NOVELTY - The wind turbine has an effective source of electro-magnetic (EM) radiation transmits an EM radiation signal. A receiver (32) receives the EM radiation signal. The receiver spaced apart from the effective source in a spanwise direction of blade (24a,24b,24c). The effective source and receiver are arranged such that deflection of blade results in a straight-line distance between effective source and receiver varying. A processor determines a parameter indicative of blade deflection and blade loading on the basis of irradiance of EM radiation signal received by receiver.    USE - Wind turbine for determining parameter indicative of blade deflection and/or blade loading.    ADVANTAGE - The amount of background noise of the wind turbine is reduced. The operation of the wind turbine is improved. The accuracy is improved with which the deflection of the blade tip relative to the root is determined. The diode lasers are compact and light, and convenient for use with a wind turbine. The use of a non-electrically conducting waveguide reduces the amount of electrically conducting apparatus present in the wind turbine thus reduces the likelihood of the wind turbine being struck by lightning in diverse weather conditions. The configuration of the wind turbine improves the accessibility for maintenance purposes. The weight of the system is reduced. The use of a protective housing protects the receivers from environmental pollutants such as dust which could result in erroneous results and reduces the amount of maintenance required. The accuracy of the determined bending angle of the blade tip is improved by positioning the transmitter in closer proximity to the blade tip.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for a method for determining a parameter indicative of blade deflection and blade loading of a rotor blade of a wind turbine.    DESCRIPTION OF DRAWING(S) - The drawing shows a perspective view of a rotor-hub assembly as featured in a horizontal axis wind turbine.    Rotor-hub assembly (22)    Blades (24a,24b,24c)    Central hub (26)    Direction arrow (28)    Receiver (32)