The column
After being accelerated away from the source, the electron beam is directed through the centre of the electron optical column. The column controls how much current passes through, focusses the beam, and scans it across the region of interest on the sample. It achieves this with three components: the condenser lens, the objective lens, and the scanning coils.
Electron optical system
The condenser lens is the first part of the optical system after the source. It converges the electrons into a tightly focussed beam, usually onto a circular aperture of a known size. This aperture helps to define the size of the beam as it passes through the rest of the column, ultimately determining the final beam spot size and intensity at the sample.
If the condenser lens converges the beam right at the position of the aperture, then most of the beam current will pass through, giving a high intensity, but a larger spot size (poorer resolution).
If it is converged above the aperture, then less beam will pass through, reducing intensity but smaller spot size (better resolution).
This lens is controlled by Spot Intensity on the FlexSEM.
The objective lens is the final lens in the electron optical column. It has the job of tightly converging the electron beam onto the sample. This is called focussing. When the lens converges the beam correctly at the sample surface, the image will be in focus. If it converges the beam above or below the surface, it will be out of focus.
This lens is controlled by Coarse and Fine Focus.
These are positioned just above the objective lens, and deflect the electron beam in X and Y. This component of the optical system is responsible for generating the electron beam raster than moves the beam pixel-by-pixel and line-by-line to generate the image.
The rate at which the scanning coils are changed is controlled by the Scan Speed, and the size of the area rastered is controlled by the Magnification.