3D optical metrology by digital moiré: Pixel-wise calibration refinement, grid removal, and temporal phase unwrapping

Fast, accurate three dimensional (3D) optical metrology has diverse applications in object and environment modelling. Structured-lighting techniques allow non-contacting 3D surface-shape measurement by projecting patterns of light onto an object surface, capturing images of the deformed patterns, and computing the 3D surface geometry from the captured 2D images. However, motion artifacts can still be a problem with high-speed surface-motion especially with increasing demand for higher measurement resolution and accuracy. To avoid motion artifacts, fast 2D image acquisition of projected patterns is required. Fast multi-pattern projection and minimization of the number of projected patterns are two approaches for dynamic object measurement. To achieve a higher rate of switching frames, fast multi-pattern projection techniques require costly projector hardware modification or new designs of projection systems to increase the projection rate beyond the capabilities of off-the-shelf projectors. Even if these disadvantages were acceptable (higher cost, complex hardware), and even if the rate of acquisition achievable with current systems were fast enough to avoid errors, minimization of the number of captured frames required will still contribute to reduce further the effect of object motion on measurement accuracy and to enable capture of higher object dynamics. Development of an optical 3D metrology method that minimizes the number of projected patterns while maintaining accurate 3D surface-shape measurement of objects with continuous and discontinuous surface geometry has remained a challenge. Capture of a single image-frame instead of multiple frames would be advantageous for measuring moving or deforming objects. Since accurate measurement generally requires multiple phase-shifted images, imbedding multiple patterns into a single projected composite pattern is one approach to achieve accurate single-frame 3D surface-shape measurement. The main limitations of existing single-frame methods based on composite patterns are poor resolution, small range of gray-level intensity due to collection of multiple patterns in one image, and degradation of the extracted patterns because of modulation and demodulation processes on the captured composite pattern image. To benefit from the advantages of multi-pattern projection of phase-shifted fringes and single-frame techniques, without combining phase-shifted patterns into one frame, digital moiré was used. Moiré patterns are generated by projecting a grid pattern onto the object, capturing a single frame, and in a post-process, superimposing a synthetic grid of the same frequency as in the captured image. Phase-shifting is carried out as a post-process by digitally shifting the synthetic grid across the captured image. The useful moiré patterns, which contain object shape information, are contaminated with a high-frequency grid lines that must be removed. After performing grid removal, computation of a phase map, and phase-to-height mapping, 3D object shape can be computed. The advantage of digital moiré provides an opportunity to decrease the number of projected patterns. However, in previous attempts to apply digital phase-shifting moiré to perform 3D surface-shape measurement, there have been significant limitations. To address the limitation of previous system-calibration techniques based on direct measurement of optical-setup parameters, a moiré-wavelength based phase-to-height mapping system-calibration method was developed. The moiré-wavelength refinement performs pixel-wise computation of the moiré wavelength based on the measured height (depth). In measurement of a flat plate at different depths, the range of root-mean-square (RMS) error was reduced from 0.334 to 0.828 mm using a single global wavelength across all pixels, to 0.204 to 0.261 mm using the new pixel-wise moiré-wavelength refinement. To address the limitations of previous grid removal techniques (precise mechanical grid translation, multiple-frame capture, moiré-pattern blurring, and measurement artifacts), a new grid removal technique was developed for single-frame digital moiré using combined stationary wavelet and Fourier transforms (SWT-FFT). This approach removes high frequency grid both straight and curved lines, without moiré-pattern artifacts, blurring, and degradation, and was an improvement compared to previous techniques. To address the limitations of the high number of projected patterns and captured images of temporal phase unwrapping (TPU) in fringe projection, and the low signal-to-noise ratio of the extended phase map of TPU in digital moiré, improved methods using two-image and three-image TPU in digital phase-shifting moiré were developed. For measurement of a pair of hemispherical objects with true radii 50.80 mm by two-image TPU digital moiré, least-squares fitted spheres to the measured 3D point clouds had errors of 0.03 mm and 0.06 mm, respectively (sphere fitting standard deviations 0.15 mm and 0.14 mm), and the centre-to-centre distance measurement between hemispheres had an error of 0.19 mm. The number of captured images required by this new method is one third that for three-wavelength heterodyne temporal phase unwrapping by fringe projection techniques, which would be advantageous in measuring dynamic objects, either moving or deforming

