Korean Journal of Optics and Photonics (한국광학회지)

Optical Society of Korea (한국광학회)
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A Study on the Improvement of Wavefront Sensing Accuracy for Shack-Hartmann Sensors

Shack-Hartmann 센서를 이용한 파면측정의 정확도 향상에 관한 연구

  • Roh, Kyung-Wan (Department of Safety Issue Research, Korea institute of Nuclear Safety) ;
  • Uhm, Tae-Kyoung (Department cf Mechanical Engineering, KAIST and Image Information Research Center) ;
  • Kim, Ji-Yeon (Department cf Mechanical Engineering, KAIST and Image Information Research Center) ;
  • Park, Sang-Hoon (Department cf Mechanical Engineering, KAIST and Image Information Research Center) ;
  • Youn, Sung-Kie (Department cf Mechanical Engineering, KAIST and Image Information Research Center) ;
  • Lee, Jun-Ho (Department of Optical Engineering, Kongju National University)
  • 노경완 (한국원자력안전기술원, 현안기술연구실) ;
  • 엄태경 (한국과학기술원 기계공학과, 영상정보특화연구센터) ;
  • 김지연 (한국과학기술원 기계공학과, 영상정보특화연구센터) ;
  • 박상훈 (한국과학기술원 기계공학과, 영상정보특화연구센터) ;
  • 윤성기 (한국과학기술원 기계공학과, 영상정보특화연구센터) ;
  • 이준호 (공주대학교 광공학과, 영상정보특화연구센터)
  • Published : 2006.10.25
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Abstract

The SharkHartmann wavefront sensors are the most popular devices to measure wavefront in the field of adaptive optics. The Shack-Hartmann sensors measure the centroids of spot irradiance distribution formed by each corresponding micro-lens. The centroids are linearly proportional to the local mean slopes of the wavefront defined within the corresponding sub-aperture. The wavefront is then reconstructed from the evaluated local mean slopes. The uncertainty of the Shack-Hartmann sensor is caused by various factors including the detector noise, the limited size of the detector, the magnitude and profile of spot irradiance distribution, etc. This paper investigates the noise propagation in two major centroid evaluation algorithms through computer simulation; 1st order moments of the irradiance algorithms i.e. center of gravity algorithm, and correlation algorithm. First, the center of gravity algorithm is shown to have relatively large dependence on the magnitudes of noises and the shape & size of irradiance sidelobes, whose effects are also shown to be minimized by optimal thresholding. Second, the correlation algorithm is shown to be robust over those effects, while its measurement accuracy is vulnerable to the size variation of the reference spot. The investigation is finally confirmed by experimental measurements of defocus wavefront aberrations using a Shack-Hartmann sensor using those two algorithms.

Shack-Hartmann 파면측정 센서는 적응광학 분야에서 파면측정용으로 가장 널리 사용되고 있다. Shack-Hartmann 센서에서 파면측정 알고리즘은 크게 점영상의 중심점 탐색 알고리즘과 파면복원 알고리즘으로 나눌 수 있다. 이 중 점영상의 중심점 탐색은 파면측정 결과에 크게 영향을 미치나 많은 연구결과에도 불구하고 최적의 중심점 측정 알고리즘 및 내부 변수 설정에 대한 일반적인 해결책은 아직 제시되지 못하고 있다. 현재, 중심점 탐색 알고리즘으로는 무게중심법(center of gravity)이 가장 널리 사용되고 있다. 본 연구에서는 중심점 탐색 알고리즘에 의한 측정 정확도 분석을 위해 무게중심법과 상관관계법(correlation)을 함께 고려하였다. 이를 위해 Shack-Hartmann 센서의 파면측정 과정을 전산모사하였고, 이로부터 중심점 탐색 알고리즘과 관련하여 무게중심법 및 상관관계법에서의 노이즈 특성을 비교, 분석하였다. 또한, 무게중심법에서 문턱값(threshold value)의 설정에 따른 파면측정의 정확도에 대한 분석을 통하여 노이즈 제거를 위한 최적의 문턱값을 제안하였다. 이와 더불어 Shack-Hartmann 센서 시스템을 구성하여 디포커스(defocus)를 발생시키고 이를 측정하는 실험을 수행함으로써 파면측정 알고리즘 및 전산모사 결과를 검증하였다.

Keywords

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