Ultrasonic fresnel lens for medical purposes - can be single or double sided or composite, with liquid filled chamber between halves

摘要

The method of transmitting ultrasonic waves for medical purposes uses a Fresnel lens. This has a series of concentric steps across its face with a convex surface at the centre. The steps are largest near the centre, decreasing in size towards the outside edge. By selecting a stage of a given height, the selected wave is superimposed in the correct phase on the focal point. The Fresnel lens may be a double lens with concentric ring shaped zones or steps, and its spherical abberration is corrected. The angle of the prism between separate zones can be varied between given limiting values. The lens is made of polystyrol. A lens with ring shaped steps on one side only can be used. Two such lenses can be combined, forming an intermediate chamber which is filled with liq. to improve the refractive index.