Theoretical Aspects of the Fresnel Focusing Technique

Recent developments of real time echographic device have shown the interest of multi transducer focusing arrays. Large focusing aperture must be synthetised to obtain a good resolution. The use of delay lines, allowing a converging cylindrical lens to be simulated, is one of the classical approach of the focusing problem in the echographic mode. A continuous variation of the focal length of such a lens allows a good focusing on all the echographic range. However, if the dimension of the lens aperture is small enough, the sequential use of a discrete number of focal lengths is sufficient to cover the entire depth of field.