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Interferometry and Radius Measurement of Spherical Sections

In the working group, the sphericity of spherical sections is measured with a Fizeau interferometer.

Beyond this, the radius of curvature is a fundamental quantity for optical elements. It can be determined by means of an aerial interferometer with focusing optics if the spherical section to be calibrated is moved along the optical axis (see Fig. 1). The interferometer detects a nearly fringe-free image in two positions. In the first position, the light beams reach the specimen perpendicular to the surface (confocal position). In the other position (cat’s eye position), the light is focused on the surface. At the focal point, the specimen’s surface acts as a nearly flat mirror, and every incoming beam is reflected symmetrically. The radius of curvature is derived from the distance between the two positions.

Prinzipskizze zur Radiusmessung
Fig. 1: Photo and schematic diagram of the radius measuring set-up

For the highly accurate determination of the two positions, they are calculated from measurement series that also analyze the wavefronts in the vicinity of the optimal positions. Depending on the apertures of the combination of the spherical section and the lens applied, uncertainties far below one micrometer can be achieved.