[show abstract][hide abstract] ABSTRACT: Measuring deformation of vibrating specimens whose dimensions are in the submillimeter range introduces a number of difficulties using laser interferometry. Normal interferometry is not suitable because of a phase ambiguity problem. In addition, the noise effect is much more serious in the measurement of small objects because a high-magnification lens is used. We present a method for full-field measurement of displacement, velocity, and acceleration of a vibrating miniature object based on image-plane digital holographic microscopy. A miniature cantilever beam is excited by a piezoelectric transducer stage with a sinusoidal configuration. A sequence of digital holograms is captured using a high-speed digital holographic microscope. Windowed Fourier analysis is applied in the spatial and spatiotemporal domains to extract the displacement, velocity and acceleration. The result shows that a combination of image-plane digital holographic microscopy and windowed Fourier analyses can be used to study vibration without encountering a phase ambiguity problem, and one can obtain instantaneous kinematic parameters on each point.
[show abstract][hide abstract] ABSTRACT: A novel optical edge projection method for surface contouring of an object with low reflectivity is presented. A structured light edge is projected onto a dark surface, and the image is captured by a CCD camera. The surface profile of the object is then evaluated by an active triangular projection technique, and a whole-field three-dimensional contour of the object is obtained by scanning the optical edge over the entire object surface. An edge detection method based on a continuous wavelet transform (CWT) is employed to determine the location of the optical edge. The method of optical edge detection is described, and characteristic details of gray-level distribution along the edge are analyzed. It is shown that the proposed wavelet edge detection method is not dependent on any threshold values; hence the true edge position can be determined without subjective selection. A black low-reflectivity object surface made from woven carbon fiber is measured, and the experimental results show that the profile of a woven carbon fiber can be obtained by the proposed method.
[show abstract][hide abstract] ABSTRACT: Shearography is an optical technique allows direct measurement of deflection derivatives. This paper presents a novel temporal phase analysis technique based on wavelet transform when shearography is applied to measure a continuously deforming object. A series of shearing speckle patterns is captured by a high-speed camera during the deformation. To avoid the phase ambiguity problem, a temporal carrier is generated by a piezoelectrical transducer (PZT) stage in one beam of the modified Michelson interferometer. The intensity variation of each pixel on recorded images is then analyzed along time axis by a robust mathematical tool - complex Morlet wavelet transform. After the temporal carrier is removed, the absolute phase change representing the first-order derivative of the continuous deformation is obtained without the need of temporal or spatial phase unwrapping process. The results obtained by wavelet transform are compared with those from temporal Fourier transform. Accepted version
[show abstract][hide abstract] ABSTRACT: A temporal wavelet analysis algorithm is proposed for shadow-moiré-based three-dimensional surface profiling on objects having discontinuous height steps. A grating is positioned close to an object, and its shadow is observed through the grating. The moiré fringe patterns vary when the grating is in-plane rotating. A series of fringe patterns are captured by a CCD camera at different rotating angles. Phase values are evaluated point by point with the continuous wavelet transform. From the phase values of each point on the object, the distance between the object and the grating can be retrieved. The surface profile is obtained without temporal or spatial phase unwrapping. This technique is applicable to objects with discontinuous height steps, which are impossible to measure with conventional shadow moiré topography. Two specimens are tested to demonstrate the validity of the method: One is an object with a height step of 1.6 mm, and another is a small coin with unevenness of less than 0.2 mm. The experimental results are compared with test results by use of the mechanical stylus method.
[show abstract][hide abstract] ABSTRACT: A novel optical edge projection method is proposed for surface contouring of an object with low reflectivity. A structured light edge is projected onto a dark surface, and the image is captured by a CCD camera. The object contour is evaluated using active triangular projection algorithm, and a whole field 3D contour of the object is obtained by scanning the optical edge over the entire object surface. The proposed method is applied to a black low-reflective object made of weaved carbon fiber. The results which are verified with conventional phase shifting fringe projection technique show that an accurate profile of a specimen can be obtained. The proposed method is also applied to surface contouring of a small component. Accepted version
[show abstract][hide abstract] ABSTRACT: Fringe projection technique is a non-contact, full field shape measurement method. The object depth information is recorded as sinusoidal or square wave fringe patterns. The phase-shifting technique or Fourier transform method can be used to extract wrapped phase data and a continuous phase distribution can be retrieved by a phase unwrapping process. In this paper, a shape measurement method using one frame projected sawtooth fringe pattern is proposed. A computer-generated sawtooth pattern is projected by a programmable liquid crystal display (LCD) projector. The deformed sawtooth pattern encoding object shape is converted to a wrapped phase map without using phase shifting or Fourier transform and a continuous phase distribution is retrieved by a quality-guided phase unwrapping algorithm. A special phase quality map, capable of detecting invalid shadowed phase data, is proposed to facilitate the unwrapping process.
