The paper presents a custom-designed laser triangulation based metrology system, which enables high precision surface displacement measurement of various material types with a single sensor configuration. Laser structuring applications require material surface alignment relative to the laser focus position where fabrication conditions are optimal. The measurement system utilizes a high-quality UV wavelength laser beam (primarily used for structuring purposes) with automatic control of its intensity. The laser source operates in a continuous wave (CW) mode during the measurement process, whereas the UV wavelength enables measurement of transparent materials. Robust displacement measurement of various material types was solved by introducing a new approach of structured light projection and its centroid detection. A high resolution 2D galvanometric scanning system is used for dynamic symmetrical pattern projection, which is proven to reduce the effects of material surface related errors and speckle noise. Furthermore, a “double curve fitting” (DCF) centroid detection algorithm, where Gaussian curves are fitted to radial cross sections of the acquired pattern, and an ellipse is fitted to their peak positions, was introduced. The method includes subsurface scattering compensation, which proves crucial for translucent material measurement, where incident light penetrates into the material surface and causes uneven light intensity distribution of the acquired pattern. Experimental results have shown that the metrology system is robust to laser intensity variation and material type, with measurement bias lower than 50 μm and standard deviation lower than ±6.3 μm for all materials. The developed probe has been integrated into commercial LPKF laser structuring systems.

Date Published: 6 September 2013
PDF: 10 pages
Proc. SPIE 8839, Dimensional Optical Metrology and Inspection for Practical Applications II, 88390M (6 September 2013); doi: 10.1117/12.2022838

Published in SPIE Proceedings Vol. 8839:
Dimensional Optical Metrology and Inspection for Practical Applications II
Kevin G. Harding; Peisen S. Huang; Toru Yoshizawa, Editor(s)