• Journal of Biomedical Engineering Research
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• v.26 no.1
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• pp.31-35
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• 2005

A novel synthetic aperture method for real-time three-way dynamic focusing is proposed, which provides lateral beam patterns represented as the product of Fourier transforms of transmit subaperture, receive subaperture, and a synthetic window function. In the proposed method, all array elements are fired individually and for each firing echo signals are recorded from all elements of a receive subaperture moving along an array with the transmit element. The three-way dynamic focusing is then achieved by employing a synthetic aperture algorithm for two-way dynamic focusing and a synthetic focusing method for transmit dynamic focusing. Both theoretical analysis and computer simulation results show that the proposed method produces ultrasound beams with improved lateral resolution at all depths compared to the conventional phased array imaging and synthetic aperture focusing methods.

• Korean Journal of Optics and Photonics
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• v.18 no.6
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• pp.375-382
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• 2007

The Shack-Hartmann wavefront sensor is composed of a lenslet array generating the spot images from which local slope is calculated and overall wavefront is measured. Generally the principle of wavefront reconstruction is that the spot centroid of each lenslet array is calculated from pixel intensity values in its subaperture, and then overall wavefront is reconstructed by the local slope of the wavefront obtained by deviations from reference positions. Hence the spot image of each lenslet array has to remain in its subaperture for exact measurement of the wavefront. However the spot of each lenslet array deviates from its subaperture area when a wavefront with large local slopes enters the Shack-Hartmann sensor. In this research, we propose a spot image searching method that finds the area of each measured spot image flexibly and determines the centroid of each spot in its area Also the algorithms that match these centroids to their reference points unequivocally, even if some of them are situated off the allocated subaperture, are proposed. Finally we verify the proposed algorithm with the test of a defocus measurement through experimental setup for the Shack-Hartmann wavefront sensor. It has been shown that the proposed algorithm can expand the dynamic range without additional devices.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.19 no.5
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• pp.796-804
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• 1994

This paper presents Inverse SAR imaging algorithm for a unknown velocity target and a real ISAR data is processed and applied to the algorithm. The real ISAR data is obtained by transmitting a number of pulse modulated by a stepped-frequency method and the received data are undersampled. We present a method applicable for the case of a undersampled data base. In this method, the original echoed signal is mixed with a reference signal to make it unaliased, followed by being interpolated. Target`s velocity required for the algorithm is estimated via subaperture processing and after the coordinate transformation into squint-mode SAR with the estimated velocity, a recently proposed SAR/ISAR imaging algorithm derived without any approximation is utilized to produce the output image. We also propose an ISAR image scheme that is usable when a target changes its velocity during ISAR data acquisition time.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.28 no.2
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• pp.139-146
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• 2017

A ground moving target's velocity estimation algorithm applicable for a SAR-GMTI system using 2 channel displaced phase center antenna(DPCA) is proposed. In this algorithm, we assume target's across-track velocity can be estimated by along-track interferometry (ATI) and present a method to estimate target's along-track velocity. To accomplish this method, we first transform a radar-target geometry in which a moving target has zero velocity via altering a radar velocity such that target's velocity is reflected into it and next manipulate the spectral centers of the subapertures within the synthetic aperture. The validity of the proposed algorithm is demonstrated through simulation results showing the performance of the target's velocity estimation and the enhancement of reconstructed target image quality in terms of resolution and SINR.

• The Optical Journal
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• s.143
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• pp.46-53
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• 2013

비구면 광학요소는 광학계에 중요한 이점을 제공하는 반면 정밀한 측정에 제한이 많기에 정밀한 비구면의 생산이 어려운 광학 요소이다. 수년 전 부분 구경 스티칭 방법(Subaperture Stitching Interferometry, 이하 SSI$^{(R)}$)의 개발과 이 기술을 기반으로 하는 장비(SSI-A$^{(R)}$)의 도입으로 비구면 형상오차의 정밀한 측정이 가능하게 되었다. 많은 비구면들이 기존의 SSI-A의 영역에 포함되지만 이에 더 나아가 VONTM(Variable Optical Null)을 도입하면 기존의 측정 영역인 비구면도(best fit sphere에서) $100{\sim}200{\lambda}$대에서 약 $1000{\lambda}$까지 확장하여 측정할 수 있다. 본고에서는 지난 2012년 10월 23일 진행된 QED의 기술강연회에서 발표된 내용 중 VONTM의 원리와 이를 이용한 SSI$^{(R)}$ 기술이 도입된 장치인 ASI$^{(R)}$(Aspheric Stitching Interferometer)의 측정결과를 그 동안 각종 학회 등에서 발표된 자료들을 통하여 소개하도록 하겠다.

