Surface Topography Measurement of Mirror-Finished Surfaces Using Fringe-Patterned Illumination

• Main Authors: Shaowei Fu, Fang Cheng, Tegoeh Tjahjowidodo
• Format: Article
• Language: English
• Published: MDPI AG 2020-01-01
• Series: Metals
• Subjects: in-situ inspection; luminance contrast; fringe-patterned illumination; surface topography
• Online Access: https://www.mdpi.com/2075-4701/10/1/69

Mirror-finished surface products have a wide range of applications in different engineering industries, such as power generation, aerospace, semiconductors and optics. The surface topography of mirror-finished products is typically measured in a metrology laboratory, which is typically time consuming and cannot be integrated into the manufacturing process. To allow for in-situ product quality assurance and automatic tool change for manufacturing processes, a more accurate and responsive surface-measurement method is needed. For highly polished surfaces, a sub-micron surface fluctuation makes it possible to use light-scattering effects and image processing for surface texture analysis. A non-contact surface inspection system using a fringe-patterned illumination method is proposed in this paper. A predesigned pattern was projected onto the target surface, and its reflected image was captured by a camera. It was found that the surface parameters <i>Sa</i> and <i>Sq</i>, which are widely used to evaluate surface quality, are significantly correlated with luminous-intensity distribution. Another parameter, <i>Str</i>, which quantifies the uniformity of surface-texture directions due to polishing or grinding marks, was traditionally quantified after a complete-surface topographic measurement. In this research, a new approach is proposed to determine surface isotropy through a luminance-intensity distribution analysis. By rotating the test coupon, the variation of specular reflection showed correlation with the significance of surface-texture direction. The experimental results demonstrate that mirror-finished surfaces with a large deviation in luminance intensity across the pattern possess low <i>Str</i> values, which indicates low uniformity in surface texture.