Low coherence interferometric fibre based in-situ measurements of surface roughness in air and in water

In many high-value manufacturing applications there is increased demand for component surfaces with enhanced geometric complexity. In order to ensure accurate adherence to design specifications and to compensate for any deviations, an in-situ measurement system capable of providing feedback on surface geometry within reasonable range is an important tool for manufacturing processes. While there have been many sensing techniques used for monitoring the manufacturing process, many of these are limited in terms of their ability to provide accurate reliable real-time measurements, are off-line systems, or are inappropriate for the hostile machining environment. In this work, a fibre based common path low-coherence interferometric technique has been successfully developed to measure surface step heights and surface roughness, in air, but more importantly in water. Results are produced from slip gauge sets and reference roughness comparator standards, demonstrating the ability to achieve close to nominal value agreement in air, but water based step height and surface roughness Ra values depart further from nominal as a function of media refractive index and dispersion characteristics.