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|Title: ||Novel algorithms for phase extraction from one single shearographic fringe pattern and the recovery of obscured data|
|Other Titles: ||Dan fu ji guang jian qie san ban gan she tiao wen tu ti qu xiang wei suan fa de yan jiu ji que sun shu ju de xiu fu|
|Authors: ||Huang, Yuanhao (黃源浩)|
|Department: ||Department of Manufacturing Engineering and Engineering Management|
|Degree: ||Doctor of Philosophy|
|Issue Date: ||2009|
|Publisher: ||City University of Hong Kong|
|Subjects: ||Computer algorithms.|
Data recovery (Computer science)
|Notes: ||CityU Call Number: QA76.9.A43 H83 2009|
xvi, 142 leaves : ill. 30 cm.
Thesis (Ph.D.)--City University of Hong Kong, 2009.
Includes bibliographical references (leaves 122-140)
|Abstract: ||The invention of laser in 1960s creates a new realm for physicists and engineers.
This new light source provides an ideal medium for coherent optics and incubates a
series of new experimental techniques such as holographic interferometry and
shearography. Since their invention, holographic interferometry and shearography
have provided powerful experimental tools for the development of theoretical
mechanics and applications in nondestructive testing.
In its early stage, shearography mainly provides qualitative measurement since
shearographic fringe order can only be determined in integer level. With the rapid
progress of computer technology and the advent of phase shifting technique in the
1980s, shearography gradually became a powerful tool for quantitative measurement
with wide applications in both laboratory and industry. Accurate and robust as it is,
the phase shifting technique requires capturing three or more fringe patterns at each
deformed stage for phase determination, thus it is not suitable for dynamic
measurement such as vibration, impact and fast deformation. To overcome this
shortcoming, techniques based on Fourier transform or Wavelet transform which
involve only one image are investigated and developed. However, such techniques
always require generating a high frequency carrier fringe, which pose much
difficulty in experiment design. Thus there is a great desire for a technique to extract
shearographic phase information from one single fringe pattern.
In this thesis, a novel phase evaluation approach based on the idea of phase
clustering has been developed to extract phase information from one single fringe
pattern. Before the object begins to deform, four speckle images are captured and
stored. After the object starts a non-repeatable deformation, only one single speckle
image is captured for each deformed stage. With these five specklegrams, the
background information, fringe modulation and initial random phase can be
determined. Since the deformation induced phase change is a smooth phase map
which owns the property of phase clustering, it is successfully extracted from one
single deformed specklegram using the special clustering approach developed in this
A key step in applying the clustering approach for phase extraction is to
guarantee the quality of the four speckle images captured before the deformation.
These four images should be accurately phase shifted to ensure that the background
information and fringe modulation are accurately obtained. Thus a proper calibration
procedure of the phase shifter is proposed in this thesis, which helps to extract the
deformation phase correctly.
Simulated results as well as a series of experiments conducted on continuously
deforming specimens show the ability and robustness of the proposed clustering
phase retrieval method. Comparison between the proposed method and standard
four-step phase shifting method further confirms the accuracy of the proposed
The clustering phase retrieval method, however, may not work as well as phase shifting under some extreme cases such as very high fringe density or very noisy
environment. This is because that the clustering approach utilizes only information
from one deformed specklegram, while the phase shifting method has abundant
information available from four deformed images. The result is that the clustering
approach can work out a proper phase map for most of applications, but with some
minor errors such as fringe interconnection and phase residues in some small areas
for the extremely terrible situations. The fringe interconnection or phase residues
may also happen when applying phase shifting method in very hostile environment.
These erroneous data, which is called obscured data, will affect the phase
unwrapping process and make it difficult to obtain the final unwrapped phase map
for quantitative measurement.
To recover the obscured data, two effective methods have been proposed and
investigated in this thesis. The first one is to correct the erroneous areas using a
phase filtering method called phase reclustering filtering which is a nonlinear filter
specially designed for wrapped phase filtering and performs better than normal
filters. By use of the reclustering approach, most of the interconnected fringes are
separated and phase residues are cleaned. Thus the wrapped phase map becomes
workable with normal phase unwrapping algorithm. The other approach tends to cut
out areas with erroneous wrapped phase information, and then unwrap the remaining
correct wrapped phase. After that, the unavailable unwrapped phase in erroneous
areas is reconstructed from the surrounding data by a computerized tomographic
method developed in this thesis.
Simulated examples as well as a series of experiments conducted under extreme
situations are used to verify the proposed phase reclustering filtering technique and
the computerized tomographic phase reconstruction method. The results show that
both methods are helpful. Compared with current filtering methods and
reconstruction method, both proposed techniques have better performance.
The three methods developed in this thesis have together provided a complete
solution for quantitative measurement of continuously deforming object. With
further refinement and development, they may open a door for interferometric
techniques to be widely applied for ultra high-speed measurement in both laboratory
and industrial environment.|
|Online Catalog Link: ||http://lib.cityu.edu.hk/record=b2340688|
|Appears in Collections:||MEEM - Doctor of Philosophy |
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