portion 41a which forms a transition from the neck 41 to the main part of tubular outer end portion 35 forms an orifice or opening 48 that can be controlled in size by positioning the neck 41 in a desired location along the end portion 43 of the sight tube sleeve 42 to provide an adequate size opening for secondary flow of the purging fluid that comes from the pressurized chamber 14 and passageway 38 the flow of purging fluid through apertures 45 carries particles entrained in the purging fluid downstream without having the particles pass over the lens surface 28a as can be seen in fig2 there is a continuous annular gap forming a slot 50 between the lens 28 and the end surface 44 of the sight tube sleeve 42 the sight tube sleeve 42 has an interior passageway 52 that forms an outflow path for the major flow of purge air the sight tube sleeve 42 is supported rigidly with respect to the pyrometer barrel 27 through the outer tubular portion 35 tapering portion 41a and neck 41 there is no need to have bridging web or both portions extending between the sleeve 34 and the sight tube sleeve 42 the annular slot 50 provides for unobstructed flow inwardly across the lens outer surface 28a the end surface 44 is positioned so the spacing of surface 44 from the lens 28 is controlled to provide a directional path for air flow in generally radially inward direction from the edges of the lens.