Abstract: A method of fluid flow velocity measurement includes emitting a beam from a light source via a first window into a pipe through which a fluid flows, the beam illuminating the fluid flowing in the pipe, using a light detector array, which is coupled via a second window to the pipe and which is outside a field of view of the light detector, to detect light caused by scattering of the beam with particles found in the fluid, and determining a velocity of the fluid flowing in the pipe as a function of signals from the light detector array.