Xiaoyuan He

Shape measurement of small objects using LCD fringe projection with phase shifting

This paper describes the use of optical fringe projection method for three-dimensional shape measurement of small objects. In this method, sinusoidal linear fringes are projected on the object surface by a programmable liquid crystal display (LCD) projector and a long working distance microscope (LDM). The image of the fringe pattern is captured by another LDM and a CCD camera and processed by phase-shifting technique. A simple procedure is described which enables calibration of the optical setup for subsequent quantitative measurement of unknown object shapes. The method developed can also be applied to the measurement of the warp of a small component under thermal loading. This method is relatively simple and accurate, and is capable of conducting fully automated measurements.

Microscopic surface contouring by fringe projection method

Abstract This paper describes the use of optical fringe projection method for 3D surface profile and deformation measurement of micro-components. In this method, sinusoidal linear fringes are projected on a micro-component surface by a grating phase shifting projector and a long working distance microscope (LWDM). The image of the fringe pattern is captured by a high-resolution CCD camera and another LWDM and processed by phase-shifting technique. A simple procedure is described which enables calibration of the optical set-up for subsequent quantitative measurement of micro-components of unknown shapes. This method is relatively simple and accurate, and is capable of conducting fully automated measurements. In this paper, two micro-components, a micro-mirror ( 0.1 mm ×0.1 mm ) and a micro-electrode pad are used to demonstrate deformation measurement and microscopic surface contouring.

Phase extraction from a single fringe pattern based on guidance of an extreme map

A method for automatic phase extraction from a single fringe pattern based on the guidance of an extreme map is introduced. The method uses an adaptive weighted filter to reduce noise and enhance contrast and to locate the fringe extremes. Wrapped phase values are calculated by use of an arccosine function obtained from the extreme map. With this method, wrapped phase values can be efficiently demodulated from a single fringe pattern without the need for assigning fringe order or interpolating fractional fringe order. The validity of the method is demonstrated by use of closed-fringe patterns generated by digital speckle interferometry.

Real-time 3D digital image correlation method and its application in human pulse monitoring

In industrial measurements and online monitoring, full-field and high-efficiency deformation analysis has been increasingly important and highly demanded in recent years. In this paper, a fast three-dimensional digital image correlation (3D-DIC) method was proposed to implement real-time measurement. Two improvements were suggested to accelerate the computation speed without sacrificing the accuracy. First, an efficient inverse compositional Gauss-Newton (IC-GN) algorithm was developed to avoid redundant computation. Moreover, a seed point-based parallel method was extended for 3D-DIC to achieve parallel computation and faster convergence speed. The detailed process of the real-time measurement using the proposed method was also introduced. Benefiting from the efficient IC-GN algorithm and parallel processing software we developed, full-field, real-time 3D deformation monitoring was realized at a frame rate of 10 frames/s with resolution of 5000 points per frame. For validation, the displacement field of a four-point bending beam was determined by the real-time 3D-DIC. As an application, the real-time human pulse diagnosis was also performed based on the presented technique. Experimental results verify that the proposed real-time 3D-DIC is practicable and effective for traditional Chinese medicine.

Two-step phase-shifting fringe projection profilometry: intensity derivative approach

A new two-step phase-shifting fringe projection profilometry is proposed. The slowly variable background intensity of fringe patterns is removed by the use of an intensity differential algorithm. The high-resolution differential algorithm is achieved based on global interpolation of fringe gray level on a subpixel scale. Compared with the traditional three- or four-step phase-shifting method, the profile measurement is sped up with this approach. Computer simulation and experimental performance are evaluated to demonstrate the validity of the proposed measurement method. The experimental results compared with those of the four-step phase-shifting method are presented.

Enlarging the angle of view in Michelson-interferometer-based shearography by embedding a 4f system.

Digital shearography based on Michelson interferometers suffers from the disadvantage of a small angle of view due to the structure. We demonstrate a novel digital shearography system with a large angle of view. In the optical arrangement, the imaging lens is in front of the Michelson interferometer rather than behind it as in traditional digital shearography. Thus, the angle of view is no longer limited by the Michelson interferometer. The images transmitting between the separate lens and camera are accomplished by a 4f system in the new style of shearography. The influences of the 4f system on shearography are also discussed.

