  Based on results obtained with FORS1 in long-slit mode we demonstrate the
power of the VLT for the spectroscopy of faint emission line nebulae such as
the gaseous halos of diffuse ionized gas (DIG) in spiral galaxies. It is shown
that VLT spectra of DIG allow us to address the ionization and excitation
processes for the interstellar medium on galactic scales and that in the future
more detailed kinematical studies of DIG could help to constrain the origin of
the observed thick H+ layer in galaxies. The currently available
instrumentation with regard to this application is compared to other possible
designs and efficiencies for integral field spectrographs, in particular
Fabry-Perot systems.
