A detector device for detecting the passage of articles of a conveyor comprises an energy source element 8, an energy responsive element 10 and reflector means 27, 9 and 28, positioned so that a beam of energy from the source element 8 to the responsive element 10 first traverses the conveyor path substantially parallel and close to the conveyor surface and is then reflected back and fourth across the conveyor path in a plane substantially perpendicular to the conveyor surface.