The invention relates to a light-emitting apparatus (100) comprising an optical waveguide, particularly an optical fiber (1), for guiding a primary light beam (Bprim) into a light-splitting unit (101) which splits it into two or more partial light beams (B1, B3, B4) which are emitted in different directions and have different optical qualities, e.g. different spectral compositions or polarizations. The apparatus may optionally comprise a detector (4) for determining a Doppler shift (Δλi) in reflected light reentering the light-splitting unit (101). This renders it possible to measure simultaneously two or more spatially independent vector components of the flow velocity of a fluid, particularly of blood, surrounding the light-splitting unit (101).