Determination of three-dimensional displacement using two-dimensional digital image correlation.

A novel method that uses a two-dimensional (2D) digital image correlation (DIC) based on a single CCD camera to measure three-dimensional (3D) displacement and deformation is proposed. Rigid-body displacement in 3D space consists of both in-plane and out-of-plane components. The presence of an in-plane displacement component results in a shift of the center of the image displacement vector, while the slope of the image displacement vector is related to the out-of-plane displacement component. Global DIC is employed to determine the displaced position of each point on an object based on a linear distribution characteristic of the displacement vector. Speckle images with deformation introduced by 3D displacement are generated to demonstrate the feasibility of the proposed method. In the 3D rigid-body displacement, both in-plane and out-of-plane displacement components are separated by determining the intercept and slope of the image displacement vector. In the 3D deformation, a zero order displacement (pure rigid-body displacement) mode is assumed in a small subset of pixels. Simulated and experimental results demonstrate that both in-plane and out-of-plane displacements can be accurately retrieved using the proposed method.

Single-shot color fringe projection for three-dimensional shape measurement of objects with discontinuities

A simple but effective fringe projection profilometry is proposed to measure 3D shape by using one snapshot color sinusoidal fringe pattern. One color fringe pattern encoded with a sinusoidal fringe (as red component) and one uniform intensity pattern (as blue component) is projected by a digital video projector, and the deformed fringe pattern is recorded by a color CCD camera. The captured color fringe pattern is separated into its RGB components and division operation is applied to red and blue channels to reduce the variable reflection intensity. Shape information of the tested object is decoded by applying an arcsine algorithm on the normalized fringe pattern with subpixel resolution. In the case of fringe discontinuities caused by height steps, or spatially isolated surfaces, the separated blue component is binarized and used for correcting the phase demodulation. A simple and robust method is also introduced to compensate for nonlinear intensity response of the digital video projector. The experimental results demonstrate the validity of the proposed method.

Characterization of dynamic microgyroscopes by use of temporal digital image correlation

The advanced mechanical testing of microelectromechanical systems (MEMS) is necessary to provide feedback of measurements that can help the designer optimize MEMS structures and improve the reliability and stability of MEMS. We describe a digital image correlation (DIC) method for dynamic characterization of MEMS using an optical microscope with a high-speed complementary metaloxide semiconductor-based camera. The mechanical performance of a series of microgyroscopes is tested. The DIC method is employed to measure the microgyroscope in-plane displacement with subpixel accuracy. Use of the DIC method is less restrictive on the surface quality of the specimen and simplifies the measurement system. On the basis of a series of temporal digital images grabbed by a high-speed camera, the stability characteristic of the microgyroscopes is analyzed. In addition, the quality factors of the microgyroscopes are determined and agree well with other experimental methods.

Self-calibration single-lens 3D video extensometer for high-accuracy and real-time strain measurement

The accuracy of strain measurement using a common optical extensometer with two-dimensional (2D) digital image correlation (DIC) is not sufficient for experimental applications due to the effect of out-of-plane motion. Although three-dimensional (3D) DIC can measure all three components of displacement without introducing in-plane displacement errors, 3D-DIC requires the stringent synchronization between two digital cameras and requires complicated system calibration of binocular stereovision, which makes the measurement rather inconvenient. To solve the problems described above, this paper proposes a self-calibration single-lens 3D video extensometer for non-contact, non-destructive and high-accuracy strain measurement. In the established video extensometer, a single-lens 3D imaging system with a prism and two mirrors is constructed to acquire stereo images of the test sample surface, so the problems of synchronization and out-of-plane displacement can be solved easily. Moreover, a speckle-based self-calibration method which calibrates the single-lens stereo system using the reference speckle image of the specimen instead of the calibration targets is proposed, which will make the system more convenient to be used without complicated calibration. Furthermore, an efficient and robust inverse compositional Gauss-Newton algorithm combined with a robust stereo matching stage is employed to achieve high-accuracy and real-time subset-based stereo matching. Tensile tests of an Al-alloy specimen were performed to demonstrate the feasibility and effectiveness of the proposed self-calibration single-lens 3D video extensometer.