Scherzer's formula and the correction of spiral distortion in the electron microscope.

Scherzer's general expression of 1937 for the spiral distortion coefficient of electron lenses is used to calculate the spiral distortion of the rectangular-field model for the axial flux density distribution in magnetic projector lenses. The results show why it is difficult to correct the spiral distortion produced by conventional lenses. On the other hand, corresponding calculations for single-polepiece projector lenses indicate that the employment of such lenses could lead to a significant improvement of the projector stage of the electron microscope, especially that of the high voltage electron microscope.