A technique for determining the optical density to relative exposure calibration, or so-called characteristic curve, for photographic emulsions in the extreme ultra-violet (XUV) using a grazing incidence spectrograph and an XUV source of unknown emission is discussed. The authors use aluminium foils of measured thickness and hence known attenuation to the XUV radiation to produce different relative exposures of spectral lines in a spectrograph with a laser-produced plasma as the XUV source. From the corresponding optical densities of spectral lines resulting on the photographic emulsion used in the spectrograph, the optical density to relative exposure calibration for the emulsion is determined. The relative exposures for different spectral lines are related in order to build-up comprehensive characteristic curves using a novel least-squares fitting technique. Characteristic curves for Kodak-Pathe SC5 and SC7 emulsions determined for wavelengths approximately 30 AA using the method are presented.