A sensor system for detecting the surface condition of a wind turbine component having a cavity 13, comprising: a plurality of turbulence sensors 10 for giving an output indicating turbulent air flow across a surface of the wind turbine component; a memory; and a processor; wherein each turbulence sensor comprises: a sensor membrane 15, for detecting the turbulence of air flow past a surface of the wind turbine component, wherein the sensor membrane 15 is integral to the surface, and covers at least part of the cavity 13; a light source 16 located in the sensor cavity 13 for illuminating the surface of the sensor membrane 15 inside the cavity 13; and a light detector located in the cavity 13 for detecting light reflected from the surface of the membrane 15, and for providing an output to a processor, the processor determining from the output a turbulence value for the air flow across the sensor membrane; wherein the processor is adapted to determine a condition of the component surface from the output received from the turbulence sensors 10; wherein the memory is adapted to record the outputs received from the turbulence sensors 10 over a period of time, and the processor is adapted to determine a condition of the component surface based on a change in the outputs compared with past outputs; and wherein the sensor system is adapted to detect the accumulation of matter on the component surface by determining whether a predetermined number of the turbulence sensors persistently indicate turbulence. a plurality of turbulence sensors 10 for giving an output indicating turbulent air flow across a surface of the wind turbine component; a memory; and a processor; wherein each turbulence sensor comprises: a sensor membrane 15, for detecting the turbulence of air flow past a surface of the wind turbine component, wherein the sensor membrane 15 is integral to the surface, and covers at least part of the cavity 13; a light source 16 located in the sensor cavity 13 for illuminating the surface of the sensor membrane 15 inside the cavity 13; and a light detector located in the cavity 13 for detecting light reflected from the surface of the membrane 15, and for providing an output to a processor, the processor determining from the output a turbulence value for the air flow across the sensor membrane; wherein the processor is adapted to determine a condition of the component surface from the output received from the turbulence sensors 10; wherein the memory is adapted to record the outputs received from the turbulence sensors 10 over a period of time, and the processor is adapted to determine a condition of the component surface based on a change in the outputs compared with past outputs; and wherein the sensor system is adapted to detect the accumulation of matter on the component surface by determining whether a predetermined number of the turbulence sensors persistently indicate turbulence. a sensor membrane 15, for detecting the turbulence of air flow past a surface of the wind turbine component, wherein the sensor membrane 15 is integral to the surface, and covers at least part of the cavity 13; a light source 16 located in the sensor cavity 13 for illuminating the surface of the sensor membrane 15 inside the cavity 13; and a light detector located in the cavity 13 for detecting light reflected from the surface of the membrane 15, and for providing an output to a processor, the processor determining from the output a turbulence value for the air flow across the sensor membrane; wherein the processor is adapted to determine a condition of the component surface from the output received from the turbulence sensors 10; wherein the memory is adapted to record the outputs received from the turbulence sensors 10 over a period of time, and the processor is adapted to determine a condition of the component surface based on a change in the outputs compared with past outputs; and wherein the sensor system is adapted to detect the accumulation of matter on the component surface by determining whether a predetermined number of the turbulence sensors persistently indicate turbulence.