In this paper the design of a Time-Domain Full-Field OCT (TD-FF-OCT) setup for non-contact volumetric layer thickness measurement is presented and quantified in terms of achievable accuracy and performance. The capabilities of the instrument regarding its measuring accuracy are verified using foil thickness standards of different strength. Afterwards, a technical application of measuring a thin and rough varnish coated PET foil (foil thickness tfoil ≈ 150 μm, rough varnish layer thickness tvarnish ≈ 10 μm) is carried out. Since the device is designed to conduct areal measurements, the thickness can be accurately determined over several measurement points. The results are compared with results achieved by applying an alternative but destructive and more time-consuming measuring method (evaluation of microscopic images of respective foil slices produced by using a microtome). Finally, the achievements are summarized and identified optimization potential is highlighted.
ISO 25178-601 ff. define specific characteristics of individual measuring principles including coherence scanning interferometry (CSI) in -604. In the present study, we use a previously developed Universal Calibration Artefact to examine which additional information about CSI-specific metrological characteristics can be obtained by the evaluation of its measured data. In doing so, a self-built CSI as an exemplary measuring instrument is examined in a case study to test which of the material measures type ASG, AFL, ARS, ACG, AIR according to ISO 25178-70 and the chirp material measure (CIN) can be used to acquire detailed information about the characteristics that are specific to the optical setup of a CSI and their possible deviations. It can be shown that additional information about many characteristics including the properties of the light source like the wavelength or bandwidth, or information about the optical setup like the numerical aperture, can be extracted from a series of only seven measurements of the Universal Calibration Artefact.
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.
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