Technical advances in ultra-precision machining of freeform optics by function-oriented testing
Technische Vorteile bei ultrapräziser Bearbeitung von Freiformflächen von optischen Geräten mit einem funktionsorientierten Prüfen
Freeform geometries enable new systems in geometrical optics by combining the functionality of different optical components and, thus, reducing the required number of components in the system. The main problem in using that kind of freeform shaped optics is the deterministic manufacturing of these geometries and their evaluation in measurement. While ultra-precision diamond turning assisted with Fast Tool Servo Systems is shown to be a suitable process to manufacture freeform geometries for the application of reflective and refractive optical components, till recently it was difficult to assess the quality of the manufactured work pieces in means of form accuracy. While it was known that Fast Tool Servo Systems can position very accurately in the submicron range, the measurement of the produced work pieces was another issue. In the first case just the linear positioning of the mechanical axis has to be measured - in the latter case, a 3D measurement of a surface is required. The fringe deflectometry measurement technique shown in this paper fills this gap and gives feedback for implementing corrections in the manufacturing process. Surface deviations are measured and their reasons can be analyzed. With this information the causes for systematic manufacturing errors can be eliminated or compensated for. The paper exemplarily shows that approach in manufacturing, measuring and correction the production of a freeform mirror for illumination optics.