Possibility of a phase contrast electron microscope.

In-focus phase contrast has been demonstrated in the electron microscope using an arrangement analogous to that of the Zernike phase contrast light microscope. Thin carbon films with a central hole were placed in the back focal plane of the objective lens so that the scattered electrons were selectively phase-shifted by the film. The maximum phase contrast was obtained when the film thickness was adjusted to give a retardation of about pi/2 to the scattered electrons and appears to be due to the elastically scattered electrons. The observed contrast was about one-half that calculated taking into account the scattering of both the object and the phase plate and making the assumption that the inelastic scatter was incoherent. Improved phase contrast should be obtained if the nonscattered intensity is reduced by a beam stop and if phase-shifting can be accomplished by a small electrostatic lens rather than by a film. An objective lens ofthe smallest available spherical aberration is required. The in-focus phase contrast arrangement may provide useful contrast for thick (>2000 A) unstained objects in the l.0-MeV microscope. A combination contrast mode is recommended for conventional (100-kV, microscopes where amplitude contrast and enhancement of phase contrast are provided by filtering out the inelastic scatter.