“…However, depending on the surface topography, more or less systematic measurement errors may also occur. Examples are artefacts known as batwings (Harasaki and Wyant, 2000;Xie et al, 2017), phase jumps resulting from the slope effect in white-light interferometry and also laser interferometry (Schake et al, 2015), artefacts from crosstalk between neighbouring pinholes of a spinning disc of a confocal microscope (Fewer et al, 1997), and artefacts occurring by equal curvature of the wavefront and the measuring surface (Mauch et al, 2012). For an investigation of these effects it is necessary to distinguish between the real and the measured surface.…”