The 'beam section' is normally the first test plane, which would typically be the focal plane of the system. At the end of a calculation the crossing points of the rays at the beam section are displayed on the screen.
In the case of cathodes the beam section is taken as the second test plane (because the first test plane is used for another purpose, see note on cathode test plane).
The ray parameters at the beam section are listed in summary form at the end of the output data file (if the print level is high enough).
There is an option to limit the number of crossing points that are displayed on the screen (this option is sometimes useful for cathodes). The option is labelled ‘Maximum number of beam section points’ and it can be found in the data-builder under /tracing rays/field of view. If the number N that is entered is less than the total number of rays then the crossing points of the first N rays will be shown. The pre-set value of N is the maximum number of rays.
In CPO2D the beam section is a line but in CPO3D it is a square that encloses the crossing points (and this square lies in the first test plane and has sides of length at least 2.2 times the maximum distance of any of the points from their centre of gravity).
In CPO2D the list at the end of the output data file includes 2 parameters that are relevant to lens action (the angle that a ray makes with respect to the z axis, and the extrapolated axis crossing), if the beam section is perpendicular to the axis.