Development of shape-from-focus profile measuring system and algorithms using hybrid pattern projection

• Main Authors: Yu-sheng Chen, 陳譽升
• Other Authors: Liang-Chia Chen
• Format: Others
• Language: en_US
• Published: 2019
• Online Access: http://ndltd.ncl.edu.tw/handle/s39tgx

碩士 === 國立臺灣大學 === 機械工程學研究所 === 107 === In this research, a solid microscopic system is developed in the aim of achieving automated optical inspection. Such system has the capability of actively projecting the programmed pattern as well as executing vertical objective movement, along with the features of superior repeatability and adaptability. Numerous issues had emerged during the process of system establishment, and thus the encountered problems and corresponding solutions are also suggested in the article. For instance, to resolve the laterally spectral-dispersive issue caused by the digital micromirror device, a Kohler illumination module is harnessed to attain greater homogeneity and spatial integration property of the reflected beam. The primary objective of this article is to reconstruct samples which contain both specular and diffusive surfaces within a single field of view by the shape from focus method. To achieve such target, a hybrid projection method is developed by the combination of the profiles constructed by active and passive illuminated surfaces. The texture of the profile will first be assessed by a numerical method through a two-dimensional all-in-focused image of the scene, then the correct topography will be allocated to designated pixels. The method can effectively reduce the shooting noise existence within the field, thus enhancing the precision in reconstruction. Additional numerical methods are also developed in order to facilitate the measuring process. The approaches include the optimization of fringe period, a new focus measure operator, and a solution to reflectivity problem. By the application of such processes, the precision, reliability, and efficiency of the reconstruction can be further improved. From experimental results, the repeatability of the system is evaluated as 0.0047μm (1σ) from three experiments, the spatial resolution is 0.497μm/pixel, and the axial resolution lies between 1.28μm and 0.64μm, under a magnification of 20 times. Aside from mechanical performances, the comparisons between ordinary and proposed approaches are also presented, where the proposed optimal pattern and the focus measure operator have evidently better precision than the compared ones.