Article Effect of a Misidentified Centre of a Type ASG Material Measure on the Determined Topographic Spatial Resolution of an Optical Point Sensor Janik Schaude * , Andreas Christian Gröschl and Tino Hausotte   Citation: Schaude, J.; Gröschl, A. C.; Hausotte, T. Effect of a Misidentified Centre of a Type ASG Material Measure on the Determined Topographic Spatial Resolution of an Optical Point Sensor. Metrology 2022, 2, 19–32. https://doi.org/ 10.3390/metrology2010002 Academic Editor: Han Haitjema Received: 13 October 2021 Accepted: 21 December 2021 Published: 5 January 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Institute of Manufacturing Metrology, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Nägelsbachstr. 25, 91052 Erlangen, Germany; andreas.groeschl@fmt.fau.de (A.C.G.); tino.hausotte@fau.de (T.H.) * Correspondence: janik.schaude@fmt.fau.de Abstract: The article presents the determination of the topographic spatial resolution of an optical point sensor. It is quantified by the lateral period limit D LIM measured on a type ASG material measure, also called (topographic) Siemens star, with a confocal sensor following both a radial measurement and evaluation, as proposed by ISO 25178-70, and the measurement and subsequent evaluation of two line scans, proposed by the NPL Good Practice Guide. As will be shown, for the latter, an only slightly misidentified target centre of the Siemens star leads to quite significant errors of the determined D LIM . Remarkably, a misidentified target centre does not necessarily result in an overestimation of D LIM , but lower values might also be obtained. Therefore, a modified Good Practice Guide is proposed to determine D LIM more accurately, as it includes a thorough determination of the centre of the Siemens star as well. While the measurement and evaluation effort is increased slightly compared to the NPL Good Practice Guide, it is still much faster than a complete radial measurement and evaluation. Keywords: surface metrology; topographic spatial resolution; lateral period limit; type ASG material measure; Siemens star; confocal sensor; nano measuring machine 1. Introduction and Literature Review Surface texture determines the functional behaviour of a manufactured component and is therefore crucial in many different areas such as electronics, optics, medicine, information technology, or consumables [1–3]. As a result, surface metrology is an essential part of manufacturing metrology extensively dealt with within the ISO 25178 standards [4,5]. In the era of globalization and interchangeability, accurate and internationally comparable measurement results are indispensable [6]. Metrological comparability is ensured by the metrological traceability of the measurement result, which goes along with a statement about the measurement uncertainty [7,8]. The stylus method has been used for over 100 years in surface metrology [9], and due to the well understood interaction between the probe and the surface [10], it still serves as reference method for traceable surface measurements in different national metrology institutes [11–13]. Furthermore, atomic force microscopy (AFM, [14]) under tightly controlled environmental conditions and in ultrahigh vacuum enables a resolution down to the atomic scale [15,16], while metrological AFMs [17] ensure the traceability of the measurement results. Nevertheless, the measurement of soft surfaces by tactile methods is problematic due to the elastic or even plastic deformation of the workpiece caused by the applied probing force [18]. Not only are such issues irrelevant for optical methods, but the non-contact nature of probing also allows a measurement speed unprecedented by tactile methods [19]. Optical methods are therefore especially suited for in situ surface metrology [20]. The main benefit of in situ metrology is the more economical and also resource-efficient production due to the possibility to realize a very short quality control loop [21]. As process changes, e.g., alterations of the machine tool, influence surface Metrology 2022, 2, 19–32. https://doi.org/10.3390/metrology2010002 https://www.mdpi.com/journal/metrology