  Electron-optical imaging instruments like Scanning Electron Microscope (SEM)
and Transmission Electron Microscope (TEM) use specially designed solenoid
electromagnets for focusing of electron beam probe. Indicators of imaging
performance of these instruments, like spatial resolution, have strong
correlation with focal characteristics of the magnetic lenses which in turn
have been shown to be functions of the spatial distribution of axial magnetic
field generated by them. Owing to complicated design of practical lenses,
empirical mathematical expressions are deemed convenient for use in physics
based calculations of their focal properties. So, degree of closeness of such
models to the actual field distribution determines accuracy of the
calculations. Mathematical models proposed by Glaser[1] and Ramberg[1] have
historically been put into extensive use. In this paper the authors discuss one
such model with secant-hyperbolic type magnetic field distribution function,
and present a comparison among these models, with results from finite element
based field simulations as reference.
