The flame radiant emission from internal combustion engines is investigated experimentally by means of a fast spectroscopic measuring technique. This technique enables cycle resolved emission spectroscopy within a single combustion process and was applied to a DI diesel engine.The ‘cycle resolved emission spectroscopy’ (CRES) measuring technique with its optical components: optical probe, light-guide, spectrograph and ICCD-Camera is described in detail. These components were carefully designed for highest spectral transmittance for maximum signal/noise ratio in the UV and VIS wavelength region.The investigations focus on two different aspects: Broadband spectral radiation caused by thermal excited soot particles and narrowband spectral radiation caused by chemoluminescence from electronically excited OH radicals are measured. The soot radiation measurements in the visible enable calculations of the mean soot particle temperature and the mean soot volume fraction. From narrowband spectral OH radical radiation a qualitative OH radical concentration is derived.