European Metrology Research Programme (EMRP)
This project addresses the urgent need of the European optics and precision engineering industry and of basic research institutes for a significant improvement of accuracy and spatial resolution of the “absolute” surface form measurement of flats, aspheres and free-form surfaces. Absolute, in this context, refers to the measurement of optical surfaces delivering the full 3D form. Absolute form data are required, e.g. ,for the manufacturing of high-end asphere optics, strongly curved freeform surfaces used for special optics, for the calibration of measuring systems used in industry for in-production metrology, and for synchrotron and astronomical systems.
Although a variety of developed techniques is available for form metrology, absolute measurements are, up to now, only possible by employing single-point scanning methods, which scan the surface under test using specialised coordinate measuring machines. These systems have the capability to achieve traceable measurements even of strongly curved surfaces, but they lack full area measurements as only one-dimensional traces across the sample are measured. Furthermore, a full understanding and modelling of these instruments is still required to reduce uncertainty. In contrast, imaging methods like interferometry can assess a two-dimensional section of the surface under test with high resolution, but traceability to the SI unit ‘metre’ is a challenge. In case of strongly curved surfaces, only small sections of the surface can be measured and the form of the whole surface is obtained by appropriate scanning and stitching procedures.
The idea of this project is to further develop, in combination, the two different approaches of single point and imaging measurements. The combined development allows for the identification of systematic differences between the techniques and hence for the identification and subsequent elimination of systematic errors for each of them. As each approach has its specific advantages and application range, the combination will yield a complete description and characterisation of surface topographies. Comparison measurements performed at the same artefacts will help to validate the different measuring systems and will also provide improved standards for form measurement of optical surfaces which in turn are applicable to disseminate ‘optical form’ to industry for calibration of existing instruments.
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