cathode wehnelt cap anode condenser lens stage with sample objective lens intermediate lenses projector lenses fluorescent screen camera

transmission electron microscopy (TEM) und cryo-TEM Source of illumination is a thermo ionic gun consisting of a cathode (tungsten, LaB6), a wehnelt cap and an anode. Electrons produced by heating the filament are accelerated towards the anode. The electrical negatively charged wehnelt cap causes the emitted electrons to be directed towards the optical axis. The accelerating voltage between cathode and anode is between 10 and several 100 kV. The condenser lens focuses the monochromatic electrons into a coherent beam in the sample plane. With an accelerating voltage of 100 kV the thickness of the sample may not exceed 100 nm. The transmitted electrons are focused by the objective lens into a first intermediate image. The objective aperture enhances the contrast by blocking out high-angle diffracted electrons . The intermediate lenses allow the switch between imaging and diffraction mode and the adjustment of the magnification. The projector lenses generate a real and enlarged image of the sample on the phosphor screen. Images and diffraction patterns can directly be observed on the phosphor screen or via CCD camera mounted below the microscope column. They can be recorded on imaging plates or by the CCD.