The invention relates to a sensor system and method for detecting deformation in a wind turbine rotor blade, and particularly to a temperature compensated system and method. A first optic fiber 22b is attached to the rotor blade such that deformation of the component acts on the first optical fiber and causes the optical path length of the fiber to change. A second optic fiber 27 is mounted loosely in the wind turbine component in such a way that it is not subject to deformation. A light source 10, a light detector 11, and a controller 12 input light into the two optical fibers and receive signals from them. Based on the detected light, the controller can determine the optical path length based on the light signals. In different embodiments, the light source varies the input wavelength over a range of wavelengths, provides a multi wavelength light signal, such as a white light source, or inputs a pulsed light signal. In further embodiments, the second optical fiber may be omitted if temperature effects are not significant.