Efficient fringe image enhancement based on dual-tree complex wavelet transform

Author name used in this publication: William W. L. Ng2011-2012 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishe

Effective bias removal for fringe projection profilometry using the dual-tree complex wavelet transform

Author name used in this publication: Daniel Pak-Kong Lun2012-2013 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishe

A fringe projection profilometry scheme based on embedded speckle patterns and robust principal component analysis

2019 SPIE. Phase unwrapping is one of the key steps for fringe projection profilometry (FPP)-based 3D shape measurements. Conventional spatial phase unwrapping schemes are sensitive to noise and discontinuities, which may suffer from low accuracies. Temporal phase unwrapping is able to improve the reliability but often requires the acquisition of additional patterns, increasing the measurement time or hardware costs. This paper introduces a novel phase unwrapping scheme that utilizes composite patterns consisting of the superposition of standard sinusoidal patterns and randomly generated speckles. The low-rankness of the deformed sinusoidal patterns is studied. This is exploited together with the sparse nature of the speckle patterns and a robust principal component analysis (RPCA) framework is then deployed to separate the deformed fringe and speckle patterns. The cleaned fringe patterns are used for generating the wrapped phase maps using the standard procedures of phase shift profilometry (PSP) or Fourier Transform profilometry (FTP). Phase unwrapping is then achieved by matching the deformed speckle patterns that encode the phase order information. In order to correct the impulsive fringe order errors, a recently proposed postprocessing step is integrated into the proposed scheme to refine the phase unwrapping results. The analysis and simulation results demonstrate that the proposed scheme can improve the accuracy of FPP-based 3D shape measurements by effectively separating the fringe and speckle patterns

COMPRESSIVE IMAGING AND DUAL MOIRE´ LASER INTERFEROMETER AS METROLOGY TOOLS

Metrology is the science of measurement and deals with measuring different physical aspects of objects. In this research the focus has been on two basic problems that metrologists encounter. The first problem is the trade-off between the range of measurement and the corresponding resolution; measurement of physical parameters of a large object or scene accompanies by losing detailed information about small regions of the object. Indeed, instruments and techniques that perform coarse measurements are different from those that make fine measurements. This problem persists in the field of surface metrology, which deals with accurate measurement and detailed analysis of surfaces. For example, laser interferometry is used for fine measurement (in nanometer scale) while to measure the form of in object, which lies in the field of coarse measurement, a different technique like moire technique is used. We introduced a new technique to combine measurement from instruments with better resolution and smaller measurement range with those with coarser resolution and larger measurement range. We first measure the form of the object with coarse measurement techniques and then make some fine measurement for features in regions of interest. The second problem is the measurement conditions that lead to difficulties in measurement. These conditions include low light condition, large range of intensity variation, hyperspectral measurement, etc. Under low light condition there is not enough light for detector to detect light from object, which results in poor measurements. Large range of intensity variation results in a measurement with some saturated regions on the camera as well as some dark regions. We use compressive sampling based imaging systems to address these problems. Single pixel compressive imaging uses a single detector instead of array of detectors and reconstructs a complete image after several measurements. In this research we examined compressive imaging for different applications including low light imaging, high dynamic range imaging and hyperspectral imaging

Snapshot Three-Dimensional Surface Imaging With Multispectral Fringe Projection Profilometry

