Abstract: For determining flow motions in a fluid containing light scattering particles, in particular for determining the blood circulation in the superficial blood vessels in a tissue, a section of the fluid is illuminated with monochromatic light from a laser (1). Light scattered from particles in the fluid and from any surrounding stationary structures is gathered from two, at least partially separated but mutually adjacent regions of the illuminated section of the fluid and transmitted separately to two separate photodetectors (5, 5'). From the output signal of each photodetector a signal is derived, which contains the beat frequency components resulting from interference between light components received by the photodetector, which have different frequencies due to the Doppler frequency shift of the light scattered by moving particles.