With conventional optical sensors, narrow and deep features, such as air gaps on electric motors or the roundness of fine injection nozzles, are often not measurable.  Laser distance sensors, chromatic focus sensors and confocal sensors fail as a result of the lens’ aperture, and oftentimes the working distance is too small.  With the Werth Interferometer Probe (WIP), a highly precise optical fiber sensor is now available that measures via interference. The WIP uses the physical effect of interference to determine the distance between the end of the optical fiber and the work piece surface. The reference beam is reflected by the optical fiber’s end, and the measuring beam is created through the reflection from the surface of the work piece and is superimposed with the reference beam. The light is generated using a super luminescent diode (SLD).  This provides for a highly precise interferometric measurement of the position of the work piece surface. At the same time a beat signal is generated through the different SLD wavelengths.  Since this beat signal has a significantly greater wave length than light, the sensor’s measuring range is in- creased to technically useful distances.  The evaluation interferometer splits the light beam with the beat signal into two beam paths of differing path lengths.  This balances the path length difference of both light beams so that they are able to interfere.  By tilting one of the two mirrors, the axial location information of the beams (depending on the position of the work piece surface) is converted into information for lateral evaluation.  Depending on the position on the mirror at which the beam is reflected, different lateral intensities arise, producing interference patterns.  These are captured by means of a camera, and then the distance between the reference surface and the work piece surface is derived.