Fringe Projection Profilometry (FPP) is a popular method for non-contact optical surface measurements, including motion tracking. The technique derives 3D surface maps from phase maps estimated from the distortions of fringe patterns projected onto the surface of an object. Estimation of phase maps is commonly performed with spatial phase retrieval algorithms that use a series of complex data processing stages. Researchers must have advanced data analysis skills to process FPP data due to a lack of availability of simple research-oriented software tools. Chapter 2 describes a comprehensive FPP software tool called PhaseWareTM that allows novice to experienced users to perform pre-processing of fringe patterns, phase retrieval, phase unwrapping, and finally post-processing. The sequential process of acquiring fringe patterns from an object is necessary to sample the surface densely enough to accurately estimate surface profiles. Sequential fringe acquisition performs poorly if the object is in motion between fringe projections. To overcome this limitation, we developed a novel method of FPP called multispectral fringe projection profilometry (MFPP), where multiple fringe patterns are composited into a multispectral illumination pattern and a single multispectral camera is used to capture the frame. Chapter 3 introduces this new technique and shows how it can be used to perform 3D profilometry at video frame rates. Although the first attempt at MFPP significantly improved acquisition speed, it did not fully satisfy the condition for temporal phase retrieval, which requires at least three phase-shifted fringe patterns to characterize a surface. To overcome this limitation, Chapter 4 introduces an enhanced version of MFPP that utilized a specially designed multispectral illuminator to simultaneously project four p/2 phase-shifted fringe patterns onto an object. Combined with spectrally matched multispectral imaging, the refined MFPP method resulted in complete data for temporal phase retrieval using only a single camera exposure, thereby maintaining the high sampling speed for profilometry of moving objects. In conclusion, MFPP overcomes the limitations of sequential sampling imposed by FPP with temporal phase extraction without sacrificing data quality or accuracy of the reconstructed surface profiles. Since MFPP utilizes no moving parts and is based on MEMS technology, it is applicable to miniaturization for use in mobile devices and may be useful for space-constrained applications such as robotic surgery. Fringe Projection Profilometry (FPP) is a popular method for non-contact optical surface measurements such as motion tracking. The technique derives 3D surface maps from phase maps estimated from the distortions of fringe patterns projected onto the surface of the object. To estimate surface profiles accurately, sequential acquisition of fringe patterns is required; however, sequential fringe projection and acquisition perform poorly if the object is in motion during the projection. To overcome this limitation, we developed a novel method of FPP maned multispectral fringe projection profilometry (MFPP). The proposed method provides multispectral illumination patterns using a multispectral filter array (MFA) to generate multiple fringe patterns from a single illumination and capture the composite pattern using a single multispectral camera. Therefore, a single camera acquisition can provide multiple fringe patterns, and this directly increases the speed of imaging by a factor equal to the number of fringe patterns included in the composite pattern. Chapter 3 introduces this new technique and shows how it can be used to perform 3D profilometry at video frame rates. The first attempt at MFPP significantly improved acquisition speed by a factor of eight by providing eight different fringe patterns in four different directions, which permits the system to detect more morphological details. However, the phase retrieval algorithm used in this method was based on the spatial phase stepping process that had a few limitations, including high sensitive to the quality of the fringe patterns and being a global process, as it spreads the effect of the noisy pixels across the entire result. To overcome this limitation, Chapter 4 introduces an enhanced version of MFPP that utilized a specially designed multispectral illuminator to simultaneously project four p/2 phase-shifted fringe patterns onto an object. Combined with a spectrally matched multispectral camera, the refined MFPP method provided the needed data for the temporal phase retrieval algorithm using only a single camera exposure. Thus, it delivers high accuracy and pixel-wise measurement (thanks to the temporal phase stepping algorithms) while maintaining a high sampling rate for profilometry of moving objects. In conclusion, MFPP overcomes the limitations of sequential sampling imposed by FPP with temporal phase extraction without sacrificing data quality or accuracy of the reconstructed surface profiles. Since MFPP utilizes no moving parts and is based on MEMS technology, it is applicable to miniaturization for use in mobile devices and may be useful for space-constrained applications such as robotic surgery

Single Frame Profilometry With Rapid Phase Demodulation On Colour-Coded Fringes

Digital fringe projection profilometry (DFPP) is a non-contact whole-field surface profiling technique. Being able to produce dynamic measurement with high accuracy at video framerates of up to 4000 Hz, this technique is particularly useful in the biomedical field, industrial inspections and cultural heritage preservation. One primary challenge is increasing the measurement speed to achieve higher throughput and higher detectable rates of change. In this work, the applicability of De Bruijn colour-coded sinusoidal fringe projection pattern in achieving single frame profilometry was studied along with phase errors that occur due to gamma nonlinearity and colour crosstalk. Simplification of geometric model by using inherent slanted projection angle in off-the-shelf projector was also studied in this work. Therefore, a corresponding fringe analysis algorithm was developed for De Bruijn colour-coded fringe pattern to circumvent the conventional phase unwrapping techniques which are unreliable and time-consuming. A phase error compensation algorithm and a simplified geometric model were also developed to reduce the phase errors and number of model affliated parameters respectively. Three experiments were carried out to: (i) verify the applicability of the algorithms used in the proposed single-frame profilometry system; (ii) verify the applicability of the proposed phase error compensation algorithm; and (iii) compare the result of the proposed profilometry against the Mitutoyo CRYSTA-Plus M Series 196 coordinate-measuring machine (CMM). The experimental results showed that the proposed profilometry was able to reconstruct objects successfully and consistently using only a single-frame fringe image. The phase error compensation algorithm was also proven to reduce phase errors at reference level from (1.08 ± 2.32) % to (0.73 ± 0.46) % with a 95% confidence level for 8 iterations on average, producing a visually smoother reconstructed surface. In the comparison against the CMM on the reconstruction of a fan blade curved surface, the mean profile-to-profile differences were consistently recorded as 2.63 mm on average and the confidence intervals of the differences at 95% confidence level were recorded at around ± 0.20 mm. The findings prove that the proposed concept is applicable and provides an alternative method for conventional fringe analysis techniques such as transform-based algorithms or phase-shifting algorithms to advance high-speed 3D profilometry

