Abstract: A laser doppler anemometer includes a source producing a laser beam which is then split into a plurality of partial radiation beams. A common optical image-forming system illuminates a measuring volume by the partial radiation beams and receives the back-scattered signal radiation therefrom which is directed into a detector, and a partially reflecting mirror arrangement is positioned in the common optical imageforming system for the purpose of masking out scattered signal radiation leaving the common optical image-forming system. The partially reflecting mirror arrangement can comprise two mirrors disposed plane parallel to each other, and it can include light transmissive elliptical rings or strips. A pinhole diaphragm, adjustable in a direction orthogonal to the optical axis of the detector and through which the back-scattered signal radiation passes is also provided and precedes the detector.