Author: Xu, Heng
Title: Stereoscopic fourier transform profilometry
Advisors: Lun, P. K. Daniel (EIE)
Degree: M.Sc.
Year: 2015
Subject: Imaging systems.
Three-dimensional imaging.
Optical measurements.
Hong Kong Polytechnic University -- Dissertations
Department: Faculty of Engineering
Pages: vii, 58 leaves : illustrations (some color) ; 30 cm
Language: English
Abstract: In recent years, fringe projection 3-demensional (3D) modeling is one of the most popular research areas in optical metrology. It is used to reconstruct the 3D shape of an object based on its fringe image. It is used in many biomedical applications such as 3D intra-oral dental measurements, human body shape measurement; industrial and scientific applications such as reverse engineering, computer and robot vision and so on. There are many methods to acquire the shape of 3D objects. They can be divided into two types: contact and non-contact. Each of them can be further divided into many categories. The fringe projection technique is a kind of non-contact method. It requires low cost hardware setup while the speed is very fast. With the development of image processing and image acquisition technologies, the fringe projection technique will receive more and more attention. Almost all fringe projection techniques are based on the same principle and have a similar operating procedure. Firstly, one or more fringe patterns are projected onto the target object and the resulting fringe patterns as shown on the object surface are captured by a camera located at a different angle from the projector. Due to the shape of the object, the fringes will be deformed. Mathematically, it can be expressed as the change in the phase distribution of a sinusoidal function. In this way, the height information of the object can be extracted from the phase map of the deformed fringe images. In other words, in order to recover the 3D model, our job is to calculate the phase distribution. The way to calculate the phase distribution is the major difference between fringe projection techniques. Two major methods are the Fourier transform profilometry (FTP) and the Phase shifting profilometry (PSP). The PSP method requires at least three fringe images while the FTP methods only requires one, although the PSP method achieves better result and more accurate. In this work we will focus on the FTP method since it is capable of processing moving objects.
With so many advantages, fringe projection techniques still have their shortcomings. One of them is the shadow problem, which is very common in practical experiments. In practice, the fringes projected can be blocked by the object itself and generates shadow in some areas of the fringe image. Without the fringes, the phase map in the shadow areas cannot be computed accurately. So the reconstruction result in these shadow areas is not correct. To overcome the shadow problem, we developed in this project a stereoscopic FTP system, which is an extension of the original FTP method. In that method, an extra set of FTP equipment is introduced, but projecting the fringe onto the target object from a different angle. So we can generate two models, and we correct the error part of one model (due to shadow) by the correct result in the other one. This seemingly simple idea however has many problems in practical implementation. The major problem of the stereoscopic FTP is that the size and orientation of each model are always different. Under this circumstance, extra effort needs to be made for aligning the two models before merging. They are related to the calibration of the system and the registration of the models, which are the major work in this project. Experimental results show that the stereoscopic FTP system can effectively reduce the shadow problem as compared with the traditional FTP.
Rights: All rights reserved
Access: restricted access

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