Optical surface metrology plays a vital role in quality control in modern manufacturing by enabling precise, non-contact measurement of surface topography. Compared to contact methods, optical techniques offer faster measurement speeds, higher resolution, and can measure delicate materials without physical damage. At present, most optical metrology instruments remain offline, where parts must be transferred from manufacturing platforms to environmentally controlled laboratories. Offline metrology delays production and leads to manufacturing deficiencies if parts are not perfectly realigned for further processes. Therefore, it is necessary to develop compact on-machine optical sensors that are immune to environmental disturbance. Additionally, modern manufacturing technologies can generate complex surfaces with a wide texture range, from smooth to rough, which presents a challenge for surface metrology instruments, which are typically limited to measuring a particular texture scale. For example, the Focus Variation (FV) method proved its capability to measure rough surfaces, including steep and high-slope surfaces. Still, it fails to measure smooth surfaces (i.e., those with insufficient surface texture and lack of contrast), such as mirrors and silicon wafers, which are usually measured with interferometer systems. Therefore, a hierarchy of instrumentation is needed, which adds cost to manufacturers and complexity for on-machine measurement. This thesis presents a Chromatic Focus Variation (CFV) instrument for on-machine surface metrology applications. The CFV system replaces mechanical motion stages by employing a dispersive objective lens (i.e. chromatically aberrated) to axially shift the focus plane along the optical axis for vertical/depth scanning. This approach speeds up measurement and reduces the instrument size. Additionally, without mechanical motion stages, the CFV does not require regular maintenance or frequent recalibration to maintain measurement accuracy over time. A second contribution of this thesis is to enable the FV technology to measure both rough and smooth surfaces in one optical setup. The CFV system is equipped with an illumination pattern projection via a polarised pixelated phase mask placed through the light source to measure optically smooth surfaces. The CFV system with pattern projection requires no energy/power at the scanning head, which simplifies metrology integration for on-machine measurement. The thesis also provides the first quantitative estimation, to the author’s knowledge, of the immunity of focus variation instruments to vibration and uses a built-in active vibration compensation system capable of mitigating micron-scale vibration amplitudes at frequencies up to several tens of Hertz. The system design, implementation, and operation are all reported in this thesis. A detailed analysis of the optical performance of the dispersive objective lens is conducted, and the measurement performance of the proposed system is validated using the state-of-the-art commercial instruments, Alicona InfiniteFocus G5 and Bruker CountorX-200 for rough and smooth surfaces, respectively, and the measurement result shows a good agreement.
| Date of Award | 15 Apr 2026 |
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| Original language | English |
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| Sponsors | Engineering and Physical Sciences Research Council |
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| Supervisor | Hussam Muhamedsalih (Main Supervisor) & Jane Jiang (Co-Supervisor) |
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