In close range photogrammetry, a scale bar which has fixed targets on it is often used to scale a photogrammetric measurement by giving known distance(s) between the targets. It’s important to make sure that the scale bar can provide precise distances so the measurement can be scaled correctly. An effective way to calibrate a scale bar was realized by getting the microscopic images of the retro-reflective targets (RRT) at two ends of the scale bar and locating their centers, and the distance between two targets was measured by the laser interferometer, then the length of the scale bar could be calculated from geometric relations. In this calibration method, the problem of accurate location of RRT is a key to calibrate the photogrammetric scale bar. So in this paper, we focus on this problem and propose a RRT location scheme based on microscopic vision and image processing. The composition and principle of the target location system are introduced. According to the retro-reflective features of the RRTs and their image characteristics under the microscope, the image processing methods of preprocessing, feature extraction and centering are analyzed. The factors such as image magnification, illumination and offsets in the field of view, which may the significant sources of location error, have been tested and analyzed. The results help to standardize the RRT imaging conditions so the measurement repeatability can be improved. The repeated positioning accuracy is less than 0.3μm.

Date Published: 12 March 2020
PDF: 11 pages
Proc. SPIE 11439, 2019 International Conference on Optical Instruments and Technology: Optoelectronic Measurement Technology and Systems, 114390S (12 March 2020); doi: 10.1117/12.2543291

Published in SPIE Proceedings Vol. 11439:
2019 International Conference on Optical Instruments and Technology: Optoelectronic Measurement Technology and Systems
Jigui Zhu; Kexin Xu; Hai Xiao; Sen Han, Editor(s)