B.S.T.J. Briefs: AC focusing of an electron microscope objective lens

Control of phase contrast in electron microscope images is presently achieved by fine vernier control of the dc objective lens current. An incremental change δJ in lens current produces a corresponding increment δf in focal length and a focal separation in object space of δL
o
. L
o
is the distance from lens to conjugate object plane so that ΔL
o
is the axial separation of conjugate object plane and scattering specimen. The defocus phase at the image plane of a ray scattered at angle β relative to the axial ray is
$\chi ={\vert K \vert \over 2} \triangle L_{\deg}\beta^{2} \qquad \eqno{hbox{(1)}}$
with |K| = 2π/λ. The phase contrast G in the image goes as
$G \sim {\rm sin} \chi {\vert K \vert \over 2} \triangle L \deg \beta^{2} \qquad \eqno{\hbox{(2)}}$
ignoring spherical aberration, astigmatism, etc., for this purpose. Precise control of phase contrast for a spacing “a” = λβ
⁻¹
requires that the focal separation increments δL
o
be as small as 100 å and preferably less.
^{1, 2}