Surface roughness study on mirror finish surface products using patterned area illumination method

Mirror finish surface products are widely used in many industries such as aerospace, optics, semiconductor and biomedical device manufacturing. The surface roughness of mirror finish products are usually measured by off-line instruments such as coherence scanning or phase shifting interferometers which are time consuming and relatively difficult to integrate into the manufacturing equipment. In order to achieve both in-situ surface quality control and automated tool changing for the polishing process, accurate and real-time surface measurement are required. To address these in-situ measurement challenges, a surface roughness measurement system using the fringe pattern illumination method is proposed in this paper. By analysing the grayscale contrast ratio of the fringe pattern reflection image, the experimental results showed that the proposed system was able to measure different mirror finish surfaces with Sq from 10 nm to 165 nm. The experimental results demonstrated a good correlation between the grayscale contrast value and Sq. In addition, the grayscale contrast ratio was also correlated with directions of the machining marks and the projected fringes at different measurement angles. The surface texture aspect ratio parameter Str which provides information about the strength of the machining marks was experimentally evaluated and compared with grayscale contrast change. In conclusion, the proposed system was able to measure surface roughness and indicate machining pattern effects on the mirror finish surface.