Multiscale analysis and characterizations of metallic surface topographies is a crucial task in advanced manufacturing and precision engineering. In this paper, we present a multiscale measurement technique based on photometric stereo for accurately reconstructing surface topographies at different scales. Our system employs a zoom lens and LED illuminators, providing the flexibility to adjust the measurement scale. To ensure efficiency and accuracy during scale transition, a fast light source calibration method using a Lambertian sphere is proposed to obtain the position and orientation of the LED precisely and simultaneously. A surface reconstruction algorithm is designed through modeling the light propagation process, enabling the identification of surface features at different levels of detail. The system is calibrated using an Edmund USAF resolution target, which demonstrate the accuracy of our method with the minimum deviation as low as 4μm. The proposed approach is applied to investigate the multiscale surface topography produced by metal additive manufacturing (AM) technique, achieving an accurate representation from contour to roughness. The results are compared with those obtained from a commercial 3D scanner from Keyence, which validate the effectiveness of our method and show its great potential in quality control and optimization of manufacturing processes.
This paper proposed a frequency-domain-decomposition denoising algorithm for nano-scale measurement in white light interferometry (WLI). In this work, the captured correlogram is firstly divided into a series of short-time stationary signals, the phase distribution can then be derived as the sum of the corresponding phase components after Fourier transform. By applying windowed threshold filtering, the noises existed in phase map can be eliminated, and a denoised correlogram is precisely reconstructed. Afterwards, the surface height is retrieved through phase-frequency least-square fitting. In simulations, the phase noises with different levels are investigated. By comparing the noise deviations in the reconstructed phase map with the original one, the effectiveness on noise suppression of the proposed method is properly verified. In the experiments, a height step standard with calibrated values 182±2.0nm are tested, where the height deviations below 3nm and the repeatability of 0.5% has proved the robustness of our proposed method.
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