Experimental Laboratory Exercises
It is possible to put a current mirror inside the SEM and use it to look at the inside of the chamber. The mirror consists of an object that is charged to high voltage. For instance, we used a metal sphere on an insulating support. The sphere was charged by illuminating it with 30 kV electrons. After the sphere was charged, the energy of the electron beam was reduced to 3 kV. These electrons are reflected by the charged sphere. The electrons never touch the sphere; they are deflected by the negative charge on the sphere. The deflected electrons hit the side of the chamber somewhere and generate secondary electrons which are detected by the secondary electron detector.
The charged sphere discharges slowly. This caused the images to shift and distort. After a few minutes, the sphere discharged so much that the current mirror effect was lost. To make more serious experiments with this effect it would be necessary to use a stable high voltage supply to keep the sphere at a fixed potential.