A multipoint diffraction strain sensor (MDSS) in moire interferometer was developed earlier as direct approach for
measuring strain of a macro-sized object. Recently a MDSS based on a micro-moire interferometer was proposed for
measuring strain for a micro-sized object in wholefield with unique features, one of which is variable sensitivity and
measurement range. The technique is now extended to measure both tilt and non-uniform strain of a material.
The single point Optical Diffraction Strain Sensor has been extended to a patent-pending Multipoint Diffraction Strain
Sensor (MDSS) using a microlens array. The system was further extended for strain measurement with variable
sensitivity and measurement range. In this paper, the MDSS is shown to measure both tilt and non-uniform strain with a
sensitivity of 0.41 mε/pixel and 4.7 mrad/pixel respectively. As validation the strain measured by the MDSS is compared
with that by a micro-moiré interferometer with a Gabor filtering method for fringe pattern analysis, while the tilt is
compared with derivatives of the surface profile measured by a confocal microscope.
We discuss a novel whole-field optical strain sensor termed the moiré interferometric strain sensor (MISS) for simultaneous measuring of multipoint strains and whole-field contours of in-plane displacement. A high-frequency grating, attached to the surface of a specimen, is used as the displacement and strain-sensing unit. When illuminated by two collimated beams at a prescribed angle, the interference of the diffracted beams gives the whole-field deformation contours. If, on the other hand, each of the individual beams is separately imaged using a multilens CCD sensor similar to a wavefront sensor, the separation between the spot centroids for each microlens is directly proportional to the normal or shear strain component at the corresponding position on the specimen. Applications are demonstrated for uniform rotation and simulated in-plane strains.
A speckle shearing interferometric system is proposed for real-time inspecting sub-surface defects of unpolished silicon wafer. Under dynamic thermal loading, derivative distribution of out-of-plane displacement along a shear direction is measured and homogeneity and regularity of the distribution is indicator of whether impurities or voids exist under the wafer surface. During a continuous thermal stressing, a sequence of speckle patterns are obtained and phase analysis is implemented in time domain. In this paper, the validity of the method for non-destructive testing of sub-surface defects of silicon wafers has been demonstrated.
The present work relates to surface and/or subsurface defects inspection system for semiconductor industries and particularly to an inspection system for a defect such as swirl defects and particles in an unpolished silicon wafer before the wafer fabrication process by a combined fiber optic digital shearography and holography technique. The dual purpose camera described in this paper gives the possibility of using either digital shearography or holography (DSPI) techniques depending on application needs. The sub-surface defects in a wafer normally create strain concentrations subjected to loading (stressing) which are translated into anomalies in the fringe pattern. A real time technique with the use of Lab view Express 7 software is developed to detect defects in Si-wafer with the application of thermal oading as a stressing method. The results obtained by applying a real time fiber optic shearography technique are described in this paper. The method described here relates specifically to semiconductor wafers, but may be generalized to any other samples.
A novel fringe processing method is proposed to segment whole- field strain distributions by incorporating moire interferometric technique. By applying an elaborately designed filter bank, the proposed scheme is employed to measure fringe frequency and hence determine derivatives of displacement.
Moire interferometry is a powerful technique for high sensitivity in-plane deformation contouring. However, from an engineering viewpoint, the derivatives of displacement, i.e. strain, is the desired parameter. Thus there is a need to differentiate the displacement field. Optical and digital methods have been proposed for this differentiation. Optical methods provide contours which still need to be quantified, while digital methods suffer from drawbacks inherent in the digital differentiation process. In this letter we describe a novel approach of strain segmentation the moire pattern using a multi-channel Gabor filter. Appropriate filter design allows for user-specific segmentation, which is essentially in engineering design and analysis.
It is interesting and useful to note that, even without carrier, the fringe pattern can still be processed for phase retrieving by Fourier transform method. A key problem is its phase retrieving accuracy, which is the topic of this paper. Our analysis shows that in some cases this method may introduce large phase errors and should be used with care.
Computer vision inspection systems are widely used for on- line inspection and quality control to improve the finished product quality and lower the costs. In this study, a carrier grating is used to carry spatial information about defects. To demodulate the regular/irregular fringe information, a Gabor-filter demodulation method is proposed, which is better for automatically detecting and localizing fringe distortion and hence demodulation method has been implemented for testing some wineglass samples. Using Gabor channel filters the feature images from global to local over those samples regions are extracted and utilized to classify them into good and bad.
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