The project aimed to develop a preliminary configuration of an optical sensor that is able to detect sub-mum-degradations of microstructured coatings (e. g. riblets) with highest accuracy. The availability of such sensor is mandatory for the establishment of drag-reduction riblet structures on aircraft wings and/or body components. Any type of degradations of the riblet structure are unfavourable for their functionality and negatively affect the aspect of fuel savings. This makes a sensor necessary, that allows to measure the state of riblet degradation with highest accuracy in the course of aircraft maintenance.

We proposed an optical sensor whose principle function is based on a simple physical phenomenon: the scattering of light. This sensor probes the riblet surface with a coherent laser beam and analyses the scattering pattern in direction of reflection.

Particularly the angular intensity distribution of the scattering pattern is unambiguous for the shape of the microstructure. Any type of riblet-degradation is causally related to an alteration of the scattering pattern. Taking into account a periodic riblet microstructure of triangular shape with degradation, the scattering intensity distribution becomes complex. Hence, a great effort is required for the clear determination of type and state of degradation from the scattering pattern.

We addressed this task by combined theoretical and experimental studies of the scattering pattern. Theory yields calculations of the scattering pattern in the far-field for a variety of riblet shapes, spatial frequencies and for both non-degraded and degraded riblets. These calculations were compared with experimental studies on differently coated riblet structures, i.e. on real structures. Parameters that are characteristic for the device function (accuracy and signal-to-noise ratio) were defined. The project ended with a preliminary configuration for an optical sensor with optimum specifications.