A wind turbine comprising a number of blades including at least a first wind turbine blade extending substantially radially from a hub on a main shaft having a substantially horizontal centre axis, the blades together with the hub constituting a rotor with a rotor plane, and which can be put into rotation by wind, and each blade having an innermost part comprising a blade root of the blade and an outermost part comprising a blade tip of the blade, wherein the wind turbine comprises an optical measurement system comprising a light source, such as a laser, an optical transmitter part (675, 676, 677), an optical receiver part, and a signal processor, wherein - the light source is optically coupled to the optical transmitter part, - the optical transmitter part comprises an emission point and is adapted for emitting light in a probing direction from said emission point, - the optical receiver part comprises a receiving point and a detector, wherein the optical receiver part is adapted for receiving a reflected part of light from a probing region along the probing direction at the receiving point and directing said reflected part of light to the detector so as to generate a signal from the detector based on the received, reflected light, and - the signal processor is adapted to determine at least a first velocity component of the inflow from the signal generated by the optical receiver part, - the emission point of the optical transmitter part is located in the first wind turbine blade at a first radial distance from the substantially horizontal centre axis, and - the receiving point of the optical transmitter part is located in the first wind turbine blade at a second radial distance from the substantially horizontal centre axis, the first wind turbine blade further comprises adjustable flow altering means, such as distributed actuators, flaps or microtabs, for adjusting an aerodynamic parameter of the blade and located in a third radial distance from the hub, the adjustable flow altering means being controlled by a controlling means, and wherein the controlling means are adapted to receive a signal from the signal processor, the signal from the signal processor being based on at least the first velocity component, the wind turbine being characterised in that the third radial position is substantially identical to the first radial position, and wherein the optical measurement system is adapted to emit at least a first probing beam and a second probing beam, wherein the first probing beam and the second probing beams form a probing angle lying in an interval of 5-90 degrees. - the light source is optically coupled to the optical transmitter part, - the optical transmitter part comprises an emission point and is adapted for emitting light in a probing direction from said emission point, - the optical receiver part comprises a receiving point and a detector, wherein the optical receiver part is adapted for receiving a reflected part of light from a probing region along the probing direction at the receiving point and directing said reflected part of light to the detector so as to generate a signal from the detector based on the received, reflected light, and - the signal processor is adapted to determine at least a first velocity component of the inflow from the signal generated by the optical receiver part, - the emission point of the optical transmitter part is located in the first wind turbine blade at a first radial distance from the substantially horizontal centre axis, and - the receiving point of the optical transmitter part is located in the first wind turbine blade at a second radial distance from the substantially horizontal centre axis, the first wind turbine blade further comprises adjustable flow altering means, such as distributed actuators, flaps or microtabs, for adjusting an aerodynamic parameter of the blade and located in a third radial distance from the hub, the adjustable flow altering means being controlled by a controlling means, and wherein the controlling means are adapted to receive a signal from the signal processor, the signal from the signal processor being based on at least the first velocity component, the wind turbine being characterised in that the third radial position is substantially identical to the first radial position, and wherein the optical measurement system is adapted to emit at least a first probing beam and a second probing beam, wherein the first probing beam and the second probing beams form a probing angle lying in an interval of 5-90 degrees.