Reference surfaces for asphere and freeform measuring instruments
Characterizing measuring instruments for non-spherical optical surfaces
While it is possible to measure and trace back spherical surfaces very accurately, a traceable measurement of nonspherical surfaces with uncertainties in the nanometer range is difficult. In order to check the accuracy of the measuring instruments used for this purpose, special calibration surfaces have been developed. They have well-defined form properties that can be measured using established measuring instruments. These form properties must then also be reflected in the surface shape determined by the measuring device for aspherical/ freeform surfaces.
So-called multiradius surfaces, which combine sphere segments of different radii, have proven their worth. The respective radii of the individual spherical sections and the small but unavoidable deviations from the perfect spherical shape (sphericity) can, for example, be measured with Fizeau interferometers.
Thanks to close cooperation between the manufacturing and measuring departments, reference surfaces have been manufactured with great accuracy in PTB᾽s Scientific Instrumentation Department by means of diamond turning. The deviation from the design form was reduced to about 20 nm root-mean-square (rms). A nickel-phosphorus coating also makes the surfaces suitable for tactile measuring systems with low measuring force. The first test consisted in checking an optical asphere measurement system by means of such a multiradius surface. The sphericity measurements deviate from the calibrations by less than 10 nm rms.
Multiradius surfaces are therefore a suitable tool for characterizing measuring instruments for aspherical and freeform surfaces. In order to make this tool accessible to all users, the possibility of manufacturing reference surfaces outside PTB with the necessary accuracy is currently being investigated.
Contact
Ines Fortmeierbr /> Department 4.2
Imaging and Wave Optics
Phone: +49 531 592-4211
ines.fortmeier(at)ptb.de
Scientific publication
I. Fortmeier, M. Schulz, R. Meeß, Traceability of form measurements of freeform surfaces: metrological reference surfaces. Optical Engineering 58(9), 092602 (2019)