It is a challenge for any optical method to measure objects with a large range of reflectivity variation across the surface. Image saturation results in incorrect intensities in captured fringe pattern images, leading to phase and measurement errors. This paper presents a new adaptive digital fringe projection technique which avoids image saturation and has a high signal to noise ratio (SNR) in the three-dimensional (3-D) shape measurement of objects that has a large range of reflectivity variation across the surface. Compared to previous high dynamic range 3-D scan methods using many exposures and fringe pattern projections, which consumes a lot of time, the proposed technique uses only two preliminary steps of fringe pattern projection and image capture to generate the adapted fringe patterns, by adaptively adjusting the pixel-wise intensity of the projected fringe patterns based on the saturated pixels in the captured images of the surface being measured. For the bright regions due to high surface reflectivity and high illumination by the ambient light and surfaces interreflections, the projected intensity is reduced just to be low enough to avoid image saturation. Simultaneously, the maximum intensity of 255 is used for those dark regions with low surface reflectivity to maintain high SNR. Our experiments demonstrate that the proposed technique can achieve higher 3-D measurement accuracy across a surface with a large range of reflectivity variation.
The calculation of the real contact area between rough surfaces is an important prerequisite for studying the contact characteristics of mechanical joints. However, it is challenging to accurately calculate the real contact area between rough surfaces by existing theoretical and experimental methods. Therefore, an efficient and practical equivalent calculation method for the real contact area between three-dimensional(3D) rough surfaces based on two-dimensional(2D) profiles is presented in this paper. The main advantage of the presented method as compared with those available in the literature is that the real contact area can be predicted only by the section of the 3D rough surfaces in contact, which greatly reduces the computational cost. The sampling strategy of sections is studied to improve the accuracy of the proposed method. The accuracy of the proposed equivalent calculation method is verified by numerical simulation. The results show that the proposed equivalent calculation method can efficiently predict the real contact area between rough surfaces within the scope of engineering application.
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