The measurement of mid-spatial frequency (MSF) in ultra-precision machining is crucial for assessing the quality and performance of machined surfaces. MSF refers to the frequency range of surface irregularities between low-frequency form errors and high-frequency roughness. The sources that contribute to MSF errors during diamond turning are vibrations and dynamic instabilities, tool wear and deflection during cutting, inconsistent feed rates, variation in material properties, incorrect machine settings/process parameters, material removal mechanism employed (e.g., ductile or brittle removal). Controlling and measuring mid-spatial frequencies in the diamond-turning process is essential for meeting stringent optical specifications in various applications, such as lens manufacturing for imaging systems, telescopes, laser systems, etc. Inspecting MSF errors offline or after the manufacturing process is a common practice in the quality control of optical surfaces. However, there is a growing interest in incorporating on-machine metrology to detect and address MSF errors. One of the latest developments is a dual-mode on-machine metrology (OMM) system that simultaneously measures surface form and roughness without requiring the optical path's reconfiguration to switch between laser interferometer mode and LED interference microscopy mode. This study uses OMM to study the influence of process parameters and their impact on the mid-spatial frequencies during diamond turning. OMM provides real-time feedback, which helps in adjusting machining parameters to correct deviations and maintain the desired mid-spatial frequencies.
A single-layer SiO2 film is deposited using ion-assisted electron beam evaporation technique, and the deposited film is characterized using variable angle spectroscopic ellipsometry, UV–Vis–NIR spectrophotometer, coherence correlation interferometer, and Abbe refractometer as well as image processing techniques to investigate its optical and surface properties. The surface quality of the film in terms of average roughness, kurtosis, skewness, and power spectral density (PSD) is analyzed using interferometer and image processing. The refractive index of the SiO2 film is found to increase from 1.452 to 1.482 at 550 nm with increase in film thickness. This is corroborated by Abbe refractometer findings where the film refractive index is found to be 1.46490 and 1.48226 for a film thickness of 100 and 400 nm, respectively. The SiO2 film also results in reduction in average surface roughness from 35 nm of the uncoated substrate to 0.15 nm of the coated surface. Statistical indicators of surface quality extract from interferometric images such as kurtosis, skewness, and PSD and also exhibit sharp decline in their respective values of the coated surface as compared to the uncoated substrate indicating improvement in the surface quality after coating.
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