Embodiments include a flow measuring system for measuring characteristics of a fluid flow. The flow measuring system can include a pair of rotating members rotating synchronously to the fluid flow. At least one rotating member can include a normally-reflecting portion and a non-normally reflecting portion. The normally-reflecting portion reflects an incident beam in a direction parallel to the normal direction of the plane of the normally-reflecting portion. The non-normally reflecting portion reflects the incident beam in a direction non-parallel to the normal direction of the plane of the normally-reflecting portion. The flow measuring system measures the optical characteristics of a beam reflected by the normally-reflecting portion and determines rotational characteristics of the rotating member. The flow measuring system determines characteristics of the flow based on the rotational characteristics of the rotating member.