Abstract: An apparatus is provided for detecting presence of transient particulate in gas within a duct (1) comprises (i) at least one emitter (3) of illumination (4) capable of being swept over essentially the entire cross-section of the duct from outside and the duct, and (ii) at least one detector (6) for detecting presence and position of any sparkle of the illumination from any particulate within the illumination as the beam is being swept over the cross-section of the duct, wherein the detector is configured to be mounted externally of a duct as the beam is being swept over the cross-section of the duct and outside the zone projecting the swept area of the duct.