A key problem when using the derotator is to ensure the coaxial alignment between the optical axis of the derotator and the rotation axis of the measured object. Moreover, accurate measurement of the rotation axis is the prerequisite of achieving the coaxial alignment. In this paper, we propose a strategy for estimation of the rotation axis with the assistant of an auxiliary laser and a binocular stereo vision sensor. When a laser source is temporarily attached to the measured object or the rotor shaft, the rotation of the laser will generate a ruled surface. The binocular sensor can accurately measure the coordinates of laser dots in three-dimensional (3D) space. The pose vector of the laser in each certain rotation angle can be estimated with measured 3D coordinates of the laser dots. Finally, the rotation axis of the object can be estimated with multiple pose vectors considering the constraint of the fixed axis rotation. The validity and precision of this proposed strategy are demonstrated by means of experiments.
This work proposes a two-step phase-shifting algorithm as an improvement of fringe projection profilometry. Considering the working process of fringe projection, the captured fringe image is formulated with two variables, i.e. surface reflectivity and phase value. And a phase shift of 3π/2 is introduced to get the two-step phase-shifting. After appropriate variable substitution, expressions of two fringe images can be transformed into two equations corresponding to a line and a circle respectively. With this circle-line model, the characteristic of solution and the phase error due to non-zero ambient light are analyzed. Then the approach of error compensation is proposed based on estimation of the real fringe contrast and non-linear least square optimization. The validity of the proposed approach is demonstrated with both simulations and experiments.
In this paper, we have reported design and analysis of logic NOR and XNOR gates based on two dimensional (2D) photonic crystals at a wavelength of 1550nm. All the logic gates based on the phenomenon of interference and selfcollimation effect. In proposed structure, we control the output by adjusting phase difference to achieve constructive or destructive interference. The working of these logic gates is analyzed by the FDTD method.
Nonlinear intensity response, namely gamma effect, of the projector-camera setup introduces phase error in phase-shifting profilometry. This paper presents a comparison of three phase error compensation methods: active, passive and adaptive, using a universal phase error model. The active method calibrates a gamma factor to modify the projected fringe patterns; the passive method implement an iterative procedure to work out an optimal phase map; the adaptive method compensate phase error based on Hilbert transform without any auxiliary conditions. Comparison Experiments were implemented in three and four phase-shifting steps, which demonstrated that the active method provided an excellent performance regardless the phase-shifting step, yet the passive method might fail when the phase error was large; the adaptive method could be in the same level as the passive method in four phase-shifting step.
With the increasing integration level of components in modern electronic devices, three-dimensional automated optical inspection has been widely used in the manufacturing process of electronic and communication industries to improve the product quality. In this paper, we develop a three-dimensional inspection and metrology system for semiconductor components with fringe projection profilometry, which is composed of industry camera, telecentric lens and projection module. This system is used to measure the height, flatness, volume, shape, coplanarity for quality checking. To detect the discontinuous parts in the internal surface of semiconductor components, we employ the fringes with multiple spatial frequencies to avoid the measurement ambiguity. The complete three-dimensional information of semiconductor component is obtained by fusing the absolute phase maps from different views. The practical inspection results show that the depth resolution of our system reaches 10 μm . This system can be further embedded for the online inspection of various electronic and communication products.
Fringe projection profilometry (FPP) has been widely used for three dimensional (3D) imaging and measurement. The fringe acquisition of FPP mainly depends on the diffuse light from the surface of objects, thus the characteristics of object surface have significant influence on phase calculation. One of the essential factors related to phase precision is modulation index, which has a direct relationship with the surface reflectivity. This paper presents a comparative study which focuses on the modulation index of different materials. The distribution of modulation index for different samples is statistical analyzed, which leads to the conclusion that the modulation index is determined by the diffuse reflectivity rather than the type of materials. This work is helpful to the development of effective de-noising algorithms to improve the measurement accuracy.
KEYWORDS: Calibration, 3D modeling, Imaging systems, 3D metrology, 3D image processing, Stereoscopy, Systems modeling, Microscopy, Optical metrology, Distortion
Fringe projection 3D microscopy (FP-3DM) plays an important role in micro-machining and micro-fabrication. FP-3DM may be realized with quite different arrangements and principles, which make people confused to select an appropriate one for their specific application. This paper introduces the ray-based general imaging model to describe the FP-3DM, which has the potential to get a unified expression for different system arrangements. Meanwhile the dedicated calibration procedure is also presented to realize quantitative 3D imaging. The validity and accuracy of proposed calibration approach is demonstrated with experiments.
