• Title, Summary, Keyword: Autofocus

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Performance Improvement of SAR Autofocus Based on Partition Processing (분할처리 기반 SAR 자동초점 기법의 성능 개선)

  • Shin, Hee-Sub;Ok, Jae-Woo;Kim, Jin-Woo;Lee, Jae-Min
    • The Journal of Korean Institute of Electromagnetic Engineering and Science
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    • v.28 no.7
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    • pp.580-583
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    • 2017
  • To compensate the degraded SAR image due to the residual errors and the spatial variant errors remaining after the motion compensation in the airborne SAR, we have introduced the autofocus method based on the partition processing. Thus, after we perform the spatial partition for the spotlight SAR data and the time partition for the stripmap SAR data, we reconstruct the subpatch images for the partitioned data. Then, we perform the local autofocus with the suitability analysis process for the phase errors estimated by the autofocus. Moreover, if the estimated phase errors are not properly compensated for the subpatch images, we perform the phase compensation method with the weight to the estimated phase error close to the degraded subpatch image to increase the SAR image quality.

Numerical Calculation for Autofocus of Zoom Lenses by Using Gaussian Brackets (가우스 괄호법을 이용한 줌 렌즈의 조출량에 대한 수치해석 계산법)

  • Jo, Jae-Heung;Lee, Do-Kyung;Lee, Sang-On;Ryu, Jae-Myung;Kang, Geon-Mo;Lee, Hae-Jin
    • Korean Journal of Optics and Photonics
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    • v.20 no.3
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    • pp.166-174
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    • 2009
  • When the object distance of a zoom lens with finite object distances is varied, we can fix the image at a fixed image plane by moving only one zoom lens group (autofocus group) without moving all zoom lens groups for the autofocus. We theoretically formulated and numerically calculated the moving distances of the autofocus group by using Gaussian brackets and a paraxial ray tracing method. The solutions of this method can be consistently and flexibly used in the initial design for the moving distance of autofocus group within these zoom loci in all types of zoom lens. Finally, in order to verify the usefulness of this method, we show that the moving distance of an autofocus group can be rapidly and diversely obtained in one example of $M_{5n}$ zoom lens type.

Minimum-Entropy-Based Autofocus Method for Real SAR Images (실제 SAR 영상에서의 최소 엔트로피 기반의 자동 초점 기법 연구)

  • Hwang, Jeonghun;Shin, Hyun-Ik;Kim, Whan-Woo
    • The Journal of Korean Institute of Electromagnetic Engineering and Science
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    • v.29 no.5
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    • pp.366-374
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    • 2018
  • In cases of airborne equipped with SAR, because the occurrence of motion is inevitable, it is necessary to apply autofocus techniques to SAR images to improve the image performance degradations caused by residual errors. Herein, a robust autofocus algorithm based on the minimum entropy criteria is proposed for the real SAR data in the spotlight mode. The convergence condition of the phase error estimation is checked at every iteration and if it is violated, the size of the phase error estimation is adjusted to the convergence condition. The real SAR raw data is used to demonstrate the excellent performance of the proposed algorithm.

Modified WLS Autofocus Algorithm for a Spotlight Mode SAR Image Formation (스포트라이트 모드 SAR 영상 형성에서의 수정된 가중치 최소 자승기법에 의한 자동 초점 알고리즘)

  • Hwang, Jeonghun;Shin, Hyun-Ik;Kim, Whan-Woo
    • The Journal of Korean Institute of Electromagnetic Engineering and Science
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    • v.28 no.11
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    • pp.894-901
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    • 2017
  • In the existence of motion, azimuth phase error due to accuracy limitation of GPS/IMU and system delay is unavoidable and it is essential to apply autofocus to estimate and compensate the azimuth phase error. In this paper, autofocus algorithm using MWLS(Modified WLS) is proposed. It shows the robust performance compared with original WLS using new target selection/sorting metric and iterative azimuth phase estimation technique. SAR raw data obtained in a captive flight test is used to validate the performance of the proposed algorithm.

PGA Implementation Technique for Stripmap SAR Signal Processing (Stripmap SAR 신호처리를 위한 PGA 적용 기법)

  • Yoon, Sang-Ho;Koh, Bo-Yeon;Kong, Young-Kyun;Shin, Hee-Sub
    • Korean Journal of Remote Sensing
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    • v.27 no.2
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    • pp.151-161
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    • 2011
  • PGA(Phase Gradient Autofocus) is a representative autofocus technique to improve the SAR(Synthetic Aperture Radar) image quality. PGA can estimate high order phase errors and have good robustness in noisy environments. However, PGA is not suitable to apply to the stripmap mode data directly because it is based on the spotlight mode operation. In this paper, the PGA implementation technique for stripmap mode data and the method of ROI(Region of Interest) selection that affects severely on PGA performance have been proposed. The proposed technique was verified by the point target simulation first, and was applied to the real SAR signal data acquired by the flight test. Finally, the significant improvements in focusing quality were shown in the processed SAR images using the proposed method.

