Residual flatness and scale calibration for a point autofocus surface topography measuring instrument

Point autofocus instruments are often used for measuring the surface topography of objects with complex geometry. Determining the metrological characteristics of the instrument is key to ensuring a traceable areal surface topography measurement. In this work, several metrological characteristics, as outlined in ISO/FDIS 25178-600, are determined for a commercial point autofocus instrument, including flatness deviation, the amplification and linearity of the lateral and vertical axes, and the perpendicularity between the axes. Calibrated material measures including an optical flat, step heights and areal cross gratings are used to determine the metrological characteristics. The impact of the point autofocus operating principle and the evaluation method on the metrological characteristics is discussed.