Accurate calibration method for a structured light system

System calibration, which usually involves complicated and time-consuming procedures, is crucial for any 3-D shape measurement system. In this work, a novel systematic method is proposed for accurate and quick calibration of a 3-D shape measurement system we developed based on a structured light technique. The key concept is to enable the projector to "capture" images like a camera, thus making the calibration of a projector the same as that of a camera. With this new concept, the calibration of structured light systems becomes essentially the same as the calibration of traditional stereovision systems, which is well established. The calibration method is fast, robust, and accurate. It significantly simplifies the calibration and recalibration procedures of structured light systems. This work describes the principle of the proposed method and presents some experimental results that demonstrate its performance.

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.

Phase-shifting method effectively attains high resolution and high accuracy in analyzing phase information on a projected grating. It is difficult to apply dynamically, however, because it usually requires several images. We developed a camera based on digital micromirror device (DMD) technology for high-speed phase analysis and shape measurement, called a DMD reflection-type CCD camera or DMD camera. Implementing DMD technology enables accurate control of intensity reaching the imaging detector of a camera. Moiré is used to adjust pixel-to-pixel correspondence highly accuracy. We introduce a controllable high-speed DMD operation board to control DMD mirrors at high-speed, so each DMD mirror operates as a controllable high-speed shutter for the corresponding CCD pixel. In experiments with an application, we conducted DMD integrated phase-shifting method using correlation to analyze phase distributions of projected gratings from images recorded by the DMD camera. We then discuss principles and experimental results under dynamic conditions. This paper is a revised version of one presented at the SPIE International Symposium on Optomechatronic Technologies (ISOT 2005), December 4-7, 2005 at the Sapporo Convention Center, Japan. That paper appears in SPIE Proceedings Vol.6049, 60490I.

We present the Frequency Shift Method, a new structured light technique allowing 3D shape acquisition in the presence of strong interreflections. The intensity signal of each camera pixel is represented by one or several peaks in the Fourier domain. If there is no interreflection, only a single peak appears, otherwise several peaks are present. Each peak represents a projector pixel participating in the illumination of the surface point imaged at the camera pixel. In the baseline version of the proposed approach, the number of patterns required is proportional to the number of projector pixels. We also propose a modification that significantly reduces the required number of patterns by subdividing and encoding the projector into multiple virtual low-resolution projectors. A method based on dynamic programming is used to separate direct and indirect illuminations. Our experimental results illustrate the effectiveness of our method compared to existing ones.

This paper presents the implementation and optimization of the SCME (Spatial Channel Model Extension) on the GPP (General Purpose Processor) platform using SSE (Streaming SIMD Extensions) instructions and multithreading technique. The SCME which is the expansion of the SCM (Spatial Channel Model) introduces the concept of “mid-paths” and can support up to 100 MHz bandwidths in 2~5 GHz frequency range. In this paper we demonstrate detailed computational analysis of the SCME. In order to reduce the latency, it is very significant to optimize the program. Three optimization methods are proposed: SSE instructions, multithreading and linear interpolation algorithm. SSE instructions and multithreading can solve this problem via increasing the computing parallel degree. Linear interpolation algorithm can reduce the complexity effectively by considering the effectiveness of the sampling rate. From the experiment result we can get that the SSE instructions with the help of IPP (Integrated Performance Primitives) library function can reduce the delay to 1/5 when operating in 32-bit floating-point, multithreading can reduce the delay to 2/5, meanwhile linear interpolation algorithm can reduce the delay to 1/20 with large amount data and ration of linear interpolation L=512. When the three optimization methods are utilized simultaneously, the delay can be cut off remarkably compared with no optimization method utilized.

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.

