Determination of the metrological characteristics of optical surface topography measuring instruments R. K. Leach, C. L. Giusca National Physical Laboratory, Engineering Measurement Division, Teddington, Middlesex, UK TW11 0LW ABSTRACT 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. Keywords: areal surface topography, optical instruments, calibration, uncertainty, traceability, good practice 1. INTRODUCTION An areal surface topography measuring instrument provides a three dimensional (3D) map of a surface. The 3D map is made up of a set of points measured with respect to three orthogonal length scales. The scales of an areal surface topography measuring instrument are nominally aligned to the axes of a Cartesian coordinate system. The axes are physically realized by various components that are part of the metrological loop of the instrument. Hence, the quality and the mutual position of these components partially confer the quality of the coordinate measurements. The coordinate measurements produced by areal surface topography measuring instruments are also affected by other influence factors such as ambient temperature, mechanical noise and electrical noise. The effect of a single influence factor, or a combination of influence factors, on the quality of the areal measurements are quantified by experimentally determining the metrological characteristics of the instrument. Typically these characteristics include the noise of the instrument; the linearity, amplification and resolution of the scales; the deviation from flatness of the areal reference and the squareness of the axes [1], [2]. The magnitude of the influence factors on an areal measurement can be different for different sizes of measuring area and sampling distance, that is to say the measurement bandwidth. The choice of measurement bandwidth is application dependent and is based on the selection of S-filters and L-filters/F-operators, each having a range of preset values termed nesting indexes [3], [4]. The calibration of the instrument is usually performed using the same conditions as those used on a daily basis. Presented in this paper is a way of calibrating the scales of areal surface topography measuring instruments. The calibration of the scales involves a series of relatively simple tasks that are performed to evaluate the magnitude of the uncertainty associated with the metrological characteristics of the instruments assuming well-defined measuring conditions. The calibration process also necessitates the use of material measures designed for calibrating surface topography measuring instruments. Here, a coherence scanning interferometer (CSI) has been used as an example instrument [5], but the principles apply to many types of areal surface topography measuring instruments, including stylus instruments, imaging confocal microscopes and focus-variation microscopes. The CSI used was equipped with a one mega-pixel camera, a Mirau type 50× magnification objective lens (more objective lenses were available but only the 50× is used as an example of the process) with a working field of view of 0.35 mm by 0.35 mm and a 0.1 mm range Optical Micro- and Nanometrology IV, edited by Christophe Gorecki, Anand K. Asundi, Wolfgang Osten, Proc. of SPIE Vol. 8430, 84300Q · © 2012 SPIE · CCC code: 0277-786X/12/$18 · doi: 10.1117/12.921452 Proc. of SPIE Vol. 8430 84300Q-1 Downloaded from SPIE Digital Library on 09 May 2012 to 139.143.5.160. Terms of Use: http://spiedl.org/terms