KEYWORDS: Imaging systems, 3D image processing, 3D metrology, 3D modeling, Calibration, Cameras, 3D acquisition, Systems modeling, Stereoscopy, Prototyping
Dedicated prototype systems for 3D imaging and modeling (3DIM) are presented. The 3D imaging systems are based on the principle of phase-aided active stereo, which have been developed in our laboratory over the past few years. The reported 3D imaging prototypes range from single 3D sensor to a kind of optical measurement network composed of multiple node 3D-sensors. To enable these 3D imaging systems, we briefly discuss the corresponding calibration techniques for both single sensor and multi-sensor optical measurement network, allowing good performance of the 3DIM prototype systems in terms of measurement accuracy and repeatability. Furthermore, two case studies including the generation of high quality color model of movable cultural heritage and photo booth from body scanning are presented to demonstrate our approach.
KEYWORDS: 3D image processing, 3D metrology, Metrology, Imaging systems, Stereoscopy, Data storage, Cameras, Computer security, Computer networks, 3D image reconstruction
In this paper, the establishment of a remote laboratory for phase-aided 3D microscopic imaging and metrology is presented. Proposed remote laboratory consists of three major components, including the network-based infrastructure for remote control and data management, the identity verification scheme for user authentication and management, and the local experimental system for phase-aided 3D microscopic imaging and metrology. The virtual network computer (VNC) is introduced to remotely control the 3D microscopic imaging system. Data storage and management are handled through the open source project eSciDoc. Considering the security of remote laboratory, the fingerprint is used for authentication with an optical joint transform correlation (JTC) system. The phase-aided fringe projection 3D microscope (FP-3DM), which can be remotely controlled, is employed to achieve the 3D imaging and metrology of micro objects.
We present a fingerprint authentication scheme based on the optical joint transform correlator (JTC) and further describe its application to the remote access control of a Network-based Remote Laboratory (NRL). It is built to share a 3D microscopy system of our realistic laboratory in Shenzhen University with the remote co-researchers in Stuttgart University. In this article, we would like to focus on the involved security issues, mainly on the verification of various remote visitors to our NRL. By making use of the JTC-based optical pattern recognition technique as well as the Personal Identification Number (PIN), we are able to achieve the aim of authentication and access control for any remote visitors. Note that only the authorized remote visitors could be guided to the Virtual Network Computer (VNC), a cross-platform software, which allows the remote visitor to access the desktop applications and visually manipulate the instruments of our NRL through the internet. Specifically to say, when a remote visitor attempts to access to our NRL, a PIN is mandatory required in advance, which is followed by fingerprint capturing and verification. Only if both the PIN and the fingerprint are correct, can one be regarded as an authorized visitor, and then he/she would get the authority to visit our NRL by the VNC. It is also worth noting that the aforementioned “two-step verification” strategy could be further applied to verify the identity levels of various remote visitors, and therefore realize the purpose of diversified visitor management.
KEYWORDS: Cameras, Image processing, Error analysis, Digital image processing, Imaging systems, Machine vision, Device simulation, Signal to noise ratio, Current controlled current source, Computer simulations
Circular targets are widely used in machine vision. The localization of circle center plays a crucial role in machine vision applications. In the process of camera imaging, the circles change to the ellipses in the image plane of camera because of perspective transformation. The center of ellipse usually does not coincide with the projected center of the circle, leading to a deviation of circle center. Based on perspective transformation and analytic geometry we present a new approach, in which the concentric circular targets are adopted and the true projective position of the circular target can be determined accurately. Both simulation and experiment results show that the proposed method is valid and robust. The true positions of the circular centers can be localized by proposed method without the center deviations.
KEYWORDS: Gaussian filters, Computer simulations, Detection and tracking algorithms, Machine vision, Edge detection, Image filtering, Digital image processing, 3D modeling, Signal to noise ratio, Optoelectronics
Circular target is one of the most commonly used artificial markers in machine vision. An alternative method for center location of circular targets with sub-pixel accuracy based on surface fitting is presented in this paper. The gray level distribution around the image of the circular target is modeled starting form one-dimensional step edge smoothed with Gaussian filter, and then extending to two-dimensional case by means of variable substitution of the elliptical rotation. The surface model aforementioned is a non-elementary function, so an approximate expression is found subsequently to make the numerical computation executable. The parameters of the surface model are estimated with algebraic least square fitting, from which the accurate center location can be calculated. The experiment results show the proposed method is more robust to image degradation comparing with the most commonly used method.