A novel key frames matching approach for human locomotion interpolation is proposed in this paper. Firstly, locomotion data is transformed from body coordinate system to world coordinate system based on recursive algorithm; secondly, features of two feet and step distance are extracted; thirdly, key frames are extracted based on feature curve and the locomotion sequence is segmented; fourthly, a new approach for key frames matching is put forward; finally, transition frames are interpolated between the matched key frames using quaternion Slerp (Spherical linear interpolation) algorithm and linear interpolation algorithm. The main contributions of this paper are: i) different locomotion sequences can be synthesized in a controllable manner with good effect; ii) to extract locomotion features conveniently, locomotion data in body coordinate system is transformed to world coordinate system; iii) to analyze locomotion sequences intuitively and accurately, key frames are extracted and locomotion sequences are segmented based on feature curve analysis; iv) to avoid illogical jump of traditional interpolation between key frames, a new key frames matching strategy is put forward and only the key frames between two continuous segments can be chosen as matching frames. Experimental results based on the CMU MoCap database show that the proposed method can confirm the logical correctness and naturalness in the junction of two locomotion sequences.

Diagnostics are crucial to the operation of all particle accelerator facilities. They provide a means of optimising beam parameters and monitoring beam quality for users. However, for the next generation of accelerators, operating in extreme parameter spaces, most current beam diagnostics will fail; either through material failure or lack of resolution. This thesis presents three new diagnostic concepts to address this imminent diagnostic need. These novel diagnostics are supported by a simulation-based algorithm, successfully benchmarked throughout this work. All work has been conducted by the Author unless otherwise stated and referenced. The first technique measures sub-micron transverse beam profiles using optical transition radiation (OTR). Simulations were used to design a new imaging system, which was then installed at KEK (Japan). OTR images were then collected for a range of beam sizes. Comparisons were then made with simulations to successfully achieve sub-micron beam size measurements. Validation of these results was found in the agreement across different imaging systems for fixed beam parameters, providing a self-consistency check for the algorithm in lieu of alternate corroboration. The next diagnostic is a bunch length monitor for ∼<100 fs particle bunches; based on imaging the spatial distribution of coherent transition radiation (CTR) in the THz regime. Simulations demonstrated a relationship between image width and intensity, with bunch length. A THz imaging system was then installed at MAX IV (Sweden). CTR images were taken for a range of compressions. Maximum compression was successfully identified by monitoring relative changes in the properties of the images. Bunch lengths were then acquired from these images through comparison with simulations. Finally, the optical performance of a Digital Micro-mirror Device (DMD) was assessed. Image resolution produced by a DMD was quantified by measuring the Point Spread Function (PSF) of an imaging system, which was capable of remotely replacing a DMD with a planar mirror. The PSFs were measured using the sub-micron focus of a laser. Results show no measurable alteration to a PSF with the introduction of a DMD. This provides a foundation for the development of future diagnostics centred around this device. This thesis provides several validated proof-of-principle experimental results and supporting simulations. Future work has been identified for each topic, only possible following the results presented here, which will provide an array of novel next generation diagnostics.

Noncontacting shape measurement for 3-D objects is important in automated manufacturing, component quality control, reverse engineering, etc. Phase-shifting method effectively attains high resolution and high accuracy in analyzing phase information on a projected grating. This technique, however, is difficult to be applied to dynamic condition since usually it requires several images. We developed a digital micromirror device (DMD) camera for phase analysis. In this paper, an one-shot shape and deformation measurement method using a DMD camera is proposed. The DMD camera consists of a DMD, a CCD, and two imaging lenses. In optics of the DMD camera, individual DMD mirrors correspond to individual CCD pixels. The DMD on-off binary pattern can be changed more than 6900 times in one second using an accessory light modulator package (ALP) controller board. Individual DMD mirrors operate as controllable high-speed shutters for corresponding CCD pixels. The four phase-shifted images can be recorded within one frame of the CCD camera. Therefore, shape measurement in dynamic condition can be realized without using a high-speed camera. In order to obtain the 3-D information of shape, multiple reference planes are used to calibrate the relationship between phase value and 3D coordinates. The principle and hardware are presented. In an experiment under dynamic condition, 3D shape information can be analyzed from only one captured image.

