A high-resolution ultraviolet–visible Fourier transform spectrometer has been used to record spectra from a Grimm-type glow discharge source with and without supplementary microwave excitation. Various cathodes were used (copper, aluminium, iron and chromium) with argon or neon as the carrier gas. True line profiles were recorded directly, so that line shapes could be studied easily. Close lines could be distinguished and identifications made unambiguously, even when major changes in relative intensities occurred. In this paper, information is presented on the effects of the excitation conditions on the iron and chromium spectra, using relative intensity measurements on a very large number of spectral lines. The results emphasize that any ‘excitation temperatures’ deduced from data on a limited number of lines are meaningless. The contribution of charge exchange processes to the excitation of ionic lines is discussed in detail. Line profile measurements will be reported in a subsequent paper.