A Study on Autofocus Method for Back-Projection Algorithm under the Squint Mode in Synthetic Aperture Radar (스퀸트 모드 SAR 영상 형성을 위한 역투영 알고리즘에서의 자동초점 기법 적용 연구)

  • Hwang, Jeonghun;Kim, Whan-Woo
    • Journal of the Institute of Electronics and Information Engineers
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    • v.54 no.7
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    • pp.81-89
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    • 2017
  • Autofocus(AF) Method is essential to overcome the performance degradation due to motion measurement errors under airborne SAR environment. In this paper, back-projection algorithm(BPA) is applied to SAR raw data acquired under the squinted mode, and preprocessing algorithm of AF for BPA is investigated. To apply AF to SAR image effectively, image backplane rotation method and doppler location alignment function for BPA are proposed. The proposed method is applied to SAR raw data acquired in a flight test and shows excellent performance improvement in real data.

DCT-Based Energy-Ratio Measure for Autofocus in Digital Camera (이산 코사인 변환 계수의 에너지 비를 사용한 디지털 카메라용 초점 간 연산자)

  • Lee, Sang-Yong
    • Journal of the Institute of Electronics Engineers of Korea SP
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    • v.45 no.6
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    • pp.88-94
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    • 2008
  • A DCT-based energy-ratio measure for autofocus in digital camera is proposed in this paper. This measure, namely AC2DC1 and AC5DC1, determines the sharpness of an image using a ratio between AC and DC energy in the DCT domain. This method is derived from energy analysis of DCT coefficients. Autofocus score calculation method is used to assess the performance of the proposed measure and to compare it with other measures. Experimental results under various conditions verify the robustness of the proposed focus measure for the Gaussian as well as impulsive noises.

Analysis of Performance for Entropy-Based ISAR Autofocus Technique (엔트로피 기반의 ISAR 자동 초점 기법에 대한 성능 분석)

  • Bae, Jun-Woo;Kim, Kyung-Tae;Lee, Jin-Ho;Im, Jeong-Heom
    • The Journal of Korean Institute of Electromagnetic Engineering and Science
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    • v.17 no.12
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    • pp.1249-1258
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    • 2006
  • Two-dimensional(2-D) radar images, namely, ISAR images from a maneuvering target include unwanted phase errors due to the target's motion. These phase errors make ISAR images to be blurred. The ISAR autofocus technique is required in order to remove these unwanted phase errors. Unless those unwanted phase errors produced by the target's motion are removed prior to target identification, we cannot expect a reliable target identification performance. In this paper, we use the entropy-based ISAR autofocus technique which consists of two steps: range alignment and phase adjustment. We analyze a relationship between the number of sampling point and a image quality in a range alignment algorithm and also analyze a technique for reducing computation time of the SSA(Stage-by-Stage Approachng) algorithm in a phase adjustment.

Autofocus of Infinity-Corrected Optical Microscopes by Confocal Principle and Fiber Source Modulation Technique (공초점 원리와 광섬유 광원 변조를 이용한 무한보정 현미경 자동초점)

  • Park, Jung-Jae;Kim, Seung-Woo;Lee, Ho-Jae
    • Korean Journal of Optics and Photonics
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    • v.15 no.6
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    • pp.583-590
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    • 2004
  • The autofocus is one of the important processes in the automated vision inspection or measurements using optical microscopes, because it influences the measuring accuracy. In this paper, we used the confocal microscope configuration based on not a pinhole but a single-mode optical fiber. A single mode fiber has the functions of source and detector by applying the reciprocal scheme. As a result, we acquired a simple system configuration and easy alignment of the optical axis. Also, we embodied a fast autofocus system by acquiring the focus error signal through a source modulation technique. The source modulation technique can effectively reduce physical disturbances compared with objective lens modulation, and it is easily applicable to general optical microscopes. The focus error signal was measured with respect to the modulation amplitude, reflectance of the specimen and inclination angle of the measuring surface. The performance of the proposed autofocus system was verified through autofocusing flat mirror surface. In addition, we confirmed that source modulation rarely degrades the depth resolution by the comparison between the FWHMs of axial response curves.