The F01 is a sensor head type with focused probe beam. This allows to measure displacements of small targets. Furthermore, the sensor heads provide high angular tolerances. After pre-collimation, an integrated beam splitter splits the light into a reference and probe beam as shown in Figure 1. The reference beam is reflected off the reference mirror which is coated to one side of the beam splitter cube. The probe beam is focused by a lens system and exits the head. The front surface of the beam splitter marks the absolute zero position of every PICOSCALE measurement as here the probe and reference beam are of equal length. Thus, the working distance (WD) is not equal to the distance from the head (i.e. focal length F) since the lens system is only integrated into the probe beam.