Abstract
A round robin comparison of freeform form measurements was carried out by the project partners and stakeholders of a European metrology research project. Altogether six measuring instruments were considered: five different (pointwise and areal) optical devices and one tactile device. Three optical freeform surfaces were used for the comparison measurements, where two specimens were measured by five instruments and one specimen by four instruments. In this paper, the evaluation methods and results of this round robin are presented for the three freeform surfaces made from a temperature-stable material, Super Invar ®. The freeforms had diameters of 40 mm, 50 mm and 100 mm and best-fit radii of 39.75 mm (convex), 40.9 mm (convex) and 423.5 mm (concave). For comparison, the bilateral pointwise differences between the available measurements were calculated. The root-mean-square values of these differences ranged from 15 nm to 110 nm (neglecting spherical contributions) and provided an insight into the status of typical freeform measurement capabilities for optical surfaces.
Highlights
Aspherical and freeform optics are essential parts of many modern optical imaging systems [1]
Most of the specimens used in these comparison studies were conventional spherical or aspherical surfaces made of glass
We present measurements provided by the Czech Institute of Plasma Physics (IPP), the German company Mahr GmbH (Mahr), the German Institute of Technical Optics at the University of Stuttgart (USTUTT), the German national metrology institute, the Physikalisch-Technische Bundesanstalt (PTB) and the National Metrology Institute of Japan (NMIJ)
Summary
Aspherical and freeform optics are essential parts of many modern optical imaging systems [1]. The processed measurements after subtracting the design form and an individual best-fit sphere formed the input data for the bilateral differences shown in Fig. 5 for the two-radii specimen, in Fig. 9 for the convex toroidal surface and in Fig. 13 for the concave toroidal surface.
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