KEYWORDS: Fringe analysis, Fourier transforms, Linear filtering, Neural networks, Phase shifts, Optical engineering, 3D modeling, Image denoising, 3D metrology, Video
Extracting phase from a single-frame fringe pattern is a key and challenging problem in fringe projection 3D measurement, especially for dynamic 3D measurement. We propose a single-shot phase extraction approach based on a low-pass filter with a well-trained image denoiser. Various comparative experiments have verified the effectiveness of the proposed method. More importantly, we associate the single-shot phase extraction problem with image denoising using deep learning. Thus more existing well-designed deep neural network models can be reused in the proposed method, without having to design a new model.
KEYWORDS: Speckle, Speckle pattern, 3D modeling, Cameras, Clouds, Image fusion, Error analysis, High dynamic range imaging, 3D metrology, Fringe analysis
Speckle projection provides rich textures for correlation, and has been widely used in three-dimensional(3D) reconstruction. However, for objects with high dynamic range(HDR) surface, conventional uniform speckle projection usually causes over-exposure and over-dark regions simultaneously in the captured images, leading to miss-match and errors in 3D result. It is difficult to perform defect elimination via adjustment of speckle intensity globally or the camera exposure time. To tackle this problem, this paper proposed a novel adaptive speckle projection method to distinguish the appropriate projection intensity of specific parts in the speckle pattern, thereby avoiding over-exposure, while the dark regions not being affected. First, uniform intensity patterns of multiple gray-levels are projected onto the surface of testing object, the appropriate projection intensity at each pixel position in the camera coordinate system is calculated, and the saturated area in the captured image is marked. Then, a set of orthogonal fringe patterns are projected onto the testing object to establish the coordinates mapping relationship between the camera and the projection system, and the adaptive speckle pattern under the projection coordinate system is generated. Finally, the generated adaptive speckle pattern is used to scan the testing object, and the spatial-temporal correlation algorithm is used for 3D shape retrieval. Experimental results demonstrate feasibility of 3D shape reconstruction of HDR surfaces with the proposed method, and obvious advantages compared with the traditional methods in terms of reconstruction completion and measurement accuracy. Keywords: adaptive speckle, high dynamic range, coordinates mapping, spatial-temporal correlation, 3D reconstruction
A three-dimensional (3D) face profilometry based on facial landmarks and a multi-view system is proposed. Theoretically, at least three fringe patterns are required in phase-shifting profilometry, but the stereo relative phase pair obtained by a spatial unwrapping algorithm cannot be used for stereo matching directly because the starting points of phase unwrapping are not on the same fringe period order, so it will result in incorrect disparity. We proposed an innovative method that utilizes facial landmarks as prior knowledge to adjust the relative phase pair to the same phase reference for accurate stereo matching through a phase adjustment algorithm to determine the accurate fringe order difference. The three-step phase-shifting patterns are projected in a looping sequence, and each three-adjacent fringe stereo pair is treated as a group to obtain one frame of a 3D face, which means that the 3D face output of our proposed method can achieve the same frame rate as the camera. The accuracy evaluation and dynamic 3D facial expressions reconstruction verify the success of the proposed method.
Acquiring the three-dimensional (3-D) surface geometry of objects with a full-frame resolution is of great concern in many applications. This paper reports a 3-D measurement scheme based on single-frame pattern projection in the combination of random binary encoding and color encoding. Three random binary encoding patterns generated by a computer embedded in three channels of a color pattern lead to a color binary encoding pattern. Two color cameras with a stereo-vision arrangement simultaneously capture the measured scene under the proposed encoding structured illumination. From captured images, three encoding images are extracted and analyzed using the extended spatial–temporal correlation algorithm for 3-D reconstruction. Theoretical explanation and analysis concerning the encoding principle and reconstruction algorithm, followed by experiments for reconstructing 3-D geometry of stationary and dynamic scenes show the feasibility and practicality of the proposed method.
