The design of three-element electrostatic electron lenses

A procedure is described by which the focal properties of a three-element lens can be determined in terms of the properties of two-element lenses. Examples are given in some detail of the design of lenses to meet specific requirements and a graph is presented which can be used to design a lens of unit magnification which will work over a specified extreme voltage ratio in the range 6·25:1 to 170:1. The range of applicability of the method is discussed and a comparison made with measurements by Imhof and Read where only a limited agreement is expected. This comparison supports both the method and its limitations.