Method and apparatus for applying (15) radiation through one of members (17; 19, 20) to be joined by heat welding to enter layer(s) (18) interposed between the two members (17; 19, 20) relies on inclusions through thickness of the layer(s) (18) to be responsive to said radiation to raise temperature through thickness and promote bonds at interfaces with surfaces of said members (17; 19, 20). Resulting is assessed using thermal imaging (16) relying on different radiation from the layer(s) (18) from that for heating, specifically mid-infrared compared with near visible.