• Proceedings of the KIEE Conference
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• 2007.07a
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• pp.341-342
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• 2007

3T MRI system에서 고해상도 영상을 얻기 위해서는 magnetic field를 균일하게 만들어야 한다. 특히 초고속 영상 기법인 Spiral scan 방식과 EPI scan 방식에서는 이미지 영역에서의 magnetic field의 inhomogeneity에 의해 영상의 왜곡이 심해진다. 본 논문에서는 magnetic field의 inhomogeneity를 단시간 내에 측정하기 위해 fast spin echo방식의 pulse sequence 제안하고, magnetic field를 분석하기 위한 field map의 구성과, field pattern의 효과적인 분석을 통하여, magnetic field를 균일하게 만드는 방법을 제안한다.

• Investigative Magnetic Resonance Imaging
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• v.11 no.2
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• pp.95-102
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• 2007

Purpose: To acquire high-resolution spiral-scan images at higher magnetic field, high homogeneous magnetic field is needed. Field inhomogeneity mapping and in-vivo shimming are important for rapid imaging such as spiral-scan imaging. The rapid scanning sequences are very susceptible to inhomogeneity. In this paper, we proposed a higher-order shimming method to obtain homogeneous magnetic field. Materials and Methods: To reduce measurement time for field inhomogeneity mapping, simultaneous axial/ sagittal, and coronal acquisitions are done using multi-slice based Fast Spin echo sequence. Acquired field inhomogeneity map is analyzed using the spherical harmonic functions, and shim currents are obtained by the multiplication of the pseudo-inverse of the field pattern with the inhomogeneity map. Results: Since the field inhomogeneity is increasing in proportion to the magnetic field, higher order shimming to reduce the inhomogeneity becomes more important in high field imaging. The shimming technique in which axial, sagittal, and coronal section inhomogeneity maps are obtained in one scan is developed, and the shimming method based on the analysis of spherical harmonics of the imhomogenity map is applied. The proposed technique is applicable to a localized shimming as well. High resolution spiral-scan imaging was successfully obtained with the proposed higher order shimming. Conclusion: Proposed pulse sequence for rapid measurement of inhomogeneity map and higher order shimming based on the inhomogeneity map work very well at 3 Tesla MRI system. With the proposed higher order shimming and localized higher order shimming techniques, high resolution spiral-scan images are successfully obtained at 3 T MRI system.

• KIEE International Transactions on Electrophysics and Applications
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• v.3C no.6
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• pp.246-251
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• 2003

We report the theory of thin-sample Z -scan for materials, viz. diffusion-dominated photorefractives, having a nonlinearly induced phase that may be proportional to the spatial derivative of the intensity profile. The on-axis far-field intensity is approximately an even function of the scan distance on different positive and negative values for phase shift $\Delta$$\Phi$$_{o}$. In case of positive phase shift, the Z -scan graph shows a minimum and two maxima, while for the negative value, only one minimum is observed. The fact is that far-field beam profiles display beam distortion and shift of the peak as compared with Kerr-type or photovoltaic nonlinearities.s.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.19 no.11
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• pp.1-7
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• 2018

In a microcolumn, a miniaturized electrostatic deflector is often adopted to scan an electron beam. Usually, a double octupole deflector is used because it can avoid excessive spherical aberrations by controlling the electron beam path close to the optical axis of the objective lens and has a wide scan field. Studies on microcolumns have been performed to improve the low throughput of an electron column through multiple column applications. On the other hand, as the number of microcolumns increases, the number of wires connected to the components of the microcolumn increases. This will result in practical problems during the process of connecting the wires to electronic controllers outside of the vacuum chamber. To reduce this problem, modified quadrupole and octupole deflectors were examined through simulation analysis by selecting an ultraminiaturized microcolumn with the Einzel lens eliminated. The modified deflectors were designed changing the size of each electrode of the conventional Si octupole deflector. The variations of the scan field and electric field strength were studied by changing the size of active electrodes to which the deflection voltage was to be applied. The scan field increased linearly with increasing deflection voltage. The scan field of the quadrupole deflector and the electric field strength at the center were calculated to be approximately 1.3 ~ 2.0 times larger than those of the octupole deflector depending on the electrode size.

• Proceedings of the KSRS Conference
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• 2007.10a
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• pp.195-198
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• 2007

GOCI(Geostationary Ocean Color Imager) is the core paryload of the geostationary satellite COMS(Communication, Ocean and Meteological Satellite) for ocean monitoring. It is scheduled to be launched at the end of 2008. GOCI observes ocean color around the Korean Peninsula over $2500km{\times}2500km$ area. Whole field of view is divided into 16 solts and scan mechanism enables to point each slot position. Tilted two-axis scan method is used to observe entire field of view with great pointing stability. Vignetting of the optical system appears when the partial obscuration by intermediate optical components occurs. It leads to the variation of the illumination in the image and gradual fading near the edge of the field. It should be prohibited for the stable radiometric performances. In this work, vignetting analysis of GOCI optical system is performed. For the systematic approach, GOCI optical system is divided into scan mechanism part and telescope part. Vignetting analysis of each part is performed and each result is combined for the overall vignetting performances. The analyzed results can be applied to the selection of slot acquisition angle of scan mechanism to minimize vignetting effects.

• Proceedings of the Optical Society of Korea Conference
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• 2006.02a
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• pp.233-234
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• 2006

• Journal of the Korea Academia-Industrial cooperation Society
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• v.10 no.10
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• pp.2628-2633
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• 2009

In a double deflector employed microcolumn, the variation of FOV and scan field width was investigated in mode of conversely biased double deflector to eliminate barrel distortion caused by aberration. The relationship between biased voltage of each deflector and electron emission tip voltage was studied for the maximum FOV and scan field width. The limitation and the linearity of zooming current image are also estimated as a function of electron emission tip voltage.

• 제어로봇시스템학회:학술대회논문집
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• 2005.06a
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• pp.1830-1835
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• 2005

It is important to estimate the distribution of intensity of a magnetic field for application of magnetic method to industrial nondestructive evaluation. Magnetic camera provides the distribution of a quantitative magnetic field with homogeneous lift-off and same spatial resolution. And it is possible to interpret the distribution of the magnetic field when the dipole model is introduced. This study introduces the numerical and experimental considering of the quantitative evaluation of several size and shapes of the cracks using the magnetic field images of the scan type magnetic camera.

• Korean Journal of Optics and Photonics
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• v.9 no.5
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• pp.342-347
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• 1998

We present a theoretical analysis of Gaussian beam propagation in nonlinear Kerr media by using aberration-free approximation and Huygens-Fresnel diffraction integral and obtain a simple analytic formular for Z-scan characteristics. Z-scan experiments are carried out on amorphous $As_2S_3$ thin film and compared with the theory developed, showing good agreement. The sign and the value of ${\gamma}$ have been measured at 633 nm to be $+8.65{\times}10^{-6}\textrm{cm}^2/W$. We also measured the far-field intensity profiles, which confirm again self-focusing effect.

• Journal of KIBIM
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• v.6 no.4
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• pp.18-26
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• 2016

Recently, for the purpose of maintenance of facilities and energy, there have been growing cases of the 3D image scan-based reverse design technology mostly in the manufacturing field. In the MEP field, because of differences between design and physical model, the reverse technology has been utilized in factory facilities such as a semiconductor factory. Because 3D point clouds from scanning include accurate 3D object information, the efficiency of management works related to the complex MEP facilities can be enhanced. In this study, the reverse technology was surveyed, and the MEP facility management based on 3D image scanning was analyzed. Based on the results, a method of 3D image scan-based MEP facility management was proposed.