The present invention provides a copolycarbonate that has a low photoelastic constant and has fewer foamed cells and that is in particular suitable for use in optical fields. The control unit provides a normal guidance, instructing a progress direction guidance for the branch point, when none of the approach and exit routes is determined to correspond to a learned road link. The implantable medical device may include a target that creates an internal magnetic field for the compass to interact with. A weld region fixedly secures the base edge of the band to the cap plate.