The double condenser system or illumination system consists of two or more lenses and an aperture. It is used in both SEM and TEM. Its function is to control spot size and beam convergence.

Two or more lenses can act together (as shown here) and their ray diagrams can be constructed using the thin lens approximation for each of them.

The diagram opposite shows the ray diagram for the double condenser system. The black dots represent the focal point of each lens.