A New Reflection Lens for Acoustic Microscopy

In ultrasonic NDE it is convenient to have a plane interface between the focussing probe and the test object. We describe an acoustic lens with a planar front face, based on total internal reflection from a curved surface. It is closely related, in principle, to a type of X-ray lens which relies on reflection at glancing incidence from a paraboloidal surface. The lens is in the shape of a truncated paraboloid of revolution. A transducer on the back face emits plane waves which are focussed by reflection from the curved surface, and come to a focus some distance in front of the lens. Theoretical studies show the lens to have a narrow annular angular aperture. This property is advantageous for internal focussing with long depth of field within solids. The theoretical results lead to a detailed appraisal of the focussing properties. lens are in satisfactory agreement with the predicted results. An alternative version of the lens is in the form of an axicon, in which longitudinal: shear mode conversion is used in order to gain a factor of two in the numerical aperture of the lens. Experiments carried out on a 4 MHz