[show abstract][hide abstract] ABSTRACT: A temporal wavelet analysis method is proposed for velocity, displacement, and three-dimensional surface-profile measurement of a continuously deforming object by use of the shadow moiré technique. A grating is placed close to a deforming object, and its shadow is observed through the grating. The moiré fringe patterns, generated by the interference of the grating lines and their shadows, are captured by a high-speed CCD camera with a telecentric gauging lens. Instantaneous frequency of gray-value variation is evaluated point by point with the continuous wavelet transform. From the instantaneous frequency of each point on the object, the velocity, displacement, and high-quality surface profile at different instants can be retrieved. In this application, two specimens are tested to demonstrate the validity of the proposed method: One is a small coin with a rigid body motion, and the other is a simply supported beam subjected to a central point load. The results are compared with those obtained from temporal Fourier-transform and mechanical stylus methods.
[show abstract][hide abstract] ABSTRACT: A simple temporal analysis algorithm (phase scanning method) is proposed for 3-D surface profile measurement of a vibrating object based on shadow moiré technique. A grating is positioned close to a low-frequency vibrating object and its shadow is observed through the grating. The moiré fringe patterns, generated by the interference of grating lines and shadow lines, are captured by a high-speed CCD camera with a telecentric gauging lens. Phase values are evaluated point by point using phase scanning method. From the phase values of each point on the object, the high-quality surface profile of the specimen at different instants of vibration can be retrieved. In this application, two specimens are tested to demonstrate the validity of the method, one is a spherical cap with height of 4 mm, the other is a small coin with unevenness of less than 0.2 mm. The experimental results are compared with test results using mechanical stylus method. Accepted version
[show abstract][hide abstract] ABSTRACT: In electronic speckle pattern interferometry (ESPI), speckles are information carriers as well as noise that hinders the extraction of high quality phase. This paper presents a phase extraction method based on the statistical property of speckles. Assuming that speckle related phase is a random variable having a uniform distribution, the grey level variance of a number of pixels is found to be related to the modulation intensity of a speckle pattern. The relation is used to establish a connection between the phase to be measured and the variance of grey level difference between two speckle patterns. Subsequently, a phase map wrapped in [0,π) is extracted. In order to obtain a standard 2π wrapped phase map, an initial one step phase shift is introduced. The phase value of a pixel under consideration is obtained from the grey levels of its N×N neighbouring pixels. The optimal value of N is obtained based on a qualitative analysis of the initial results. With an appropriate value of N, an accuracy of 1% can be achieved. Accepted version
[show abstract][hide abstract] ABSTRACT: The quality-guided algorithm is a method widely used in phase unwrapping. The algorithm uses a quality map to guide its unwrapping process, and its validity depends on whether the quality map can truly reflect phase quality. In fringe projection surface profilometry, discontinuous surface structure, low surface reflectivity, and saturation of the image-recording system are sources of unreliable phase data. To facilitate the unwrapping process, we demonstrate an accurate quality map based on spatial fringe modulation, which is extracted from a single fringe pattern. Compared with temporal fringe modulation, the new criterion is more sensitive to spatial structure changes and less dependent on illumination conditions.
[show abstract][hide abstract] ABSTRACT: In recent years, optical interferometry has been applied to whole-field, non-contact measurement of vibrating or continuously-deforming objects. In optical dynamic measurement, an interferogram sequence is obtained by a highspeed camera. Retrieving dynamic phase values from this interferogram sequence leads to a precise measurement of different kinematic and deformation parameters of a continuously-deforming or vibrating object. In this paper, this interferogram sequence is classified into two types: (i) intensity variation; and (ii) exponential phase signal. Different temporal phase retrieving techniques, such as Hilbert transform, Fourier transform, windowed Fourier transform and wavelet transform are applied to extract the phase from a simulated signal. The advantage and drawback of each algorithm are discussed. In addition, a new method based on the combination of Fourier transform and windowed Fourier transform is proposed and the simulation shows it can eliminate the noise and evaluate the phase more accurately. Published version