Sketch of the two polarimetric measurements for GRAVITY. Left: in the first measurement, the light from the metrology laser is used and measured with a polarimeter. Right: the light from the calibration unit is used and recorded on the spectrometer. In both cases, the light direction is indicated by arrows. The light path is the same as shown in Fig. 10, with the reflection for the Tip-Tilt and Piston control just shown as a change in direction in the light path.