Accuracy improvement of the stereoscopic measuring system based on the holographicoptical element and spatial frequency

• Main Authors: Wei-Ren Chen, 陳威任
• Other Authors: 施錫富
• Format: Others
• Language: zh-TW
• Published: 2015
• Online Access: http://ndltd.ncl.edu.tw/handle/97259902067600714672

碩士 === 國立中興大學 === 機械工程學系所 === 103 === Based on previous studies, this study utilizes a holographic optical element(HOE) to replace the two dimensional diffractive optical element (DOE) of the infrared emission system inside the Kinect sensor, so as to correct the projected light field with obvious distortion and uneven intensity distribution. Besides, this study adopts a novel depth measuring method by using spatial frequency to perform the depth measure of an object and discusses the its setup and measuring precision. First, the infrared module of the Kinect sensor was analyzed to realize the element composition and operating mechanism. Owing to serious distortion and uneven intensity distribution in the light field after being projected through the two-dimensional DOE this study improves the problems by designing the light field shape of the first diffraction order and controlling its etching depth of the HOE. After analyzing the light field range projected by the Kinect system, the optical simulation software was applied to the HOE design. Two phase polynomials were separately designed for the diffraction position and light field size, respectively. They were then merged as one to describe the free-form surface of the HOE. The curve fitting and binary sampling were used to approximate the free-form surface and depict the structural pattern on the HOE surface. Then, the lithographic and etching processes were utilized to fabricate the HOE on glass. Since the designed pattern is a layered arc shape with lines of wide to thin from bottom to top, it will make the HOE have different structural depths and unable to get better diffraction efficiencies by using common dry etching processes. Therefore, the research attempts an etching process under higher vacuum to provide the gas molecules longer free-path within the cavity to reduce the molecular collision, and makes the incident ions more vertical to get consistent depths at different widths. On the depth measuring, the speckle patterns projected by the Kinect infrared module were converted to the spatial frequency distribution for measuring. By establishing the mathematical relationship, the experimental results were compared with the estimated values for testing the validity. Furthermore, the influences from the images resolution, focus position and capturing area were also summarized. Under current conditions, the depth measuring accuracy of 1 cm could be well achieved.