Aspheric lens design

In this paper, a method is proposed to design aspheric lenses, with which a transducer may have less thickness and be able to extend its range of focal zone. In the past, the simple lens maker's formula was used to design no more than a spherical or cylindrical shape lens to carry out beam focusing. The drawback of these two types of lenses is their limited range of focal zone. Studies show that an aspheric lens may enlarge the range of focal zone and requires less thickness, which could reduce the attenuation of sound waves propagating through it. Therefore, a "Vari-Focus Lens Shape Equation" is raised in this paper as the foundation for aspheric lens design. By giving a "Lens Focus Distribution (LFD) function" and based on thin lens assumption, calculation results show a wider range of focal zone. The interested focal length and focal range can be obtained by choosing a proper LFD function. Calculations for beam profiles on aspheric lenses and a single curvature lens were compared. It shows that aspheric lens gave better uniformity on axial pressure distribution, wider range of focal zone, and less thickness required.