Projector nonlinearity is a common problem for digital structured light-based three-dimensional (3-D) shape measurement techniques. A temporal-spatial binary encoding method is presented for the purpose of eluding it. We build a 3-D shape measurement scheme by combining our proposed method with phase measurement profiling. A standard sinusoidal fringe pattern is divided into more than two binary fringe patterns using specially designed temporal and spatial binary encoding rule based on intensity hierarchic quantification, and then are in-focus projected onto the measured object at a time sequence to reconstruct a frame phase-shifting fringe image. On account of the projected binary fringe pattern strictly consisting of zeros and ones, the influence of the projector nonlinearity on the measurement result can be effectively ruled out and simultaneously enables high-quality sinusoidality. In-depth investigations by theoretical analysis and experiments are conducted to demonstrate the performance of this method.
The proposed four-quadrant Moiré alignment scheme to detect the misalignment between mask and wafer for proximity lithography can achieve the alignment accuracy with nanometer level. When implementing the scheme, however, the distribution of Moiré fringes associated with the mask–wafer gap indeed goes against the alignment, making the gap optimization highly urgent. The optimization model is established, and numerical simulation as well as experimental verification is also provided. Furthermore, an alignment accuracy of ∼3 nm with the illumination wavelength of 632.8 nm is experimentally attained. Simultaneously, the design mechanism of alignment marks for improving the availability of the alignment scheme is discussed.
Effects of grating marks parameters on alignment precision and scope are investigated in this paper. In the lithography
alignment method based on moiré fringe, gratings are especially used as alignment marks. However, the rational design
of grating marks for this approach to realize high-precision alignment is of great importance. In order to improve the
feasibility of the alignment method, effects of several physical parameters of grating marks on alignment precision are
analyzed by numerical calculation. The results imply that qualities of grating marks, such as size of period and ratio
between two gratings, have an important impact on alignment precision and scope.
The application of dual-grating moiré fringe is introduced in lithography alignment. Minute angular displacement between the two alignment marks will lead to the tilt of moiré fringe, which has an influence in high-accuracy alignment. Proposed is a novel measurement method of angular displacement based on the phase of moiré fringe. The relationship between phase of moiré fringe and angular displacement is analyzed. Both simulation and experiment indicate that an angular displacement can be determined to achieve a linear displacement measurement with error of nanometer level.
The relative position of wafer and mask can be calculated by information of Moiré fringe during alignment, a maskless
lithography alignment method based on circular gratings Moiré fringes phase-shifting technique is proposed in this paper.
Circular grating Moiré fringes have characteristics of measuring simultaneously angular displacement and line
displacement. Location information of wafer in alignment can be real-time reflected in spatial phase of Moiré fringes. A
digital micromirror device controlled by a computer is used to generate phase-shifting grating labels, and phase-shifting
Moiré fringes will be formed by superposition of phase-shifting grating labels with grating label on the wafer. The
position information of wafer can be abstained by phase analysis using Fourier transform method combined with
phase-shifting technique,and gives feedback to the displacement stage to realize alignment. The theory basis of this
method is emphatically introduced. Also, application of this method in maskless lithography alignment is analyzed in
detail. Simulation results show that this method is high in accuracy, simple in operation and simple in algorithm. It
provides a feasible method for lithography alignment technique.
Lithography is one of the most important and complicated key equipments for the Integrated Circuit (IC) manufacture.
The ultra-precision stage is the important subsystem of lithography and its motion performance impacts directly on the
resolution and throughput of lithography. In this paper, a robust and high response speed control strategy for dual-stage
manipulator is presented. The coarse/fine dual-stage uses the linear motor driven coarse (macro) positioning stage and
the Lorenz plane motor driven fine (micro) positioning stage. By adopting merits of both coarse and fine actuator, a
desirable system having the capacity of large workspace with high resolution of motion is enabled. The feedback
controller is constructed so that the fine stage tracks the coarse stage errors. The controller is robustly designed as the
master-slave control strategy. In addition, the position decoupling which translates fine stage’s machine position
command into its actual position command are discussed and, as a result, the overall coarse/fine dual-stage servo system
exhibits robust and high response speed performance. Simulation shows that master-slave controller can much decrease
positioning error and improve response speed of the coarse/fine dual-stage system.
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