KEYWORDS: Calibration, Cameras, 3D image processing, 3D acquisition, Distortion, Imaging systems, 3D metrology, 3D modeling, Projection systems, Image processing
In this paper, we proposed a novel method for correcting the 2D calibration target. Firstly, we captured
multiple images of the inaccurate calibration target from multi-views and located the coordinates of
those circle landmarks in these images. Secondly, homonymous landmarks in different images could be
detected by a scheme for a special topology relation. Thirdly, we could accurately reconstruct the 3D
coordinates of landmarks with a scale constraint using bundle adjustment strategy. And finally, the
scale was computed from an accurate distance between any two landmarks. Then we could obtain the
truly coordinates of landmarks, which multiplied by the scale. The experimental results validated that
our method is efficient, high-precision, low-cost and easy-implementation, which can be widely
applied in vision measurement and system calibration.
It is usually difficult to calibrate the 3-D vision inspection system that may be employed to measure the large-scale
engineering objects. One of the challenges is how to in-situ build-up a large and precise calibration target. In this paper,
we present a calibration target reconstruction strategy to solve such a problem. First, we choose one of the engineering
objects to be inspected as a calibration target, on which we paste coded marks on the object surface. Next, we locate and
decode marks to get homologous points. From multiple camera images, the fundamental matrix between adjacent images
can be estimated, and then the essential matrix can be derived with priori known camera intrinsic parameters and
decomposed to obtain camera extrinsic parameters. Finally, we are able to obtain the initial 3D coordinates with
binocular stereo vision reconstruction, and then optimize them with the bundle adjustment by considering the lens
distortions, leading to a high-precision calibration target. This reconstruction strategy has been applied to the inspection
of an industrial project, from which the proposed method is successfully validated.
KEYWORDS: Image registration, Error analysis, Optimization (mathematics), Range image registration, Metrology, 3D metrology, Detection and tracking algorithms, Optoelectronics, 3D image processing, 3D modeling
With the improvements in range image registration techniques, this paper focuses on error analysis of two registration methods being generally applied in industry metrology including the algorithm comparison, matching error, computing complexity and different application areas. One method is iterative closest points, by which beautiful matching results with little error can be achieved. However some limitations influence its application in automatic and fast metrology. The other method is based on landmarks. We also present a algorithm for registering multiple range-images with non-coding landmarks, including the landmarks' auto-identification and sub-pixel location, 3D rigid motion, point pattern matching, global iterative optimization techniques et al. The registering results by the two methods are illustrated and a thorough error analysis is performed.
Circular targets are commonly used in vision measurement and photogrammetry. Due to the asymmetric projection, the geometric centroid of the ellipse projection and the true projection of the target center are not identical, which leads to a systematic center location error. A method to correct the center location error is presented in this paper. Surface normal directions of circular targets are determined by camera calibration in advance. Then the correction values of the geometric centroids are calculated with space analytic geometry. The experimental results show the improvement of accuracy can be achieved after error correction by our method.
As a crucial part of active three dimensional(3D) vision system based on fringe projection technique, correspondence
search between two adjacent range images would directly influence on the accuracy of matching and fusion procedure of
the depth data. The conventional sub-pixel matching method by means of phase correlation can achieve a high level of
accuracy while it would also be associated with a time consuming procedure due to the requirement of capturing two
orthogonal series of sinusoidal fringes. Another technique utilizing linear interpolation algorithm based on the fringe
projection with single direction might cause accuracy declination as a result of the simplification in imaging model. A
novel method under the framework of linear interpolation was proposed. This approach makes use of absolute phase
values and parameters of epipolar line as two kinds of feature points for correspondence search, leading to a significant
improvement on the measurement accuracy of the depth data. Theoretical analysis and experiment results demonstrate
the validity of presented approach.
A novel method for the coarse registration of range images is proposed. This approach is based on texture-feature
recognition. As the development of optical digitizing technique, it is now able to acquire the range images and associated
texture images sequentially or simultaneously. It's possible to identify the range feature points through texture feature
points. Scale Invariant Feature Transform (SIFT) is an efficient method for texture feature generation. SIFT transforms
texture image into a large collection of local feature vectors, each of which is invariant to image scaling, translation, and
rotation. The mismatched correspondence pairs can be discarded using random sample consensus algorithm based on
epipolar geometry constraint. We select more than three well-registered texture-feature pairs, with which we could find
the associated range-feature pairs of the range images. Initial pose estimation of the two involved range images can be
computed by these range pairs, and the fine registration is implemented using iterative closest point (ICP) algorithm. Our
approach utilizes the texture information to register the range images, leading to a technique that can be automatically
performed while the influence of 3D noise can be avoided. The experiment results demonstrate that the proposed
approach is efficient and robust for the registration of multiple range images.
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