디지털 홀로그래피를 이용한 CT시험편의 파단면 조도 측정

Abstract

이 논문에서는 디지털 홀로그래피를 이용하여 물체의 표면 거칠기를 측정하여 기계적 응용을 증대하고자 기술하였다. 먼저 기존의 표면 거칠기를 측정하는 접촉식 방법이나 분해능의 한계를 극복하는 단계로, 디지털 홀로그래피에 짧은 간섭거리를 가진 광원을 사용함으로써 Resolution target을 측정하여 본 논문의 분해능을 검증하였다. 그 후 CT시험편이 파단에 이르기까지 파단면의 각 위치별로 모멘트의 변화에 따라 파단면 내에서 거칠기를 측정하였다. 이 실험에서는 디지털 홀로그래피 반사형 간섭계를 이용하여 홀로그램 이미지를 얻었으며, 수치적 재생이 이루어졌다. 초점이 맞는 파단면에 진폭(Intensity) 정보 뿐 아니라, 위상정보도 이용함으로써 파단면의 형상을 정밀하게 측정하였다.|Digital holography is an imaging technique offering both sub-wavelength resolution and real time observation capabilities. The reconstruction of the wavefront from the hologram provides the amplitude and the absolute phase of the wave diffracted by the microscopic objects. Absolute phase contrast yields longitudinal accuracies as low as one nanometer in air or even less in dielectric media. The lateral accuracy and the corresponding resolution can be kept at a sub-micron level by the use of a high Numerical Aperture (N.A.) microscope Objective (M.O.). The principles of hologram formation, acquisition and wavefront reconstruction from digital holograms, acquired in a non-scanned modality, are described in details. The role of the M.O. in the capture of high spatial frequencies components of diffracted light and their restitution for high fidelity imaging is developed. A variety of applications of this new type of optical microscopy are described: material research, surface and interface sciences, microtechnologies, micro-optics and MOEMS. For reconstruction of fracture surface roughness of CT specimen, the hologram is need to expressed parameters and reconstruction basic theory. There are three main reconstruction algorithms most widely used. Each method has different properties with respect to available reconstruction distance, resolution of reconstructed image, and computational load. This is very important for reliable reconstruction to choose the appropriate method in a given situation. In a accordance with, the reconstruction algorithm for this thesis is choose the angular spectrum method. Because this method correctly reconstructed near wavefields for more diffractive that the micro variation of object surface. In this thesis, the variation of outside condition is appeared to the temperature and surface roughness of specimen. In the accordance with, the evolution is detected to Infrared camera, system of Michelson interferometer. This condition is associated with each other. The thesis found it the association. That research isn't attempt to other technician so it has important things. In this paper, the specimen variation according to outside condition measured the quantitative evaluation. The system of interferometer measured 10-9 unit. So, the nano variation and crack of object will be detectable. That result is very useful in the nondestructive testing method in industry.