Abstract: A diagnostic apparatus and method for measuring motion of flowing solid and fluid particles that utilizes a differential doppler system and makes measurements parallel to the optical axis of the system. The system includes a laser generating a light beam directed toward a light beam splitter. The splitter separates the light beams into a plurality of beams that are focused in a crossing and focusing area. Receiving optics aligned with a photodetector sense light scattered by particles moving in the focusing area. The photodetector converts the light signals to electrical signals. A signal mixer coupled to the photodetectors operates to produce a difference electrical signal established by the difference between the frequencies of the electrical signals produced by the photodetector. The difference electrical signal is converted with signal processors to a voltage proportional to the velocity of particles moving through the focusing area.