Measurment of aspheres and freeforms

The use of aspheric and freeform surfaces becomes more and more important in the design of modern optical systems. These surfaces offer additional degrees of freedom to the optical design, allowing to improve the optical imaging as well as to reduce the number of surfaces needed for an optical design. However the fabrication and testing of such surfaces is still a difficult task. At the ITO we developed and patented the so called Tilted Wave Interferometer [1][2][3] (TWI) which makes it possible to measure these kinds of surfaces. The main advantage of the TWI as compared with other available systems is the short measurement time. Furthermore, the TWI has the possibility to not only test rotationally symmetric aspheres, but also freeform surfaces, a feature rarely found in commercially available systems. To verify the measurement results of the TWI, we compared them with the results from two different measurement methods. As test surface we used an asphere with a diameter of 40 mm. The deviation of the surface from the spherical form was 600 µm peak to valley, with a maximum slope of 8°. The surface was measured on the TWI (fig. 3), on a Zygo Verifire Asphere which is another interferometric system (fig. 2), as well as on a tactile UA3P measurement machine form Panasonic (fig. 1). It could be shown that the measurement results of the TWI correspond to those of the two commercial systems. The advantage of the TWI is, the huge enhancement in the time needed for the measurement. Unlike the UA3P which needed 42 minutes for the measurement or the Zygo interferometer, where the process took 8 minutes, the TWI only needed about 30 seconds to measure the whole surface. The possibility for a fast measurement is of high importance for the introduction of a system into the automated fabrication of optical elements. In a running BMBF project we bring the interferometer from its laboratory state into a demonstrator setup, which will be working under industrial conditions. Further we are working on the extension of the measurement process to freeform surfaces, as well as to stitching, which is necessary for the measurement of larger surfaces.

We thank the Bundesministerium für Bildung und Forschung for the support (project Mesofrei, (FKZN 13N10854).