Structured light system calibration with unidirectional fringe patterns

3-D shape measurement systems by contactless method are required in the quality inspections of metal molds and electronic parts in industrial fields. A grating projection method with phase-shifting method has advantages of high precision and high speed. Recently, the size of a BGA (ball grid array) becomes smaller. So the pitch of a grating pattern projected onto the specimen should be smaller. In conventional method, fringe pattern is projected using an imaging lens. The focal depth becomes smaller in the case of reduced projection. It is therefore difficult to project a grating pattern with small pitch onto an object with large incident angles. Authors recently proposed a light source stepping method using a linear LED device. It is easy to shrink the projected grating pitch with a lens because this projection method does not use an imaging lens. The pitch of the projected grating depends on the width of the light source. There is a limit to shrink the projected grating pitch according to the size of the LED chip. In this paper, a small pitch fringe projection method with multiple linear fiber arrays for 3D shape measurement is proposed. The width of the fiber array is 30μm. It is one digit smaller than the width of the LED chip. The experimental result of 3-D shape measurement with small pitch projection with large incident angles is shown.

Fringe projection profilometry technique has attracted more and more attention in the computer vision and found wide applications in the areas of high-precision measurement. This paper presents an accurate and robust system calibration method to formulate the phase-to-height conversion. This method uses the Levenberg-Marquardt nonlinear optimization algorithm to fit the phase value of a virtual reference plane, which is parallel to the line between the optical centers of the camera and the projector. The final expression just has three coefficients. With no need to manually adjust the position of components, several experiments were conducted to evaluate the performance of proposed method and demonstrated that the novel system calibration outperforms state-of-the-art methods in terms of stability and accuracy.

This paper presents a patented three-dimensional (3D) shape measurement system, ReCreator, which is mainly composed of two CCD cameras and a random pattern projector with fixed geometry. It can work in the binocular mode, where the stereo image pair is utilized to reconstruct the 3D point cloud in the common field of view of the two cameras; while in the monocular mode, each single camera and the random pattern projector constitute a measuring unit respectively to alleviate the highlight and occlusion effect. The design and the calibration of the random pattern projector, which is the key ingredient for enabling the system, are discussed in detail. The system algorithms are partially implemented with NVIDIA Graphics Processing Units (GPU) in the Compute Unified Device Architecture (CUDA) programming environment, which proves effective to accelerate the computation. Experiments are conducted to validate the precision, rapidity and data integrity of the measurement system.

In phased array ultrasonic sector scanning testing, echoes from defects near the detection surface are often overlapped with interface echoes and cannot be characterized. Based on linear acoustic theory, a mathematical model combining the background subtraction and the sum of squares differences algorithm is established to extract the characteristics of near-detection-surface defect echoes. The upper surface of the artificial notch defect is used as a near-detection-surface defect for detection. Specially, linear interpolation algorithm is used to suppress the effect of residual interface echo on the defect echo feature extraction and the effect of different interpolation factors is analyzed. The defect location and size were calculated based on the extracted defect characteristics. The simulation and experimental results indicate that the model can effectively extract the near-detection-surface defect echoes characteristics. Furthermore, when the interpolated signal reaches at least 500 MHz, residual interface echoes can be effectively suppressed. More importantly, according to the extracted defect echo characteristics, the localization and quantitative accuracies of near-surface defects can reach 0.2 mm and 0.3 mm, respectively. These findings provide an effective strategy for phased array ultrasonic sector scanning testing of near-detection-surface defects.

In this paper, a new direct calculation method of frost ice shape on the blade surface of horizontal axis wind turbine is proposed. Using linear interpolation algorithm, the airfoil ice shape obtained by LEWICE 2D software or ice wind tunnel experiment was fitted with equidistant step length in the first and fourth quadrants and equidistant step length in the second and third quadrants. The key point coordinates of ice shapes on cross-sections along the span-wise were mapped into lagging and flapping surfaces through the mathematical dimension reduction, respectively. The polynomial fitting was used to deal with ice projection points of multiple sections in lagging and flapping surfaces, and then the blade’s frost ice shape was obtained. By calculating the sum of squared residuals of the polar diameter at the same polar angle, the errors between experimental and airfoil frost shape fitting methods, experimental and FENSAP, and blade frost shape formation methods and FENSAP were analyzed. The results show that the new method is in good agreement with the ice shape of FENSAP simulation results and experimental results. The residual sum of squares is small. This method makes the analysis of frost ice morphology of wind turbine blades do not need to consider interdisciplinary. The calculation process is simple and reliable.