The invention comprises a turbulence sensor and sensor system for determining the condition of a wind turbine blade. A turbulence sensor comprises a sensor membrane provided in the surface of a wind turbine blade. A light source and light detector provided in a sensor cavity in the blade illuminate the sensor membrane and detect the light reflected back. The reflected light is mixed with non-reflected light to give an interference pattern indicative of the quality of the airflow, whether it is laminar or turbulent. The turbulence sensor can be used in a sensor system for detecting the accumulation of unwanted matter, such as dirt or ice, on the blade.