A wind turbine blade measurement system for optically determining a torsion of a wind turbine blade is disclosed. The wind turbine blade measurement system comprises: a wind turbine blade, which is configured to be mounted to a hub of a wind turbine, a first camera, and an auxiliary camera. The first camera is mounted in a fixed position on a support structure on an exterior surface of the root section of the wind turbine blade and arranged so as to measure along the spanwise direction of the wind turbine blade. The auxiliary camera is arranged at a position outside of the wind turbine blade, the auxiliary camera being arranged so as to being able to carry out measurements of a plurality of sets of markers arranged on the surface of the wind turbine blade and an orientation of at least one of the support structure and the first camera.