Papers by Keyword: Phase Unwrapping

Abstract: Optical interferometry methods are widely used for measuring microdisplacement with nanometer accuracy. However, most commercially available optical interferometry systems are large and expensive for manufacturing applications. In this study, we report the development of a low-cost portable optical interferometry microscope for factory use. The light source was a tungsten–halogen white lamp with an optical filter. The microscope has an objective lens with a numerical aperture of 0.3, a magnifying power of 10, and field depth of 3.056 μm. Interference images were collected with an NTSC CCD-video camera. The resolution of the interference image is 320 × 240 pixels and stored in BMP format. To obtain phase shifted interferometry images, a piezoelectric actuator was used to monitor the table movement along the optical axis. The total cost of all system parts is approximately 7000 to 8000 US dollars. To evaluate the basic performance of the developed interferometry microscope, we measured a steel ball, the penetration mark of a Rockwell scale hardness indenter, and a gauge block surface with a bump. The developed interferometry microscope can measure continuous and gently sloping surfaces. The processing time is approximately 10–20 s.

Abstract: Accurate information of microscopic topography is very important for efficacy assessment of a surface texture of skin health. Due to the limitations of the direct visual assessment of skin microscopic topography, an optical dermastocopy is very common to be used as skin imaging device to magnify skin topography based on a white light reflection. The limitation of this method is its poor spatial resolution to quantify skin topography. In this work, microscopic skin imaging based on phase shifting method is configured using a DLP pico-projector with LED illumination and a handheld digital microscope. As illuminator for the digital microscope, the DLP projector is programmed to generate patterned light on skin surface. Image processing is required in providing accurate information of surface topography. The first step, a wrapped phase shifting must be extracted from acquired intensity images. The second step is obtaining unwrapped phase image, which is a critical process because it must be recovered from wrapped phase shifting that containednoise. Finally, phase offset due to multiples of 2π during phase unwrapping must be removed. Early experiments on simple object are carried out to test the level of distortion of fringe in several variations of contrast and also to test the performance of the system on several frequency variations. The test results indicate the depth proportion obtained from absolute phase image has the same trend as the proportion of direct measurement. Implementation on the skin surface profile performed on three test areas: the back of the hand and knuckle creases. Based on quantitative and qualitative analysis,our proposed scheme of skin imaging based on phase shifting is promising for surface profile measurement and imaging of the skin.

Abstract: InSAR phase unwrapping is one of the key technologies in precise differential interferometry measurement. However, when the noise is more, wrapped phase easily make the unwrapping result’s errors more. To solve this problem, phase error is compensated, and extended Kalman filter gain is limited within a certain range, error is suppressed in a relatively small area. Finally, extended Kalman filter smooth reduce the errors’ spread. Using interference data of ESA ERS-1 satellite to do experiments, it show that this method is superior to least squares method, quality guided method and original Kalman filter method in unwrapping precision and quality, it has higher stability.

Abstract: This paper presents a novel image acquisition technology for colorless raised characters. First, a grating image modulated with the height information of raised character is captured through a vision system; then, the Fourier transform method is used to get the phase of the grating image; finally, a rational design phase unwrapping method is designed to reconstruct the grayscale image of raised characters. Experiments show that the proposed method can get the well-separated images of raised characters.

Abstract: A phase unwrapping algorithm for 3D shape measurement of discontinous surface is presented. The proposed phase unwrapping algorithm can overcome the phase discontinuous problem.Three channel values in HSV space are completely employed to calculate the pattern color information intensity, the relative position relationship between adjacent pixels hue channel value is used to decide absolute phase fringe order. The proposed unwrapping algorithm can be applied to the measurement for 3d object with large height steps and/or spatially isolated surfaces. The correctness of the proposed algorithm is validated by our simulated experiment.

Abstract: Phase unwrapping (PU) is a difficult task commonly found in applications involving interferometric synthetic aperture radar (InSAR), magnetic resonance imaging (MRI) and optical surface profile measurements; all of which involve mathematically ill-posed problems. Conventional algorithms exhibit strong shortcomings in PU when phase discontinuity flaws exist. To simulate these situations, we are custom-designed test data with a phase discontinuity flaw. This simulated data is a 3D Gaussian distribution with an arc-shaped notch as a phase flaw. PU is carried out by Bayesian inference and MRF (Markov Random Field) modeling. A graph cut algorithm is employed for optimization with respect to energy minimization. Three other conventional algorithms are also employed and their PU performance is compared. The results show the good performance and effectiveness of the Bayesian MRF modeling method. These experimental results are important references for phase unwrapping problems when phase discontinuities exist.

Abstract: The kalman filtering phase unwrapping is a state estimation problem. It can realize phase unwrapping and noise elimination at the same time, and calculate the real phase by establishing the state space model and vector observation model. In the steep terrain, the conventional kalman filtering algorithm unwrapping results are often not accurate, easy to cause the error transfer. Aiming at this problem, the weighted kalman filter phase unwrapping algorithm based on the phase derivative variance map is proposed. The values of the phase derivative variance maps are applied to determine the noise variance in the observation equation, then the weighted kalman filter is used to unwrap phase, this can increase the accuracy of the results. Finally, experiments are carried out in the InSAR data application under the condition of steep terrain, and with the conventional kalman filtering phase unwrapping algorithm are compared, the effectiveness of the proposed method is verified.

Abstract: The profilometry of three dimensional objectives based on phase approaching of projected grating is widely used in various fields of application engineering. In this paper, two cameras are equipped in our system to obtain fast and static measurement compared to the conventional measuring system with single camera which needs rotated measurement. Algorithm of phase unwrapping based on reliability is also adopted to improve the performance which avoid the breakage of projected grating. Experimental data indicates the measurement error of this Two-camera phase measuring profilimetry system is within 1%. This new profilometry system has great potential application value in the profile measurement of static three dimensional targets.

Abstract: A phase-shifting algorithm, called a (4,4) algorithm, which takes four phase-shifting interferograms before a specimen is deformed and four interferograms after a specimen is deformed, is presented first. This method is most widely used for phase extraction. Its drawback limited it to be used in dynamic measurements. Also shown is an algorithm called a (4,1) algorithm that takes four phase-shifting interferograms before a specimen is deformed and one interferogram after a specimen is deformed. Because a high-speed camera can be used to record the dynamic interferogram of the specimen, this algorithm has the potential to retain the phase-shifting capability for ESPI in dynamic measurements. The quality of the phase map obtained using (4,1) algorithm is quite lower compared to using (4,4) algorithm. In order to obtain high-quality phase map in dynamic measurements, a direct-correlation algorithm was integrated with the (4,1) algorithm to form DC-(4,1) algorithm which is shown to improve significantly the quality of the phase maps. The theoretical and experimental aspects of this newly developed technique, which can extend ESPI to areas such as high-speed dynamic measurements, are examined in detail.

Abstract: In this paper, we present an optical profilometric technique for 3D shape measurement--Fourier Transform Profilometry (FTP). In contrast to other optical techniques, the grating pattern will in its spatial frequency fields as well as in its space-signal fields which projected onto the object surface and was Fourier-transformed and processed. This technique only requires a single fringe pattern up to no more than two to reproduce 3D object shape. FTP has a much higher sensitivity to the fringe quality than other techniques. We design a system which has a high-resolution system composed of a video projector and a digital camera. A fringe pattern of programming by computer is projected onto the free surface and its diagram is recorded by the camera. The deformed fringe pattern arising from the surface deformations is later compared to the unreformed (reference) one, leading to a phase map from which the free surface can be reconstructed after some series of processes.