• Proceedings of the KOSOMBE Conference
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• v.1998 no.11
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• pp.219-220
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• 1998

Although a single-element synthetic aperture system can produce high-resolution beam profile, it is not a highly practical system because of its low signal to noise ratio against conventional system's. A multi-element synthetic aperture processing has been proposed with defocusing in this paper. A multi element subaperture defocused to emulate a single element spatial response with high acoustic power. The results have higher signal to noise and better contrast resolution than conventional synthetic aperture method.

• Journal of Biomedical Engineering Research
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• v.22 no.4
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• pp.321-330
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• 2001

In this paper, we propose a new synthetic aperture focusing scheme for improving the lateral resolution which is one of the most important factors determining the quality of ultrasound imaging. The proposed scheme enables full round-trip dynamic focusing with approximately limited property. This properties are obtained through transmitting plane waves of which the traveling angle varies with the receive subaperture position, as opposed to stepping the spherical wave source across an array in other synthetic aperture focusing schemes, and employing dynamic focusing in receive. In this paper, the properties of the proposed scheme is analyzed in which a hypothetical infinite line source is used to transmit the plane waves and verified through computer simulation results. Also, we show that the proposed scheme is realizable with an array transducer with a finite aperture size. In summary, it is shown through comparison between the field contours of the proposed scheme and the conventional scheme that the proposed scheme can improve greatly the lateral resolution of ultrasound imaging.

• Journal of the Korean Society of Manufacturing Process Engineers
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• v.10 no.2
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• pp.104-109
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• 2011

I present a method of aspheric surface profile measurement using 2nd derivative of local area profile. This method is based on the principle of curvature sensor which measures the local 2nd derivative under test along a line. The profile is then reconstructed from the data on the each point. Unlike subaperture-stiching method and slope detection method, 2nd derivative method has strong points from a geometric point of view in measuring the aspheric surface profile. The second derivative terms of surface profile is an intrinsic property of the test piece, which is independent of its position and tip-tilt motion. The curvature is measured at every local area with high accuracy and high lateral resolution by using White-light scanning interferometry.

• Journal of Biomedical Engineering Research
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• v.27 no.1
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• pp.22-29
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• 2006

This paper proposes an efficient array beamforming method using spatial matched filtering for ultrasound imaging. In the proposed method, ultrasound waves are transmitted from an array subaperture with fixed transmit focus as in conventional array imaging. At receive, radio frequency (RF) echo signals from each receive channel are passed through a spatial matched filter that is constructed based on the system transmit-receive spatial impulse response. The filtered echo signals are then summed. The filter remaps and spatially registers the acoustic energy from each element so that the pulse-echo impulse response of the summed output is focused with acceptably low side lobes. Analytical beam pattern analysis and simulation results using a linear array show that the proposed spatial filtering method can provide more improved spatial resolution and contrast-to-noise ratio (CNR) compared with conventional dynamic receive focusing (DRF) method by implementing two-way dynamically focused beam pattern throughout the field.

• Journal of Biomedical Engineering Research
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• v.23 no.5
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• pp.351-364
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• 2002

Conventional 3D ultrasound imaging using mechanical ID arrays suffers from poor elevation resolution due to the limited depth-of-focus (DOF). On the other hand, 3D imaging systems using 2D phased arrays have a large number of active channels and hence require a very expensive and bulky beamforming hardware. To overcome these limitations, a new real-time volumetric imaging method using curved 2-D arrays is presented, in which a small subaperture, consisting of 256 elements, moves across the array surface to scan a volume of interest. For this purpose, a 2-D curved array is designed which consists of 90$\times$46 elements with 1.5λ inter-element spacing and has the same view angles along both the lateral and elevation directions as those of a commercial mechanical 1-D array. In the proposed method, transmit and receive subapertures are constructed by cutting the four corners of a rectangular aperture to obtain a required image qualify with a small number of active channels. In addition the receive subaperture size is increased by using a sparse array scheme that uses every other elements in both directions. To suppress the grating lobes elevated due to the increase in clement spacing, fold-over array scheme is adopted in transmit, which doubles the effective size of a transmit aperture in each direction. Computer simulation results show that the proposed method can provide almost the same and greatly improved resolutions in the lateral and elevation directions, respectively compared with the conventional 3D imaging with a mechanical 1-D array.