Measurement of mid-spatial frequencies of diamond turned optics by using dual-mode snapshot interferometry

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.