The method of election-pass filtering and image fusion based on the wavelet transform is presented for eliminating
the fringes in the continuous wave (CW) Terahertz (THz) Images. The CW THz images have the appearance of
sinusoidal waves covering the image with standing wave patterns. The patterns result from reflected inside of the
imaging system interfering with the images coming from the sample. The pattern interferes with contrast
enhancement techniques, and can make the analysis of small areas difficult. An analysis is taken on the traditional
method dealing with this problem by Frequency Domain Filtering and notch filtering, and the disadvantages of the
traditional methods are shown. The Wavelet Transform is chosen to deal with the THz images and eliminate
interference fringes. In the method, when the size of object is larger than one interference fringe, the character of
the object can also be reserved. And the image fusion based on the Wavelet Transform is used to combine the
background and the target with pixel-level fusion, which is proposed to improve the speed of identification and the
quality of the fused image. Comparing the image restored via wavelet transform with other images, it can be found
that the scope of continuous wave THz imaging's application can be extended and clearer images can be acquired.
The method about how to distinguish the object from the standing wave, eliminate the wave patterns in CW THz images
based on phase analysis is reported. The standing wave of CW THz images result from the interference of the wave-front
reflected from the object and other optics in the system. Therefore, the information of the surface of the object and the
background is included in the interference pattern of the images. Because the objects have different Phase Gradients
from the background, they could be extracted from the image by utilizing the phase information of the interferential
waves. The waves of the image can be removed with Election-pass filter, and the result of the filtering would not
deteriorate the result because there is no object in the image. At last, the image of object and the image without standing
wave can be fused together to make the image better for eyes. The scope of CW THz imaging's application can be
extended and the quality of images can be improved by applying this approach.
Continuous THz wave (CW THz) has been widely used in imaging field. But for security screening such as inspection at
the airport, the speed of the imaging calls for an improvement since the former CW image systems which scan point to
point could not satisfy. To increase the image speed, we proposed a fast CW THz image system in which a galvanometer
is introduced for the first time. The galvanometer makes the coming beam reflected in different angles by vibrating at a
certain frequency which can significantly decrease the image acquisition time compare to point scan THz imaging. A big
hyperbolic polyethylene lens is also used in the system to collect all the beams on to the target. A Gunn oscillator and a
corresponding Schottky diode are the source and detector respectively. The image we get has ideal resolution. And after
image processing, the images looked not only clear but also realistic. The system has more practicality because it is
designed in reflection geometry instead of transmission geometry. Moreover, the source and detector in our system do
not as ponderous as gas laser which has been used in many THz imaging system previously. Example of measurements
of weapons concealed behind the cloth and box are presented and discussed. A compact high speed THz imaging system
is expectable which will have a widely application in security field.
A novel image block matching algorithm based on motion vectors of correlative pixels in oblique direction is presented
for digital image stabilization. Digital image stabilization is the new generation of image stabilization technology which
can obtains the information of relative motion between frames of dynamic image sequences through the method of
digital image processing. The image matching algorithm is one of the digital image stabilization methods. But when the
grey values of pixels in the matching image block don't have obvious change or the shaking frame has rotational motion
with the referenced frame, the matching effect of the traditional algorithm is not ideal. The novel matching algorithm
calculated in 45 degrees oblique direction is presented. The matching parameters in this method contain transverse
vectors and vertical vectors in the image blocks at the same time. More veracious matching information can be obtained
after making correlative operation in the oblique direction. The novel method can make sure that matching precision can
be less than 0.5 pixels when the excursion value in shaking frame are 40 pixels. When the image sequences have
rotational motion less than 15 degree, the right matching blocks also can be obtained in this algorithm.
An improved corner detection algorithm based on SUSAN principle is proposed. Because SUSAN operator is hard to
distinguish the corner from some special points on the digital image edges, a double template is constructed. It extracts
potential corners by SUSAN operator and then decides the accurate location of corners by a 5×5 template. Meanwhile,
an adaptive selection of gray threshold t is proposed on the basis of the local gray discreteness of pixel. The experiment
results show that the improved algorithm further raises the accuracy of corner detection and is more suitable for
application in digital image processing.
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