Novel method for structured light system calibration

System calibration is crucial for any 3-D shape measurement system. An accurate method is proposed to calibrate a 3-D shape measurement system based on a structured light technique. The projector is treated as a camera to unify the calibration procedures of a structured light system and a well-established stereo vision system. The key to realizing this method is to establish a highly accurate correspondence between camera pixels and projector pixels and generate digital micromirror device (DMD) image sets for projector calibration. A phase-shifting method is used to accomplish this task. A precalibrated lookup table and a linear interpolation algorithm are proposed to improve the accuracy of the generated DMD image, and accordingly improve the accuracy of the projector calibration and the 3-D measurement. Some experimental results are presented to demonstrate the performance of the system calibration.

Aiming at the problem of the low accuracy of projector calibration in a structured light system, an improved projector calibration method is proposed in this paper. One of the key ideas is to estimate the sub-pixel coordinates in the projector image plane using local random sample consensus (RANSAC). A bundle adjustment (BA) algorithm is adopted to optimize the calibration parameters to further improve the accuracy and robustness of the projector calibration. After system calibration and epipolar rectification, the mapping relationship between the pixel coordinates and the absolute phase in the projector image plane is established by using cubic polynomial fitting, and the disparity is rapidly solved by using the mapping relationship, which not only ensures the measurement accuracy, but also improves the measurement efficiency. The experimental results demonstrated that the average re-projection error after optimization is reduced to 0.03 pixels, and the proposed method is suitable for high-speed 3D reconstruction without the time-consuming homogenous point searching.

Structured light system calibration with unidirectional fringe patterns

In fringe projection profilometry, system calibration serves as a critical prerequisite for ensuring measurement precision. During calibration procedures, the elimination of specular reflection-induced saturated image poses significant challenges due to inherent surface reflectance variations. Therefore, a calibration technique utilizing adaptive polarization fringe projection is proposed in this paper. The proposed method analyzes gray-level information model from the polarization camera and employs a dichotomizing search strategy to determine optimal polarization projection intensity for saturated pixels. Furthermore, precise correspondence between image pixels and projection pixels is established. Then, adaptive polarization projection fringe patterns are generated. The proposed method enables the acquisition of high-quality calibration images, effectively enhancing system calibration accuracy. Compared with conventional calibration methods under natural conditions, the proposed method improves calibration accuracy while achieving a 9.6% improvement in measurement precision.

Structured light systems are a simple and efficient tool in the field of 3D reconstruction. Existing calibration methods of structured light systems are either not precise or elastic. In this paper, an easy to use method is proposed. The calibration of a structured light system can accomplished with a good accuracy. The projector projects a checkerboard pattern rendered by OpenGL functions onto a planar board, and the coordinates of corner points in the projector image plane are adjusted to the target position with an iterative algorithm. Thus, the projector can view the printed chessboard pattern. The calibration of a projector becomes essentially the same as that of a monocular camera. Meanwhile, the projector calibration doesn't depend on a pre-calibrated camera, which makes errors of the camera calibration cannot effect the projector calibration. Experimental results show that the mean re-projection error of the projector is less than 1 pixel.

The cone beam computed tomography (CBCT) guided small animal radiation research platform (SARRP) has been developed for focal tumor irradiation, allowing laboratory researchers to test basic biological hypotheses that can modify radiotherapy outcomes in ways that were not feasible previously. CBCT provides excellent bone to soft tissue contrast, but is incapable of differentiating tumors from surrounding soft tissue. Bioluminescence tomography (BLT), in contrast, allows direct visualization of even subpalpable tumors and quantitative evaluation of tumor response. Integration of BLT with CBCT offers complementary image information, with CBCT delineating anatomic structures and BLT differentiating luminescent tumors. This study is to develop a systematic method to calibrate an integrated CBCT and BLT imaging system which can be adopted onboard the SARRP to guide focal tumor irradiation. The integrated imaging system consists of CBCT, diffuse optical tomography (DOT), and BLT. The anatomy acquired from CBCT and optical properties acquired from DOT serve as a priori information for the subsequent BLT reconstruction. Phantoms were designed and procedures were developed to calibrate the CBCT, DOT/BLT, and the entire integrated system. Geometrical calibration was performed to calibrate the CBCT system. Flat field correction was performed to correct the nonuniform response of the optical imaging system. Absolute emittance calibration was performed to convert the camera readout to the emittance at the phantom or animal surface, which enabled the direct reconstruction of the bioluminescence source strength. Phantom and mouse imaging were performed to validate the calibration. All calibration procedures were successfully performed. Both CBCT of a thin wire and a euthanized mouse revealed no spatial artifact, validating the accuracy of the CBCT calibration. The absolute emittance calibration was validated with a 650 nm laser source, resulting in a 3.0% difference between simulated and measured signal. The calibration of the entire system was confirmed through the CBCT and BLT reconstruction of a bioluminescence source placed inside a tissue-simulating optical phantom. Using a spatial region constraint, the source position was reconstructed with less than 1 mm error and the source strength reconstructed with less than 24% error. A practical and systematic method has been developed to calibrate an integrated x-ray and optical tomography imaging system, including the respective CBCT and optical tomography system calibration and the geometrical calibration of the entire system. The method can be modified and adopted to calibrate CBCT and optical tomography systems that are operated independently or hybrid x-ray and optical tomography imaging systems.

