Contrast enhancement in the phase plate transmission electron microscopy using an objective lens with a long focal length.

A new optical condition using an objective lens (OL) of a long focal length (objective mini lens: OM) was tested to enhance image contrast in phase plate transmission electron microscopy (P-TEM). A phase plate was set on the selected area aperture plane where diffraction patterns were formed under the optical condition using the OM. A phase shift by the phase plate was added to the electron waves to visualize phase objects. The application of the OM to the P-TEM should provide higher phase contrast than that obtained by the OL for the phase objects. One of the reasons for the contrast enhancement is that high-angle scattering electron waves which would give the background intensity were not used for image formation due to the large spherical aberration. Another reason is that the cut-on frequency above which the phase shift was added by the phase plate could be smaller using the OL with a long focal length. Experimental results and model calculations showed the contrast enhancement of the biological specimens using the OM.