Phase contrast imaging

Phase contrast imaging with a transmission electron microscope (TEM). Physicists in Germany have taken a crucial step toward achieving higher-resolution images of biological samples and other “weak phase” objects typically made of low-weight elements like hydrogen, carbon, nitrogen, and oxygen. In a TEM, most of the electrons pass through the thin electron-transparent sample without scattering; those that are scattered have their phase shifted. Scattered and unscattered electron waves are then focused and recombined into an image. Unfortunately, for weak-phase objects the phase shifting is slight and results in poor contrast. Scientists at the University of Karlsruhe and the Max Planck Institute of Biophysics in Frankfurt have fabricated a special microstructured electrostatic lens. When placed in the objective lens’s back focal plane and with a voltage applied, the lens shifts the phase of the unscattered wave by 90°, as shown in the diffractogram, but leaves the scattered wave unshifted. The lens is called a Boersch phase plate in honor of Hans Boersch, who proposed the technique in 1947. (K. Schultheiss et al. Rev. Sci. Instrum. 77 , 033701, 2006 http://dx.doi.org/10.1063/1.2179411 .)