Magnetic Electron Microscope Projector Lenses

The results of an earlier paper are extended to the special case of the electron microscope projector and similar type lenses It is shown that the focal properties of symmetrical projector lenses with different ratios of gap width S to lens bore D can be represented by a single curve. This is then applied to the discussion of various cases of practical interest. The calculated results are found to be in excellent agreement with measurements by Ruska The question of image distortion as a function of design parameters and lens excitation is then discussed; it is shown that all the types of lenses investigated can be operated at a point where they are free from `radial distortion'. A consideration of scaling operations shows that there is little difference between the optimum performance of lenses of different geometry, but equal focal length. Certain results of use in practical lens design are combined in a design chart from which the relevant design or operational parameters can be read directly. In conclusion, design rules are given for the optimum design of projector lenses covering a wide range of magnifications