1,222,355. Ultrasonic probes; ultrasonic inspection devices; acoustic lenses. S.T.D. SERVICES Ltd. 18 March, 1968 [21 March, 1967], No. 13198/67. Headings H4D, H4J and H4X. A probe for use in ultrasonic flaw detection incorporates a flat piezo-electric transducer 10 (Fig. 1) and a sound refractive lens 12 in acoustic contact with the transducer 10, the lens 12 being shaped to produce a focused ultrasonic beam which is of elongate cross-section with a substantially even intensity distribution across the cross-section at the focus. The transducer 10 is a narrow lead zirconate titanate element (20mm. by 5 mm.) supported on abase 11 of tungsten loaded epoxy material. The lens 12 is of polymethyl methacrylate and has a face 12a of convex part-spherical form and a face 12b of concave part-cylindrical form. Face 12a is acoustically coupled to transducer 10 by sound transmitting adhesive 13. The axis of the cylindrical surface is parallel to the length of the transducer 10. A line focused beam is produced having a long narrow cross-section. With the probe transducer axis parallel to the length of expected longitudinal flaws, a flaw echo signal, produced by reflection of waves from the same or a different probe, will vary markedly with flaw length of the same order of length as the transducer, as well as with flaw depth. An associated flaw detection instrument is calibrated with a " standard flaw." On detection of an unacceptable flaw an alarm is sounded or the test piece is marked. A shorter flaw of the same depth will not be indicated, nor will a shallower flaw of the same length. The instrument will indicate all flaws longer and deeper than standard and those which are longer or deeper and not significantly shallower or shorter. A flaw which is considerably shorter than standard but also considerably deeper will be indicated. Line focusing may alternatively be obtained with square and circular transducer wafers. In another arrangement the lens 14 (Fig. 5) has a planar face coupled to an elongate transducer 10 by adhesive 13 and the other lens face is convex in the direction of the transducer length and concave in the transverse direction.