Review of post-process optical form metrology for industrial-grade metal additive manufactured parts

The scope of this review is to investigate the main post-process optical form measurement technologies available in industry today and to determine whether they are applicable to industrial-grade metal additive manufactured parts. An in-depth review of the operation of optical three-dimensional form measurement technologies applicable to metal additive manufacturing is presented, with a focus on their fundamental limitations. Looking into the future, some alternative candidate measurement technologies potentially applicable to metal additive manufacturing will be discussed, which either provide higher accuracy than currently available techniques but lack measurement volume, or inversely, which operate in the appropriate measurement volume but are not currently accurate enough to be used for industrial measurement

Three-dimensional geometry characterization using structured light fields

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

Optical thin film measurement by interferometric fringe projection and fluorescence stimulated emission

The introduction of a new technique for metrology of thin liquid films to give both the profile of the exterior surface and information on the thickness of the film is the main focus of this research. The proposed approach is based on the use of fringe projection system with a narrow band laser illumination and a high concentration of fluorescent dye dissolved in the fluid in order to generate fluorescence emission from minimum thickness of the film (i.e. the top few microns). The method relies on calculation of an interference phase term and the modulation depth of the fringes created by means of a twin fibre configuration. The characterisation of candidate fluorescent dyes in terms of absorption, related to the depth of penetration of the incident light into the dye and their fluorescence emission efficiency is presented and their application in full field imaging experiments is evaluated. A strong focus of the technique proposed is its flexibility and versatility allowing its extension to phase stepping techniques applied to determine the (fringe) phase map from static and dynamic fluids. Some experiments are carried out using the best dye solution in terms of fluorescence emission and light depth penetration. On the basis of the phase-height relationship achieved during the calibration process, the proposed measurement system is applied for the shape measurement of some static fluids. The profile of the exterior surface of these fluids is investigated by means of phasestepping technique and the resolution of the measurements is estimated. Furthermore a flow rig set-up based on inclined system (gravity assisted) is presented in order to test the shape measurement system in presence of real liquid flows. Different liquid flow thicknesses are processed and analysed. Example data will be included from some fluid films of known geometry in order to validate the method

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

Development of temporal phase analysis techniques in optical measurement

Ph.DDOCTOR OF PHILOSOPH

Coherent lensless imaging techniques using terahertz radiation

Terahertz (THz) radiation denotes the portion of the electromagnetic spectrum lying between the infrared and microwave bands, corresponding to frequencies in the range 0.1-10 THz. The most intriguing feature of using THz waves is their ability to penetrate several non-conducting and optically opaque materials, such as plastics, textiles, paper, and some building materials as well as intrinsic semiconductors. While this property is also shared by microwaves, THz radiation provides a better spatial resolution thanks to the shorter wavelength, thereby imaging hidden objects with sub-millimeter resolution. The non-ionizing nature of THz radiation when it interacts with living tissues also makes THz imaging techniques promising for biomedical and biological applications. In this thesis, I focus on the development and implementation of THz imaging techniques. All the techniques presented here belong to the realm of coherent lensless imaging, aiming at reconstructing the amplitude and phase of the wavefront diffracted by an unknown object, illuminated with coherent radiation, based on measurements of the intensity of their diffraction pattern recorded with a camera. The fact that the imaging process is carried out fully computationally and without the need of lenses has a crucial impact on the experimental setup, which is therefore compact and can be better tailored to real-life applications. In particular, I am going to discuss both theoretical and experimental aspects of synthetic aperture THz off-axis digital holography, the first experimental demonstration of THz ptychography and how to image objects hidden behind weakly and strongly diffracting barriers. A potential biomedical application for such THz imaging techniques will also be suggested

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