We present numerical calculations on the field distribution in the focus of
an optical system with high numerical aperture. The diffraction integrals
which are based on the Debye approximation are derived and evaluated for a
radially polarized input field with a doughnut-shaped intensity distributio
n. It is shown that this mode focusses down to a spot size significantly sm
aller as compared to the case of linear polarization. An experimental setup
to measure the three-dimensional intensity distribution in the focal regio
n is presented, which is based on the knife-edge method and on tomographic
reconstruction.