The correction of spiral distortion in the transmission electron microscope

Abstract

The present investigation is concerned with the improvement
of the image viewing and recording system of
the transmission electron microscope, by means of a wideangle
projector lens of low distortion.: A critical investigation
of a previously proposed system for the
correction of distortion by means of conventional magnetic
lenses has been carried out; this showed that such lenses
are not capable of providing a distortion free image with
a projection semi-angle greater than about 10°. Theoretical
and experimental investigation has shown that with
projection semi-angles in the region of 30°, third order
aberration theory is inadequate and can even be misleading.
It was therefore desirable to calculate the actual axially
symmetric trajectories of the electrons forming the projected
image. Special attention was therefore paid to
the development of a computer program for calculating the
electron trajectory in a system of magnetic lenses with
the aid of the general ray equation. Such a calculation
allows one to determine the appearance of the final image
directly without recourse to aberration theory. This
direct image simulation has been achieved by using the
output of the general ray program to print out the image
of a square mesh grid on an x-y plotter. This enables a
direct comparison to be made between the calculated and
observed images. An experimental wide-angle lens has
been constructed by computer-aided design, using the above
principle. Essentially distortion-free images have been
obtained over a semi-projection angle of 30°. It is
believed that this represents an ultimate limit for a
twin-lens design of this type. The projection lens system
can find immediate application in TEMs with transmission
fluorescent screens and also in high voltage electron
mircoscopes.

Date of Award	1982
Original language	English