## Condenser lens Together with electron guns, condenser lens are parts of the **illumination system**. In most TEM, two lenses exist for electron beam condensing. - Condenser lens C1 controls the spot size (by adjusting back focal plane position), and controls the convergence angle. - Condenser lens C2 controls the brightness of the electron beam. ![[Drawing 2023-09-30 16.31.01.excalidraw.svg]] But the actual function depends. Adjusting C1 and C2 leads to the tread-off of beam coherence and brightness -- only center part passing the aperture, better coherence could be achieved but less brightness, and vice versa. When adjusting stigmator coils for condenser system, we're adjusting C2 lens. Below the C2 lens we have the condenser lens aperture. To have the proper set up, centering of the electron beam and the aperture is always required. In some machine, we may also adjust the size of condenser lens aperture to control the brightness, when we're looking at extremely small regions and do not want the sample/camera get destroyed. For those TEM with STEM function, deflector coils are installed to tilt the beam, and with two such coils combined, the beam could be shifted (tilt once then tilt back) or, combining with C2, focus a spread beam into a small point. This allows us to condense the beam on a single point other then the axis, and scan the sample. This is called [[STEM]] mode. ![[Drawing 2023-09-30 16.41.36.excalidraw.svg]]