Characterization of the sampling in optical measurements of machined surface textures

This work investigates the areal effect, a smoothing of the surface areal topography in measurements from optical instruments that arises from the use of discrete pixels. Two-dimensional and three-dimensional models for the process of acquiring areal topography data were developed and applied to three-dimensional reference surface textures of various traditional machined surfaces that were measured by an atomic force microscope. Sets of reference surface areal data were stitched by a three-dimensional stitching method prior to their use in simulations. Roughness parameters and cumulative power spectra of areal topography of the specimen surfaces measured by coherence scanning interferometry were calculated and compared to the results from the simulated profiles. The results from the simulated data profiles showed good agreement with profiles measured by a coherence scanning interferometry microscope. Finally, the relationship between the pixel size of the optical instruments and the resulting reliability of the high-frequency components of the acquired surface profiles was investigated with spectral analysis. Selection criteria for the magnification of the objective lens are proposed, which allow determination of the appropriate sensor spatial resolution for measurements based on the surface texture characteristics of the specimen.