the velocity of a fluid flowing through a flow channel , relative to the flow channel , is measured by passing parallel rays of light through the fluid perpendicular to the flow and through an artificially - introduced transfer function device such as a grating or double reflective mirror , and then focusing the light on a photodetector . the basic principle by which embodiments of the invention operate involves the spatial convolution of the light intensity pattern established by the artificially - introduced transfer function with the flowing stream . the transfer function is associated with a known spatial alteration of the light beam . the light intensity pattern resulting from variations in the properties of the flowing stream is characterized by fluctuations which are transported with the flow velocity . these variations of the flowing stream are manifested as localized changes in density , and absorptivity and reflectivity of the light as it traverses the flow channel . this convolution generates a new response function that appears at the electrical output of the photodetector which is periodic and whose base frequency is directly proportional to the flow velocity .