• 전자공학회논문지S
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• 제34S권1호
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• pp.105-114
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• 1997

The phase retrieval problem is concerned with the reconstruction of a signal or its fourier transform phase form the fourier transform magnitude of the signal. This problem does not have a unique solution, in general. If, however, the desired signal is minimum-phase, then it can be decided uniquely. This paper shows that we can make a minimum-phase signal by adding a delta function having a large value at the origin of an arbitrary one-dimensional signal, and a two-dimensional signal can be uniquely specified from its fourier transform magnitude if it is added by a delta function having a large value at the origin, and finally we can solve a two-dimensional phase retrieval problem by decomposing it into several ine-dimensional phase retrieval problems.

• KSII Transactions on Internet and Information Systems (TIIS)
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• 제14권11호
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• pp.4355-4371
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• 2020

Phase retrieval, recovering a signal from phaseless measurements, is generally considered to be an NP-hard problem. This paper adopts an amplitude-based nonconvex optimization cost function to develop a new stochastic gradient algorithm, named stochastic reweighted phase retrieval (SRPR). SRPR is a stochastic gradient iteration algorithm, which runs in two stages: First, we use a truncated sample stochastic variance reduction algorithm to initialize the objective function. The second stage is the gradient refinement stage, which uses continuous updating of the amplitude-based stochastic weighted gradient algorithm to improve the initial estimate. Because of the stochastic method, each iteration of the two stages of SRPR involves only one equation. Therefore, SRPR is simple, scalable, and fast. Compared with the state-of-the-art phase retrieval algorithm, simulation results show that SRPR has a faster convergence speed and fewer magnitude-only measurements required to reconstruct the signal, under the real- or complex- cases.

• Journal of the Optical Society of Korea
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• 제14권2호
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• pp.131-136
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• 2010

Modification of the phase gradient formulation is proposed in order to make phase retrieval less susceptible to noise. The modified formulation is derived from separation of the phase terms and the intensity modulation terms of interferograms, and subsequent differentiation to reduce the noise-induced error of the phase gradient vector. Its performance is evaluated and compared to that of the conventional formulation, and noise-robust nature is confirmed.

• Journal of the Optical Society of Korea
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• 제12권3호
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• pp.186-191
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• 2008

A phase retrieval method using an error reduction algorithm is developed for reconstructing a wavefront aberration of an 100-TW Ti:sapphire laser pulse from the measurement of a focal spot. The phase retrieval method can successfully reconstruct a wavefront aberration of a 100-TW Ti:sapphire laser pulse, and the reconstructed wavefront aberration shows a good agreement with the wavefront aberration measured with a wavefront sensor. The effect of the dynamic range and the intensity noise on the reconstruction is also investigated in reconstructing a wavefront aberration of an 100-TW Ti:sapphire laser pulse.

• 한국인지과학회:학술대회논문집
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• 한국인지과학회 2010년도 춘계학술대회
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• pp.37-41
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• 2010

The previous investigations of electroencephalogram (EEG) activity in the memory retrieval tasks demonstrated that event-related potentials (ERP) during recollection showed different durations and the peak levels from those without recollection. However, it has been unknown that recollection in memory retrieval also modulates high-frequency brain rhythms as well as establishes large-scale synchronization across different cortical areas. In this study, we examined the spectral components of the EEG signals, especially the gamma bands (20-80Hz), measured during the memory retrieval tasks. Specifically, we focused on two major spectral components: first, we evaluated the temporal patterns of the power spectral density before and after the onset of the memory retrieval task; second, we estimated phase synchrony between all possible pairs of EEG channels to evaluate large-scale synchronization. Fourteen healthy subjects performed the memory retrieval task in the virtual reality environment where they selected whether or not t he present item was seen in the previous training period. When the subjects viewed the unseen items, the middle gamma power (40-60Hz) appeared to increase 200-500ms after stimulus onset while the low gamma power (20Hz) was suppressed all the way through the post-stimulus period 150ms after onset. The degree of phase synchronization in this low gamma level, however, increased when the subjects fetched the item from memory. This suggests that phase synchrony analysis might reveal different aspects of the memory retrieval process than the gamma power, providing additional information to the inference on the brain dynamics during memory retrieval.

• Current Optics and Photonics
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• 제6권5호
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• pp.473-478
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• 2022

To overcome the capture-range problem in phase-diversity phase retrieval (PDPR), a geometrical-optics initial-estimate method is proposed to avoid a local minimum and to improve the accuracy of laser-wavefront measurement. We calculate the low-order aberrations through the geometrical-optics model, which is based on the two spot images in the propagation path of the laser, and provide it as a starting guess for the PDPR algorithm. Simulations show that this improves the accuracy of wavefront recovery by 62.17% compared to other initial values, and the iteration time with our method is reduced by 28.96%. That is, this approach can solve the capture-range problem.

