An optical system design for measuring the velocity of fluids flowing through pipes or other conduits is disclosed. The optical system is comprised of a means for delivering two beams through a window in the wall of the pipe, focused to two points aligned along an axis of the pipe and separated by a known distance, and means for detecting light that is scattered by particles carried in the fluid stream through a second window, that is disposed on the opposite side of the pipe. By measuring the time delay between detected signals, the velocity of the fluid can be determined. The delivered light beams are focused in a shallow cone of light and are blocked by an obstruction disposed behind the second window. The scattered light passes through an aperture behind the second window that surrounds the obscuration, and is focused on to a detector surface.