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Determination of the metrological characteristics of optical surface topography measuring instruments

The use of optical areal surface topography measuring instruments has increased significantly over the past ten years as industry starts to embrace the use of surface structuring to affect the function of a component. This has led to a range of optical areal surface topography measuring instruments being developed and becoming available commercially. For such instruments to be used as part of quality control during production, it is essential for them to be calibrated according to international standards. The ISO 25178 suite of specification standards on areal surface texture measurement presents a series of tests that can be used to calibrate the metrological characteristics of an areal surface texture measuring instrument (both contact and optical). Calibration artefacts and test procedures have been developed that are compliant with ISO 25178. The artefacts include crossed gratings, resolution artefacts and pseudo-random surfaces. Traceability is achieved through the NPL Areal Instrument - a primary stylus-based instrument that uses laser interferometers to measure the deflection of the stylus tip. Good practice guides on areal calibration have also been drafted for stylus instruments, coherence scanning interferometers, scanning confocal microscopes and focus variation instruments.