2D-Laser Doppler Velocimeter at a flow channel. Two pairs of laser beams, slightly different in color/wavelength, for measuring the longitudinal and vertical component of the flow velocity at the point of intersection. The object lens is fiber-optically coupled to the laser system (not shown) facilitating scanning movements of the object lens together with the mechanically connected detector at the opposite side of the flow channel. The air inside and outside the channel is loaden with smoke to show the beams (note the apparent break where the beams pass through the wall of the channel).