Real-time 3-D Reconstruction by Means of Structured Light Illumination

Structured light illumination (SLI) is the process of projecting a series of light striped patterns such that, when viewed at an angle, a digital camera can reconstruct a 3-D model of a target object\u27s surface. But by relying on a series of time multiplexed patterns, SLI is not typically associated with video applications. For this purpose of acquiring 3-D video, a common SLI technique is to drive the projector/camera pair at very high frame rates such that any object\u27s motion is small over the pattern set. But at these high frame rates, the speed at which the incoming video can be processed becomes an issue. So much so that many video-based SLI systems record camera frames to memory and then apply off-line processing. In order to overcome this processing bottleneck and produce 3-D point clouds in real-time, we present a lookup-table (LUT) based solution that in our experiments, using a 640 by 480 video stream, can generate intermediate phase data at 1063.8 frames per second and full 3-D coordinate point clouds at 228.3 frames per second. These achievements are 25 and 10 times faster than previously reported studies. At the same time, a novel dual-frequency pattern is developed which combines a high-frequency sinusoid component with a unit-frequency sinusoid component, where the high-frequency component is used to generate robust phase information and the unit-frequency component is used to reduce phase unwrapping ambiguities. Finally, we developed a gamma model for SLI, which can correct the non-linear distortion caused by the optical devices. For three-step phase measuring profilometry (PMP), analysis of the root mean squared error of the corrected phase showed a 60х reduction in phase error when the gamma calibration is performed versus 33х reduction without calibration

Three-dimensional geometry characterization using structured light fields

Tese de doutoramento. Engenharia Mecânica. Faculdade de Engenharia. Universidade do Porto. 200

IN- SITU STRUCTURED LIGHT TECHNIQUES STUDY TO INSPECT SURFACES DURING ADDITIVE MANUFACTURE

Three-dimensional (3D) shape measurement techniques play an increasingly important role in the quality control proceedures of industry, such as aerospace, bioengineering, information security, automobile, integrated circuits and so on. Additive manufacturing (AM) provide significant advantages over conventional subtractive manufacturing techniques in terms of the wide range of part geometry that can be obtained. The key metal AM technology is powder bed processing. During the AM process, powder delivery occurs thousands of times. Therefore, the assessment of delivery quality would be advantageous for the process to provide feedback for process control. After the energy source melts the powder bed, the detection of the machined surface is also a critically important criterion for the evaluation of the manufacturing quality. This thesis presents an in-situ quantitative inspection technique for the powder bed post raking and printed surface after melting, the technique uses fringe projection profilometry. In this thesis, system calibration methods, phase analysis algorithms, and error correction methods are investigated. A novel surface fitting algorithm is employed to reduce the influence of phase error and random noise during system calibration. A novel intelligent fringe projection technique using a support-vector-machine (SVM) algorithm is proposed to measure the 3D topography of high dynamic range surfaces on a layer by layer basis within the EBAM machine. A simple calibration method is used to eliminate phase errors during system calibration. The proposed in-situ inspection technique has been installed on a commercial electron beam melting (EBM) AM machine. Exemplar powder beds with defects and printed surfaces, are measured with the proposed technique. The whole inspection process lasts less than 5 seconds. Experimental results showed that the powder and the melting surface defects could be efficiently inspected using the proposed system and the measurement result could be fed back to the build process to improve the processing quality. For the inspection of highly reflective surface geometries that have been further machined post AM, phase measuring deflectometry (PMD) has been widely studied for the 3D form measurement. This thesis presents a new direct PMD (DPMD) method that measures the full-field 3D shape of complicated specular objects. A mathematical model is derived to directly relate an absolute phase map to depth data, instead of the gradient. The 3D shape of a monolithic multi-mirror array having multiple specular surfaces was measured. Experimental results show that the proposed DPMD method can obtain the full-field 3D shape of specular objects having isolated and/or discontinuous surfaces accurately and effectively. In this thesis, the fringe projection and the deflectometry techniques are studied. Two different measurement systems were used to measure different roughness surfaces. The experimental results shows the rough surfaces, reflective surfaces, and the highly reflective specular surfaces can be measured and reconstructed by the proposed methods