Logo PTB
symbolic picture: "magazines"

Reference surfaces for asphere and freeform measuring instruments

Characterizing measuring instruments for non-spherical optical surfaces

PTBnews 1.2020
Especially interesting for

manufacturers of asphere and freeform measuring instruments

optics manufacturers

Non-spherical optical surfaces, i.e. aspheres and freeform surfaces, are nowadays essential components of many modern imaging systems in industry and research. To test high-precision measuring instruments for the measurement of aspheres and freeform surfaces, special reference surfaces have been developed which consist of several spherical segments with differing radii. The first prototypes have been manufactured and calibrated at PTB with high accuracy. They were used for the first time to check an asphere measuring device within the scope of the FreeFORM EMPIR project.

Left: Representation of a multiradius surface. For illustration purposes, the spherical segments of the surface have been raised. Right: Photo of a realization. The diameter is 40 mm.

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.


Ines Fortmeierbr /> Department 4.2
Imaging and Wave Optics
Phone: +49 531 592-4211
Opens window for sending emailines.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)