3D Shape Measurement of Objects in Motion and Objects with Complex Surfaces

This thesis aims to address the issues caused by high reflective surface and object with motion in the three dimensional (3D) shape measurement based on phase shifting profilometry (PSP). Firstly, the influence of the reflectivity of the object surface on the fringe patterns is analysed. One of the essential factors related to phase precision is modulation index, which has a direct relationship with the surface reflectivity. A comparative study focusing on the modulation index of different materials is presented. The distribution of modulation index for different material samples is statistically analysed, which leads to the conclusion that the modulation index is determined by the diffuse reflectivity. Then the method based on optimized combination of multiple reflected image patterns is proposed to address the saturation issue and improve the accuracy for the reconstruction of object with high reflectivity.A set of phase shifted sinusoidal fringe patterns with different exposure time are projected to the object and then captured by camera. Then a set of masks are generated to select the data for the compositing. Maximalsignal-to-noise ratio combining model is employed to form the composite images pattern. The composite images are then used to phase mapping.Comparing to the method only using the highest intensity of pixels for compositing image, the signal noise ratio (SNR) of composite image is increased due to more efficient use of information carried by the images

A convenient look-up-table based method for the compensation of non-linear error in digital fringe projection

AbstractAlthough the structured light system that uses digital fringe projection has been widely implemented in three-dimensional surface profile measurement, the measurement system is susceptible to non-linear error. In this work, we propose a convenient look-up-table-based (LUT-based) method to compensate for the non-linear error in captured fringe patterns. Without extra calibration, this LUT-based method completely utilizes the captured fringe pattern by recording the full-field differences. Then, a phase compensation map is established to revise the measured phase. Experimental results demonstrate that this method works effectively

Three-dimensional geometry characterization using structured light fields

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

Adapted Fringe Projection Sequences for Changing Illumination Conditions on the Example of Measuring a Wrought-Hot Object Influenced by Forced Cooling

Optical 3D geometry reconstruction, or more specific, fringe projection profilometry, is a state-of-the-art technique for the measurement of the shape of objects in confined spaces or under rough environmental conditions, e.g., while inspecting a wrought-hot specimen after a forging operation. While the contact-less method enables the measurement of such an object, the results are influenced by the light deflection effect occurring due to the inhomogeneous refractive index field induced by the hot air around the measurand. However, the developed active compensation methods to fight this issue exhibits a major drawback, namely an additional cooling of the object and a subsequent transient illumination component. In this paper, we investigate the cooling and its effect on temporal phase reconstruction algorithms and take a theoretical approach to its compensation. The simulated compensation measures are transferred to a fringe projection profilometry setup and are evaluated using established and newly developed methods. The results show a significant improvement when measuring specimens under a transient illumination and are easily transferable to any kind of multi-frequency phase-shift measurement

Tri-dimensional optical inspection based on flexible image guide: first step toward 3D industrial endoscopy

International audience3D optical endoscopy is now a major challenge to allow the high resolution inspection of industrial equipments. The proposed instrument is based on a flexible image guide (70 000 fibres) and a Digital Micro mirror Device (DMD, 1024 x 768 "on-off" micro mirrors). The optical design is as follows: the light emitted by a 532 nm laser diode is dynamically structured by the DMD chip as a fringes pattern which is phase-shifted due to the active control of the DMD chip and projected onto an object on a circular field of 6 mm in diameter. Due to a telecentric and binocular arrangement that creates a stereoscopic angle, it is possible to get a depth of field of 2 mm along the optical axis without keystone distortions and few disturbances created by defocus and coma aberrations. Then, images are captured by a 1024 x 768 digital camera (not yet moved away by fibres) at 15 fps and directly used in the reconstruction algorithm to access the tri-dimensional shape of the unpainted object. The results are compared to incoherent white light results obtained with white painted mechanical objects. The lateral resolution is 31.3 µm and the RMS axial resolution is 10 µm for the laser-based design after speckle attenuation