Optical measurement techniques have received increased attention in the last few years. The main reasons for this development are the rapid improvements in data storing and processing, laser and semiconductor technology. In addition, the main feature common to all optical techniques is the non-intrusiveness, meaning that the measurements do not influence the investigated systems in any way, as conventional probes often do. This also implies the possibility of measurements where none existed before. Therefore it is expected, taking into account the further development of components for optical probes, that optical techniques will continue to gain in importance in many new fields of application.