Large scale central receiver systems typically deploy between thousands to more than a hundred thousand heliostats. During solar operation, each heliostat is aligned individually in such a way that the overall surface normal bisects the angle between the sun’s position and the aim point coordinate on the receiver. Due to various tracking error sources, achieving accurate alignment ≤1 mrad for all the heliostats with respect to the aim points on the receiver without a calibration system can be regarded as unrealistic. Therefore, a calibration system is necessary not only to improve the aiming accuracy for achieving desired flux distributions but also to reduce or eliminate spillage. An overview of current larger-scale central receiver systems (CRS), tracking error sources and the basic requirements of an ideal calibration system is presented. Leading up to the main topic, a description of general and specific terms on the topics heliostat calibration and tracking control clarifies the terminology used in this work. Various figures illustrate the signal flows along various typical components as well as the corresponding monitoring or measuring devices that indicate or measure along the signal (or effect) chain. The numerous calibration systems are described in detail and classified in groups. Two tables allow the juxtaposition of the calibration methods for a better comparison. In an assessment, the advantages and disadvantages of individual calibration methods are presented.

A structured-light system with a binary defocusing technique has the potential to have more extensive application due to its high speeds, gamma-calibration-free nature, and lack of rigid synchronization requirements between the camera and projector. However, the existing calibration methods fail to achieve high accuracy for a structured-light system with an out-of-focus projector. This paper proposes a method that can accurately calibrate a structured-light system even when the projector is not in focus, making it possible for high-accuracy and high-speed measurement with the binary defocusing method. Experiments demonstrate that our calibration approach performs consistently under different defocusing degrees, and a root-mean-square error of about 73 μm can be achieved with a calibration volume of 150(H) mm×250(W) mm×200(D)mm.

This paper presents a novel approach for the optimization of calibration parameters in structured light system (SLS). Different with conventional calibration algorithms, the proposed optimization algorithm is implemented in 3D space instead of 2D image space. The object used for parameter optimization can be a simple plane with some markers. A global optimal function is constructed to contain all the intrinsic and extrinsic parameters of the SLS. Using the primary calibration parameters by conventional methods as initial values, the optimal function can be solved by minimizing the 3D measurement errors like distance, angle between markers, and the planarity of the reference plane. Experimental results show that, 3D reconstruction accuracy can be greatly improved by the proposed approach in comparison with traditional SLS calibration methods.

A Structured Light System (SLS) is a fast and flexible optical method for measuring the 3D shape of objects. The dense point cloud resulted accurately describes the shape information of the object. The information can be used in many applications, such as generating CAD models, measuring shapes and inspecting qualities. The desired geometrical information of positions, poses, distances, angles, profiles etc. can conveniently be extracted from the measurement result. It is also possible to calculate and visualize the deviation between a measurement and a CAD model. In this paper, we present techniques, which lead to development of practical 3D machine vision systems in a different way than traditional ones. We have developed low cost and user-friendly 3D machine vision systems with off-the-shelf components based on Structured Light Systems (also known as Fringe Projection Systems) suitable for automated quality inspection and shape measurement. Two cases are presented for demonstrating the successful implementation.

Imaging fluorescence analysis is a powerful tool for the characterization of thin functional layers. Due to the development of new components such as cost-efficient and long life diode lasers and LEDs as well as sensitive cameras, the number of industrial in situ sensors based on fluorescence analysis technology increased rapidly in recent years. Of crucial importance for all these new sensors are efficient and robust methods for calibration. Although there are many examples for the calibration of laboratory setups for single specialized applications, there is no standardized method for the traceable device independent calibration of imaging fluorescence systems. This paper presents the evaluation of five different methods for the calibration of systems for quantitative fluorescence analysis. Each method is applied for the calibration of an imaging fluorescence laser scanner. In addition to characterizing the precision of the methods, the work analyzes the usability of the methods for different applications. The results show for the first time that a calibrated IR point sensor can be used for the auto calibration of high resolution imaging inline fluorescence sensors. In addition, we present a novel method for the transfer of calibration data between analysis systems with different optical setups by using a solid material fluorescence standard.

The calibration of eddy-current measurement systems is a long-standing problem in nondestructive evaluation. Eddy-current probe calibration is needed for several reasons: to compensate for different probe sensitivities, to set detection thresholds, to validate instrument setup and operation, and to perform quantitative flaw sizing.1,2 The most frequently used calibration method is to scan the probe being calibrated over simulated defects such as electrical-discharge-machined (EDM) slots, saw cuts, or laboratory-produced fatigue cracks. This method has the virtue of calibrating probe and instrument at the same time and it can be performed on the same material as that to be inspected. But it has a number of disadvantages as well. First, a large number of artifact standards must be generated, certified, and maintained, resulting in considerable expense. Second, the signals from EDM slots and saw cuts are not equivalent to the signals from actual defects.3 Third, it is questionable whether quantitative flaw sizing can be performed using such a calibration method. Even if laboratory-produced cracks were to be used for routine calibration (a prohibitively expensive option), the accuracy of calibration or quantitative sizing could be compromised by the occurrence of crack closure effects.4

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

Traditional dispersive spectrographic systems based on single slits cannot achieve dual advantages simultaneously and statically. A novel dual advantages system model for dispersive spectrographic system appears and the traditional entrance slit is replaced by a coded aperture based on orthogonal independent column codes. The extended aperture lets more energy enter the system and the independence of the codes helps to avoid degradation of the spectral resolution. On the basis of the derivation of system model, this paper emphasizes on system tolerance analysis, to find out the key factors which affect the realization and performance of the system model. First, the influences of inaccurate system adjustment and calibrations on system performance were studied. The standard methods for system adjustment and calibrations were represented. Then, simulations were carried out to study the influence of several abnormal situations of the coded aperture on system performance, which includes the existing of error fabrication and imperfect illumination of the coded aperture. The analysis result shows that the system can still function normally when the coded aperture has a fabrication error rate of 30% or severe imperfect illumination of the coded aperture.

Extrinsic calibration of a conoscopic holography system integrated in a CMM