단일 영상 공간 위상 천이 기술을 이용한 3차원 형상 측정 연구

Abstract

Optical interferometry has lots of benefits to measure 3D surface profiles of specimens as non-destructive measurement techniques. Phase shifting interferometry (PSI) has the high precision and has been widely used in optical shop testing because of its robust and reliable measurement results even though it suffers from the well-known 2π-ambiguity problem caused by the phase jumps. Low coherence scanning interferometry (LCSI) is free from the 2π-ambiguity problem because of the low coherence characteristic to localize the interference fringe, i.e. correlogram, which enables to precisely measure the surface profiles of step height specimens as well as smooth surface targets. However, the main drawback of LCSI is the scanning procedure to obtain image stack to find the peak positions of correlograms. On the other hand, multi-wavelength interferometry has been approached with the techniques of wavelength scanning and equivalent wavelength in order to avoid 2π-ambiguity problems of PSI. However, they also have the fundamental limitations of the measurement speed by scanning wavelength and temporal phase shifting. Recently, a polarization CMOS camera (PCMOS) has been used to various research fields such as polarization imaging, photoelastic measurements and 3D imaging because its single image contains four kinds of different polarized sub-images. In 3D imaging especially, the PCMOS enables to obtain four phase-shifted images at once and instantly obtain the measurement phase map without any mechanical or electrical moving parts for temporal phase shifting. Instead of using a temporal phase shifting by electrical or mechanical devices, this spatial phase shifting interferometry can measure the phase with PCMOS and confirm the measurement accuracy as similar to typical PSI. In this investigation, I propose the various techniques that combines ideas of multi-wavelength interferometry and spatial phase shifting interferometry to realize the single shot surface profiling. A color polarization CMOS camera (color-PCMOS) has been adopted to implement snapshot 3D surface profile and eliminate the 2π-ambiguity problem using RGB LD (RGB laser diode). The main advantage of the multi-color interferometry is that it overcomes 2π-ambiguity problem for reconstructing surface profiles with height step by deriving three equivalent wavelength and phases. In addition, it is possible to measure the fine surface profile of the specimen in the large vertical range with a single shot image by the principle of multi-wavelength interferometry. The successful combination of these principles can greatly enhance the current limitation of surface profiling techniques such as long measurement time and limited measurement range. On the other hand, I also propose another technique that combines ideas of two wavelength interferometry and spatial phase shifting interferometry to realize the single shot surface profiling. Based on the two wavelength interferometry, the 2π-ambiguity range can be extended by the combinations of the wavelengths and the spatial phase shifting interferometry can solve the time consumption of typical phase shifting technique by the use of the PCMOS. Applying the novel two wavelength interferometric algorithm proposed in this thesis makes it possible to measure the surface profile of the specimen with a single acquisition of the image by the PCMOS, which can obtain the phase-shifted interferograms from each of the neighboring four pixels. To verify theoretical predictions of the proposed interferometers, feasibility experiments were performed and several height step samples were measured. Practical limitations were considered and their improvements were also discussed in each of the proposed techniques.