Abstract: An assembly which comprises (a) a duct (22) for transmission of fluids, said duct having longitudinal axis, (b) at least one emitter (301) capable of exciting a beam of radiation (21) in the form of a cone having root angles ?ex and ?ey, which may be the same or different, into a cross section area of a duct to be analysed in a direction downstream of fluid flow in the duct at an angle ?e from the plane perpendicular to the longitudinal axis of the duct, the angle ?e being between 5° to 45°, (c) a detector (23, 304) sensitive to radiation radiated by scattering centres in the fluid over at least 1% of the cross sectional area of the duct, the direction of the central axis of the emitter cone being at an angle ? to the direction of the central axis of the detector cone, the detector cone having root angles ?dx and ?dy, which may be the same or different and the detector cone having angle ?d perpendicular to the longitudinal axis of the duct, the direction of the detector being downstream of fluid flow in the d