Absolute Phase Map Recovery With Digital Multiple Wavelength Fringe Projections In Three-dimensional Measurement

Along with the deeply development of the modern science technique and the progress of industrial production level, the demand for accurate three-dimension measurement has become more and more strong. As a new generation of non-contact three-dimension measurement technique, digital fringe projection (i.e. Fringe projection profilometry, FPP) is considered to be the most promising one due to its high resolution, non-destruction, fast speed and simplicity of implementation. This technique has been widely used in industrial automatic detection, product quality monitoring, machine vision, reverse design, biomedicine, etc.Consider that a set of sinusoidal fringe patterns are projected onto the surface of an object by projector, which are reflected and captured by a camera. The deformed fringe patterns captured by camera which contain the surface shape information of the object are processed by the technique of fringe pattern analysis and phase unwrapping, then the absolute phase maps are retrieved. At last, combining with the calibration results and the absolute phase maps, the three-dimension points cloud data can be worked out. The accuracy of the three-dimension shape data is directly influenced by the result of phase unwrapping, hence, phase unwrapping is one of the key steps in FPP.This thesis gives a detailed introduction on the basic principle of FPP, the classic techniques of fringe analysis and phase unwrapping. The shortcomings of the existing techniques of phase unwrapping are also discussed, especially that too many fringe patterns are needed to project and capture. The disadvantage will lead to bad real-time capability in the measurement. The research of this thesis focuses on the improvement of anti-noise capability of phase unwrapping while employ as few images as possible.Firstly, the analysis on the existing techniques of multiple-frequency temporal phase unwrapping is made, particularly the approach to recover absolute phase maps of two fringe patterns with selected spatial frequencies which suffers from limitations of strict constraint on frequency selection and low phase error bound. A novel approach is proposed to unwrap the phase maps of two fringe patterns with flexible selection of fringe spatial wavelengths. The basic principle and implementation steps of the modified method are introduced in details. A unique mapping between the wrapped phase maps and the fringe orders is explored, and a lookup table is constructed by solving some inequalities to recover the absolute phase map rapidly. The principle of fringe spatial wavelengths selection and the phase error bound are also discussed which show that the larger the g.c.m. of the two spatial wavelengths have, the better anti-noise capability can get. Experiment results are presented to confirm the effectiveness of the proposed method. Compared to the existing techniques, the proposed is simpler in its implementation and has better anti-error capability.Secondly, for the technique to recover the absolute phase maps of fringe patterns with two selected fringe spatial wavelengths, to achieve higher anti-noise capability, the proposed method requires employing the fringe patterns with longer spatial wavelengths; however, longer spatial wavelength may lead to the degradation of the signal-to-noise ratio (SNR) in the surface measurement. Hence, we proposed a new approach to unwrap the phase maps from their wrapped versions based on the use of fringes with three different spatial wavelengths which is characterized by improved anti-error capability and SNR. Therefore, while the previous method works on the two phase maps obtained from six-step PSP (thus 12 fringe patterns are needed), the proposed technique performs very well on three phase maps from three steps PSP, requiring only 9 fringe patterns and hence more efficient. The principle of fringe spatial wavelengths selection and the phase error bound are also derived in the paper.Ideally, according to the theoretical analysis, the absolute phase recovered by the three fringe method proposed above should be characterized by monotonic variance over the areas of smooth shape; however, we noted that some incorrectly unwrapped pixels may be generated regularly due to the effect of optical heterodyne. The errors could be avoided by adding more constraints on spatial wavelengths selection, but as the relative error rate is very low, the errors could be corrected by the algorithm of data interpolation or discarded by a designed quality template while rarely effect the results of three-dimension measurement.The full demonstration on the absolute phase map recovery of multiple fringe patterns with flexible selection of fringe spatial wavelengths are given in this thesis which lay the groundwork for their application in FPP.Finally, the author concludes the whole research works and looks forword to the further research in this field.