Optical shape measurements using temporal phase unwrapping

This doctoral thesis is devoted to optical shape measurements using an analysis scheme called temporal phase unwrapping. This scheme makes it possible to determine the absolute distance from a detector (usually a camera’s CCD-detector) to an object. Two different optical methods are considered for this 3-D sensing. The other first is a new interferometric method, which uses wavelength scanning. The other one is a classical triangulation method, which uses projected fringes. Both are whole field active measuring methods. The temporal phase unwrapping strategy requires a projection and acquisition of a sequence of fringe maps. In the present work, we formulate the reduced temporal phase unwrapping scheme and investigate the performance of it. With this scheme, we can calculate the phase (shape) from an arbitrary number of fringe frequencies. We found expressions that relate the physical quantities to phase errors. We also found expressions that relate the measurement accuracy and the unwrapping reliability, respectively, to the reduction in fringe sequence length. As expected, the measurement accuracy is unaffected by a shorter fringe sequence while a significant reduction in the unwrapping reliability is found, as compared to the complete negative exponential sequence. We consider both a single-channel and a multi-channel approach, letting different channels carry different phase-stepped image. In these simulations, the single channel approach proves to be the most robust one. The strength of reduced temporal phase unwrapping is demonstrated experimentally, in a projected fringe three-channel system. Instead of letting each channel carry phase-stepped images, each channel carries images with a change in fringe pitch. This significantly reduces noise, but requires an acquisition of at least three phase-stepped images. A spatio-temporal approach for real time absolute shape measurements is formulated. We experimentally show that absolute shape is measurable with a reduced number of fringe maps in exchange of spatial resolution. In combination with the multichannel-approach one need only to project and acquire one static image. The solution can therefore become a powerful measurement method for automation and assembly line work. For validation of forming simulations a methodology using projected fringes and reduced temporal phase unwrapping is presented. In this work, the shape of a long object is measured using projected fringes, reduced temporal phase unwrapping and calibration data. Local 3-D coordinates from several measurements are merged together through a feature based stitching program. The features in shape of the forming simulation are compared to the results from the shape measurements. It is also shown that the temporal phase unwrapping analysis scheme can be used to evaluate experimental data from wavelength scanning interferometry. Two unwrapping strategies are considered: fitting to a reversed exponential sequence and complex Fourier-transform ranging. The achievable accuracy for both methods ultimately depends on the tuning width, the speckle correlation, and random noise in the optical setup.Godkänd; 2003; 20061106 (haneit