Performance specification of areal surface texture instruments exemplified by a self-built WLI

The calibration of optical surface topography measuring instruments ensures the comparability of the resulting surface topographies. Such comparability is also desirable for self-built optical microscopes in order to optimize these instruments with regard to their systematic and stochastic measurement uncertainty. However, there is not yet a fixed set of standards in the field of interference microscopy as in the field of stylus instruments that describes methods for their calibration, verification, adjustment and performance specification [1] [2]. The associated standard for optical measuring instruments is still in preparation in the corresponding ISO working group. There is the guideline according to VDI/VDE 2655 Sheet 1.1, for the calibration of interference microscopes for surface texture measurement. This guideline is essentially a transfer of methods that origin from the calibration of stylus instruments [1]. Thus, it describes a profile-like calibration and does specifically consider areal properties of interference microscopes. The field of areal calibration is still work in progress in the international standardization. The ISO 25178-600 that was recently published defines the basic metrological characteristics of areal surface topography measuring instruments [3]. In our study, we describe the optimization and performance specification of a self-built white light interference microscope based on methods that were developed for instrument calibration. For the performance specification we use the metrological characteristics of ISO 25178-600 that are imaged with the help of a previously developed Universal Calibration Artefact for areal calibration [4]. The results of the metrological characteristics are compared with a commercially available white light interferometer (WLI). In order to improve the self-built interference microscope, the measurements are repeated with different configurations and a final setup is chosen that allows a specification of the performance of the self-built interference microscope for measuring areal surface texture.