"Fast error detection method for additive manufacturing process monitoring using structured light three dimensional imaging technique" (2025)

Abstract

This paper presents a novel method to speed up error detection in an additive manufacturing (AM) process by minimizing the necessary three-dimensional (3D) reconstruction and comparison. We develop a structured light 3D imaging technique that has native pixel-by-pixel mapping between the captured two-dimensional (2D) image and the reconstructed 3D point cloud. This 3D imaging technique allows error detection to be performed in the 2D image domain prior to 3D point cloud generation, which drastically reduces complexity and computational time. Compared to an existing AM error detection method based on 3D reconstruction and point cloud processing, experimental results from a material extrusion (MEX) AM process demonstrate that our proposed method significantly increases the error detection speed.

“Calibration method based on virtual phase-to-coordinate mapping with linear correction function for structured light system," (2024)

R. Vargas, L. A. Romero, S. Zhang, and A. G. Marrugo, “Calibration method based on virtual phase-to-coordinate mapping with linear correction function for structured light system,“ Optics and Lasers in Engineering, 183, 108496, (2024)

Abstract

Structured light systems are crucial in fields requiring precise measurements, such as industrial manufacturing, due to their capability for real-time reconstructions. Existing calibration models, primarily based on stereo vision (SV) and pixel-wise approaches, face limitations in accuracy, complexity, and flexibility. These challenges stem from the inability to fully compensate for lens distortions and the errors introduced by physical calibration targets. Our work introduces a novel calibration approach using a virtual phase-to-coordinate mapping with a linear correction function, aiming to enhance accuracy and reduce complexity. This method involves traditional stereo calibration, phase processing, correction with ideal planes, and fitting a pixel-wise linear correction function. By employing virtual samples for phase-coordinate pairs and computing a pixel-wise correction, our methodology overcomes physical and numerical limitations associated with existing models. The results demonstrate superior measurement precision, robustness, and consistency, surpassing conventional stereo and polynomial regression models, both within and beyond the calibrated volume. This approach offers a significant advancement in structured light system calibration, providing a practical solution to existing challenges.

Calibration of dual resolution dual camera structured light systems,“ (2024)

Y. Yang, I. Bortins, D. Baldwin, and S. Zhang, “Calibration of dual resolution dual camera structured light systems,“ Optics and Lasers in Engineering, 182, 108472, (2024)

Abstract

This paper introduces a calibration framework designed for dual-resolution dual-camera structured light systems. The calibration process for each camera-projector pair adheres to a conventional pixel-wise calibration method for structured light systems. To align the two cameras' coordinate system, the method employs a white planar object and virtual feature points. These points are generated based on the dual-directional absolute phase maps obtained from each camera. By leveraging the pixel-wise calibration results from both single camera-projector pairs, this approach determines the geometric relationship between the camera coordinates via rigid transformation. Experimental results demonstrate the proposed method's high calibration precision for each camera-projector pair and the accurate alignment of coordinates between these two cameras.

“Image-based non-isotropic point light source calibration using digital fringe projection" ( 2024)

Y-H Liao and S. Zhang, “Image-based non-isotropic point light source calibration using digital fringe projection,” Optics Express, 32(14) 25046-25061 (2024)

Abstract

Accurate light source calibration is critical in numerous applications including physics-based computer vision and graphics. We propose an image-based method for calibrating non-isotropic point light sources, addressing challenges posed by their non-radially symmetric radiant intensity distribution and the non-Lambertian properties of the calibration target. We deduce an image formation model, and capture the intensity of the calibration target at multiple poses, coupled with accurate 3D geometry acquisition using the digital fringe projection technique. Finally, we design an iterative computational framework to optimize all the light source parameters simultaneously. The experiment demonstrated the high accuracy of the calibrated light source model and showcased its application by estimating the relative reflectance of diverse surfaces.

"Adaptive focus stacking for large depth-of-field microscopic structured-light 3D imaging,” (2024)

[150] L. Chen, R. Ding and S. Zhang, “Adaptive focus stacking for large depth-of-field microscopic structured-light 3D imaging,” Applied Optics, (2024)

Abstract

This paper presents an adaptive focus stacking method for large depth-of-field (DOF) 3D microscopic structured-light imaging systems. Conventional focus stacking methods typically capture images under a series of pre-defined focus settings without considering the attributes of the measured object. Therefore, it is inefficient since some of the focus settings might be redundant. To address this problem, we first employ the focal sweep technique to reconstruct an initial rough 3D shape of the measured objects. Then, we leverage the initial 3D data to determine effective focus settings that focus the camera on the valid areas of the measured objects. Finally, we reconstruct a high-quality 3D point cloud using fringe images obtained from these effective focus settings by focus stacking. Experimental results demonstrate the success of the proposed method.

“Pixelwise calibration method for telecentric structured light system,” (2024)

Y. Yang and S. Zhang, “Pixelwise calibration method for telecentric structured light system,” Applied Optics, 63(10), (2024)

Abstract

This paper introduces a pixel-wise calibration method designed for a structured light system utilizing a camera attached with a telecentric lens. In the calibration process, a white flat surface and another flat surface with circle dots serve as the calibration targets. After deriving the properties of the pinhole projector through conventional camera calibration method using circle dots and determining the camera's attributes via 3D feature points estimation through iterative optimizations, the white surface calibration target was positioned at various poses and reconstructed with initial camera and projector calibration data. Each 3D reconstructions was fitted with an ideal virtual ideal plane that was further used to create the pixel-wise phase-to-coordinate mapping. To optimize the calibration accuracy, various angled poses of the calibration target are employed to refine the initial results. Experimental findings validate that the proposed approach offers high calibration accuracy for a structured light system using a telecentric lens.

“Unidirectional structured light system calibration with auxiliary camera and projector,” (2024)

Y. Yang, Y.-H. Liao, I. Bortins, D. P. Baldwin, and S. Zhang, “Unidirectional structured light system calibration with auxiliary camera and projector,” Optics and Lasers in Engineering, 175, 107984, (2024)

Abstract

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 visionwith 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.

"Electrically tunable lens assisted absolute phase unwrapping for large depth-of-field 3D microscopic structured-light imaging,” (2024)

L. Chen and S. Zhang, “Electrically tunable lens assisted absolute phase unwrapping for large depth-of-field 3D microscopic structured-light imaging,” Optics and Lasers in Engineering, 174, 107967 (2024)

Abstract

This paper presents an absolute phase unwrapping method for large depth-of-field (DOF) microscopic structured-light 3D imaging. We calculate the wrapped phase maps and the fringe contrast maps from fringe images captured under various focus settings realized by an electrically tunable lens. From the fringe contrast maps, we extract each in-focus pixel and determine its approximate depth value from the focus settings. The approximate depth map is then used to create an artificial phase map. The geometric-constraint-based phase unwrapping method is adopted to unwrap the phase of all in-focus pixels using the artificial phase map. The unwrapped in-focus phase map is then used to reconstruct 3D coordinates for the entire DOF with the multi-focus pin-hole model. Experimental results demonstrate that our proposed method only requires three phase-shifted fringe patterns to achieve large DOF 3D imaging.