• 전자공학회논문지B
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• 제31B권5호
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• pp.26-33
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• 1994

Phase retrieval is concerned with the reconstruction of a signal from its Fourier transform magnitude (or intensity), which arises in many areas such as X-ray crystallography, optics, astronomy, or digital signal processing. In such areas, the Fourier transform phase of the desired signal is lost while measuring Fourier transform magnitude (F.T.M.). However, if a reference 'signal is added to the desired signal, then, in the Fourier trans form magnitude of the added signal, the Fourier transform phase of the desired signal is encoded. This paper addresses uniqueness and retrieval of the encoded Fourier phase of a multidimensional signal from the Fourier transform magnitude of the added signal along with the Fourier transform magnitude of the desired signal and the information of the additive reference signal. In Part I, several conditions under which the desired signal can be uniquely specified from the two Fourier transform magnitudes and the additive reference signal are presented. In Part II, the development of non-iterative algorithms and an iterative algorithm that may be used to reconstruct the desired signal(s) is considered.

• 전자공학회논문지B
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• 제31B권5호
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• pp.34-41
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• 1994

Phase retrieval is concerned with the reconstruction of a signal from its Fourier transform magnitude (or intensity), which arises in many areas such as X-ray crystallography, optics, astronomy, or digital signal processing In such areas, the Fourier transform phase of the desired signal is lost while measuring Fourier transform magnitude (F.T.M.). However, if a reference 'signal is added to the desired signal, then, in the Fourier trans form magnitude of the added signal, the Fourier transform phase of the desired signal is encoded This paper addresses uniqueness and retrieval of the encoded Fourier phase of a multidimensional signal from the Fourier transform magnitude of the added signal along with Fourier transform magnitude of the desired signal and the information of the additive reference signal In Part I, several conditions under which the desired signal can be uniquely specified from the two Fourier transform magnitudes and the additive reference signal are presented In Part II, the development of non-iterative algorithms and an iterative algorithm that may be used to reconstruct the desired signal (s) is considered

• 한국정보과학회논문지:소프트웨어및응용
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• 제37권4호
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• pp.264-272
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• 2010

예제기반 초해상도 영상복원은 영상 패치의 대한 학습 및 검색을 통해 저해상도 영상으로부터 고해상도 영상을 복원하는 방법으로써 성능이 좋고 한 장의 저해상도 영상에 대하여도 적용 가능하다. 그러나 복원 과정에서 패치 검색에 많은 비교 연산이 요구되기 때문에 속도가 매우 느리다. 복원 속도를 향상시키기 위해서는 효과적인 패치 검색 알고리즘이 요구된다. 본 논문에서는 패치 검색에 사용 가능한 다양한 고차원 특징 검색 방법들을 실제 초해상도 영상복원 시스템에 적용하여 그 성능을 비교하였다. 또한 문자 인식 분야에서 성공적으로 적용되어왔으나 초해상도 영상복원에서는 사용되지 않았던 단계적 후보축소 방법을 패치 검색 단계에 적용할 것을 제안한다. 실험 결과 기존의 방법 중에서는 LSH가 가장 좋은성능을 나타내었다. 본 논문에서 제안한 단계적 후보 축소에 의한 패치 검색 방법은 LSH보다 더욱 우수하여 $1024{\times}1024$ 영상의 복원 시 LSH보다 최대 3.12배 빠른 복원 속도를 나타내었다.

• 전자공학회논문지CI
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• 제40권3호
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• pp.171-180
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• 2003

내용기반 이미지검색이란 색상, 형태 및 질감 등의 저-수준 특징정보를 이용하여 이미지 데이터베이스를 구축하고, 이미지에 대한 검색요구가 발생했을 때 사용자가 찾고자 하는 이미지와 유사한 이미지를 제공하는 시스템으로 정의된다. 데이터베이스의 구축시간과 사용자가 질의를 입력한 후 결과를 얻을 때까지의 반응시간을 나누어 고려할 때, 사용자는 반응시간에 보다 관심을 갖는 것이 일반적이다. 내용기반 이미지검색 시스템에서 질의이미지와 데이터베이스 내의 이미지와의 유사도 비교시간이 전체 반응시간 중에서 가장 큰 비중을 차지한다. 본 논문에서는 이러한 유사도 비교시간을 최소화하기 위해 특징벡터의 클러스터링 기법을 적용한 2단계 탐색방법을 제안한다. 실험 결과를 통해 제안하는 2단계 탐색방법으로 대용량의 이미지 데이터베이스 내의 전체 이미지에 대한 원 특징정보와 비교하는 전체검색에 비해, 동일한 적합성을 보장하면서 평균적으로 2배 이상의 검색속도 향상을 확인하였으며, 이미지의 수가 더욱 커질수록 효과적임을 입증하였다.