounded metallic truncated cone to be used for calibration purposes of a microwave imaging system. The basic idea is to demonstrate the equivalence between the cone and a cylinder having a suitable radius, in order to simplify the computation of its capacity with respect to ground. A mathematical expression for the capacitance of the uniform cylinder is presented, and its validity is confirmed by comparing the data provided by this formula with numerical values given by a commercial simulator. Starting from this analytic result, the model of the cone is presented, and a procedure for the choice of the cylinder radius is discussed in detail. This methodology can be applied to calculate the contribution to the stray capacitance of a metallic tip used for scanning probe microscopy, and specifically for microwave sensing applications. In particular, the capacitance due to the conic part of the probe can be quantified, an operation that is usually a difficult task when trying to separate it from contribution of the experimental setup. In our opinion, this issue is very important to improve the accuracy of system calibration in the scanning microwave microscopy technique.

Iterative optimization calibration method for stereo deflectometry.

An accurate system calibration method is presented in this paper to calibrate stereo deflectometry. A corresponding iterative optimization algorithm is also proposed to improve the system calibration accuracy. This merges CCD parameters and geometrical relation between CCDs and the LCD into one cost function. In this calibration technique, an optical flat acts as a reference mirror and simultaneously reflect sinusoidal fringe patterns into the two CCDs. The normal vector of the reference mirror is used as an intermediate variable to implement this iterative optimization algorithm until the root mean square of the reprojection errors converge to a minimum. The experiment demonstrates that this method can optimize all the calibration parameters and can effectively reduce reprojection error, which correspondingly improves the final reconstruction accuracy.

A Study for Efficient Methods of System Calibration between Optical and Range Sensors

Abstract. Recently, interests in 3D indoor modeling and positioning have been growing. Data fusion by using different sensors data is one of the 3D model producing methods. For a data fusion between two kinds of sensors, precise system calibration is essential. If relative geometric location of each sensor can be accurately measured with a system-calibration, it is possible to locate a pixel that corresponds to the same object in two different images, and thus, produce a more precise data-fusion. Purpose of this study is finding more efficient method of system calibration between optical and range sensor. For this purpose, experiment was designed by considering following variables, i) system calibration method, ii) testbed type, iii) and distance data(whether use it or not). So, In this study, test-bed for system calibration was designed by considering the characteristics of sensors. Also, precise simulation was done to find efficient method of system calibration, and its results were reflected in real experiment. Results of simulation show that the bundle adjustment method is more efficient than single photo resection in system calibration between range and optical sensors. And the most efficient case was when using i) the bundle adjustment with ii) the simulated data set which were obtained between 2m to 4m away from the test-bed. These results of simulation were reflected in real system calibration. Finally, real system calibration were performed and its results were compared to results of simulation. And accuracy of system calibration was evaluated by producing fusion data between range and optical sensors.

Material acceleration estimation by four-pulse tomo-PIV

A tomographic PIV system is introduced for the instantaneous measurement of the material acceleration (material derivative of velocity). The system is conceived to operate with short temporal separation (microseconds) and is therefore suitable for applications up to the high-speed flow regimes. The method of operation consists of tomographic imaging of a measurement volume using three arrays of four CCD cameras and two double-pulse laser systems. Advantages and shortcomings of the approach with respect to the most commonly used method based on light polarization are discussed. Various approaches are compared to determine the optimal utilization of four-pulse data to measure the material acceleration: Eulerian and Lagrangian schemes are compared to the recently introduced fluid trajectory correlation (FTC) technique from the authors. A synthetic image test case of a translating vortex is used to compare the schemes with and without the presence of noise. The truncation errors and sensitivity to random noise for each scheme are highlighted. A discussion is also given on the dynamic range of the schemes. The four-pulse tomographic system is used to measure the separated wake of an axisymmetric truncated base with afterbody at a Reynolds number of 68 000. The system calibration accuracy and the baseline measurement uncertainty of the velocity are evaluated with a zero-time delay test. A novel criterion is introduced to establish the relative accuracy of the material derivative measurement, based on the curl of the material acceleration field. The results indicate that the four-pulse tomo-PIV approach suits the measurement of the material acceleration using a variety of estimation schemes. In particular, the FTC technique gives the lowest error levels and is well-suited to perform accurate material acceleration measurements.